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Satellite Communications, GSM specification and explain functional architecture of GSM , Introduction to Wireless LAN and IEEE 802.11, WIRELESS LANS AND PANS, Multiplexing, MULTIPLE ACCESS TECHNIQUES

Satellite Communications

 Overview

Basics of Satellites

Types of Satellites

Capacity Allocation

 Basics : HOW DO SATELLITES WORK

Two stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.

The two stations can use a satellite as a relay station for their communication

One Earth Station sends a transmission to the satellite. This is called a Uplink.

The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.

ADVANTAGES OF SATELLITES

The advantages of satellite communication over terrestrial communication are:

The coverage area of a satellite greatly exceeds that of a terrestrial system.

Transmission cost of a satellite is independent of the distance from the center of the coverage area.

Satellite to Satellite communication is very precise.

Higher Bandwidths are available for use.

DISADVANTAGES OF SATELLITES

 The disadvantages of satellite communication:

Launching satellites into orbit is costly.

Satellite bandwidth is gradually becoming used up.

There is a larger propagation delay in satellite communication that in terrestrial communication.

FACTORS IN SATELLITE COMMUNICATION

Elevation Angle: The angle of the horizontal of the earth surface to the center line of the satellite transmission beam: 

This effects the satellites coverage area. Ideally, You want a elevation angle of 0 degrees, so the transmission beam reaches the horion visible to the satellite in all directions.

However, because of environmental factors like objects blocking the transmission, atmospheric attenuation and the earth electrical background noise, there is a minimum elevation angle of earth stations.

HOW SATELLITES ARE USED

Service Types

- Fixed service satellites(FSS)

example: point to point communication

- Broadcast Service Satellites (BSS)

example: Satellite television / radio

also called direct broadcast service(DBS).

- Mobile Service Satellites (MSS)

example: Satellite phones.

TYPES OF SATELLITES

Satellite Orbits

* GEO

* LEO

* MEO

* Molniya Orbit

* HAPs

Frequency Bands

GEOSTATIONARY EARTH ORBIT (GEO)

These satellites are in orbit 35863 km above the earth's surface along the equator.

Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth.

ADVANTAGES

* A GEO satellite's distance from earth gives it a large coverage area, almost a fourth of the earth's surface.

* GEO satellites have a 24 hour view of  a particular area.

* These factors make it ideal for satellite broadcast and other multi point applications.

DISADVANTAGES

* A GEO satellite's distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to point communication.

* GEO satellites, centered above the equator, have a difficulty broadcasting signals to near polar regions.

LOW EARTH ORBIT (LEO)

LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1500 km above the surface.

LEO satellites don't stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.

A network of LEO satellites is necessary for LEO satellites to be useful.

ADVANTAGES

* A LEO satellite's proximity to each compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.

* A LEO satellite's smaller area of coverage is less of a waste of bandwidth.

DISADVANTAGES

* A network of LEO satellites is needed, which can be costly

* LEO satellites have to compensate for Doppler shifts cause by their relative movement.

* Atmosphere  drag effects LEO satellites, causing gradual orbital deterioration.

MEDIUM EARTH ORBIT (MEO)

A MEO satellite is in orbit somewhere between 8000km and 18000 km above the earth's surface.

MEO satellites are similar to LEO satellites in functionality.

MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to 8 hours.

MEO satellites have a larger coverage area than LEO satellites.

ADVANTAGE

* A MEO  satellite's longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than a LEO network.

DISADVANTAGE

* A MEO satellite's distance gives  it a longer time delay and weaker signal than a LEO satellite, through not as bad as a GEO satellite,

FREQUENCY BANDS

Different kinds of satellites use different frequency bands

* L- Band: 1 to 2 Ghz, used by MSS

* S-Band: 2 to 4 Ghz, used by MSS, NASA, Deep space research

* C-Band: 4 to 8 Ghz, used by FSS

* X-Band: 8 to 12.5Ghz, used by FSS and in terrestrial imaging, ex: military and meteorological satellites.

* Ku-Band: 12.5 to 18 Ghz: used by FSS and BSS(DBS)

* K-Band: 18 to 26.5 GHz : used by Fss and BSS

* Ka-Band : 26.5 to 40 Ghz: used by FSS

CAPACITY ALLOCATION

 FDMA

* FAMA- FDMA

* DAMA - FDMA

 

TDMA

* Advantages over FDMA

 

FDMA

Satellite frequency is already broken into bands and is broken in to smaller channels in Frequency Division Multiple Access.

Overall bandwidth within a frequency band is increased due to frequency reuse (a frequency is used by two carriers with orthogonal polarization).

The number of sub-channels is limited by three factors:

    * Thermal noise ( too weak a signal will be effected by background noise)

    * Intermodulation noise ( too storage a signal will cause noise)

    * Crosstalk (cause by excessive frequncy reusing)

FDMA can be performed in two ways:

* Fixed-assignment multiple access(FAMA): The sub-channel assignments are of a fixed allotment. Ideal for broadcast satellite communication.

* Demand-assignment multiple access (DAMA): The sub-channel allotment changes based on demand. Ideal for point to point communication.

TDMA

TDMA (Time Division Multiple access) breaks a transmission into multiple time slots, each one dedicated to a different transmitter.

TDMA is increasingly becoming more widespread in satellite communication.

TDMA uses the same techniques (FAMA and DAMA) as FDMA does.

Advantages of TDMA over FDMA:

Digital equipment used in time division multiplexing is increasing becoming cheaper.

There are advantages in digital transmission techniques Ex error correction

Lack of inter modulation noise means increased efficiency.

 

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List out GSM specification and explain functional architecture of GSM

 

GSM Specification

 

 Uses a combination of FDMA (Frequency Division Multiple Access) and TDMA (Time Division

Multiple Access).

 

 Allocation of 50 MHz (890–915 MHz and 935–960 MHz) bandwidth in the 900 MHz frequency

band and using FDMA further divided into 124 (125 channels, 1 not used) channels each with a

carrier bandwidth of 200 KHz.

 

 Using TDMA, each of the above mentioned channels is then further divided into 8 time slots

 

 So, with the combination of FDMA and TDMA, a maximum of992 channels for transmit and

receive can be realized.

 

 

 

Frequency reuse in GSM

 

 To serve hundreds of thousands of users, the frequency must be reused and this is done

through cells.  

 

 The area to be covered is subdivided into radio zones or cells. Though in reality these cells could

be of any shape, for convenient modeling purposes these are modeled as hexagons. Base

stations are positioned at the center of these cells.

 

Figure : Cell Structure in GSM 

 

 Each cell i receive a subset of frequencies fbi from the total set assigned to the respective

mobile network.

 

 To avoid any type of co-channel interference, two neighboring cells never use the same

frequencies.

 

 Only at a distance of D (known as frequency reuse distance), the same frequency from the set

fbi can be reused. Cells with distance D from cell i, can be assigned one or all the frequencies

from the set fbi belonging to cell i.

 

 When moving from one cell to another during an ongoing conversation, an automatic channel

change occurs. This phenomenon is called handover.

 

 Handover maintains an active speech and data connection over cell boundaries.

 

 The regular repetition of frequencies in cells results in a clustering of cells. The clusters

generated in this way can consume the whole frequency band.

 

 The size of a cluster is defined by k, the number of cells in the cluster. This also defines the

frequency reuse distance D. The figure in next slideshows an example of cluster size of 4. 

 

 

GSM Architecture

 

 In System, It consists at the minimum one administrative region assigned to one MSC (Mobile

Switching Centre).

 

 Administrative region is commonly known as PLMN (Public Land Mobile Network).

 

 Each administrative region is subdivided into one or many Location Area (LA).

 

 One LA consists of many cell groups and each cell group is assigned to one BSC (Base Station

Controller).

 

 For each LA, there will be at least one BSC while cells in one BSC can belong to different LAs.

Figure 11: GSM Architecture

 

 Cells are formed by the radio areas covered by a BTS (Base Transceiver Station). Several BTSs

are controlled by one BSC.

 

 Traffic from the MS (Mobile Station) is routed through MSC. Calls originating from or

terminating in a fixed network or other mobile networks is handled by the GMSC (Gateway

MSC)

 

 For all subscribers registered with a cellular network operator, permanent data such as the

service profile is stored in the Home Location Register (HLR). The data relate to the following

information:-

 

o Authentication information like IMSI.

 

o Identification information like name, address, etc., of the subscriber.

 

o Identification information like MSISDN, etc.

 

o Billing information like prepaid or postpaid customer.

 

o Operator select denial of service to a subscriber.

 

o Handling of supplementary services like for CFU (Call Forwarding Unconditional), CFB

(Call Forwarding Busy), CFNR (Call Forwarding Not Reachable) or CFNA (Call Forwarding

Not Answered)

 

o Storage of SMS Service Center (SC) number in case the mobile is not connectable so that

whenever the mobile is connectable, a paging signal is sent to the SC

 

o Provisioning information like whether long distance and international calls allowed or

not.

 

o Provisioning information like whether roaming is enabled or not

 

o Information related to auxiliary services like Voice mail, data, fax services, etc.

 

o Information related to auxiliary services like CLI (Caller Line Identification), etc.

 

o Information related to supplementary services for call routing. In GSM network, one can

 customize the personal profile to the extent that while the subscriber is roaming in a

foreign PLMN, incoming calls can be barred. Also, outgoing international calls can be

barred, etc.

 

o Some variable information like pointer to the VLR,location area ofthe subscriber, Power

OFF status of the handset, etc.

 

 The GSM technical specifications define different entities that form the GSM network by

defining their functions and interface requirements. The GSM network can be divided into 5

main groups:-

 

 The Mobile Station(MS): This includes the Mobile Equipment(ME) and the Subscriber Identity

Module (SIM).

 

 The Base Station Subsystem (BSS): This includes the Base Transceiver Station (BTS) and the

Base Station Controller(BSC).

 

 The Network and Switching Subsystem (NSS): This includes Mobile Switching Center (MSC),

Home Location Register (HLR), Visitor Location Register (VLR), Equipment Identity Register(EIR),

and the Authentication Center(AUC).

 

 The Operation and Support Subsystem (OSS): This includes the Operation and Maintenance

Center (OMC).

 

 The data infrastructure that includes Public Switched Telephone Network (PSTN), Integrated

System Digital Network (ISDN), and the Public Data Network (PDN). 

 

 

Explain the handover procedure in GSM system OR What is

handover/handoff? How handoff is different from roaming?

 

 The process of handover or handoff within any cellular system is of great importance.

 

 It is a critical process and if performed incorrectly handover can result in the loss of the call.

 

 Dropped calls are particularly annoying to users and if the number of dropped calls rises,

customer dissatisfaction increases and they are likely to change to another network. 

 

 

Types of GSM handover

 

 Within the GSM system there are four types of handover that can be performed for GSM only

systems:

 

o Intra-BTS handover: This form of GSM handover occurs if it is required to change the

frequency or slot being used by a mobile because of interference, or other reasons.

 

o In this form of GSM handover, the mobile remains attached to the same base station

transceiver, but change the channel or slot.

 

o Inter-BTSIntraBSC handover: ThisGSMhandover or GSMhandoff occurs when the

mobile is moved out of the coverage area of one BTS but into another controlled by the

same BSC.

 

o In this instance the BSC is able to perform the handover and it assigns a new channel

and slot to the mobile, before releasing the old BTS from communicating with the

mobile.

 

o Inter-BSC handover: When the mobile is moved out of the range of cells controlled by

oneBSC, a more involved form of handover has to be performed, handing over not only

from one BTS to another but one BSC to another.

 

o For this the handover is controlled by the MSC.

 

o Inter-MSChandover:Thisformofhandoveroccurswhenchangingbetweennetworks.

 

The two MSCs involved negotiate to control the handover. 

 

 

GSM handover process

 

 Although there are several forms of GSM handover as detailed above, as far as the mobile is

concerned, they are effectively seen as very similar. There are a number of stages involved in

undertaking a GSM handover from one cell or base station to another.

 

 In GSM, which uses TDMA techniques the transmitter only transmits for one slot in eight, and

similarly the receiver only receives for one slot in eight.

 

 As a result the RF section of the mobile could be idle for 6 slots out of the total eight.

 

 This is not the case because during the slots in which it is not communicating with the BTS, it

scans the other radio channels looking for beacon frequencies that may be stronger or more

suitable.

 

 In addition to this, whenthemobile communicates with a particular BTS,one of there sponses it

makes is to send out a listof the radio channels of the beacon frequencies of neighboringBTSs

via the Broadcast Channel (BCCH).

 

 The mobile scans these and reports back the quality of the link to the BTS. In this way the

mobile assists in the handover decision and as a result this form of GSM handoveris known as

Mobile AssistedHand over (MAHO).

 

 The network knows the quality of the link between the mobile and the BTS as well as the

strength of local BTSs as reported back by the mobile.

 

 It also knows the availability of channels in the nearby cells. As a result it has all the information

it needs to be able to make a decision about whether it needs to hand the mobile over from one

BTS to another

 

 If the network decides that it is necessary for the mobile to hand over, it assigns a new channel

and time slot to the mobile. It informs the BTS and the mobile of the change.

 

 The mobile then retunes during the period it is not transmitting or receiving, i.e. in an idle

period.

 

 A key element of the GSM handover is timing and synchronization. There are a number of

possible scenarios that may occur dependent upon the level of synchronization. 

 

 

Roaming

 

 In wireless telecommunications, roaming is a general term that refers to the extending of

connectivity service in a location that is different from the home location where the service was

registered. Roaming ensures that the wireless device keeps connected tothenetwork, without

losing the connection. The term "roaming" originates from the GSM (Global System for Mobile

Communications) sphere; the term"roaming" canalso be applied to the CDMAtechnology.

 

 

Figure 12: Handoff Process 

 

 

Handoff 

 In cellular telecommunications, the term handover or handoff refers to the process of

transferring an ongoing call or data session from one channel connected to the core network to

another.

 

 In satellite communications it is the process of transferring satellite control responsibility from

one earth station to another without loss or interruption of service. 

.

Explain the importance of following identifiers with that GSM

deals with: - 1) IMEI 2) IMSI 3) MSISDN 

 

 International Mobile Station Equipment Identity (IMEI): It uniquely identifies a mobile station

internationally. It is a kind of serial number.

 

 The IMEI is allocated by the equipment manufacturer and registered by the network operator,

who stores it in the EIR.

 By means of IMEI one can recognize obsolete, stolen or non-functional equipment. The

following are the parts of an IMEI:

 

o Type Approval Code (TAC):- 6 decimal places, centrally assigned.

 

o Final Assembly Code (FAC):- 6 decimal places, assigned by the manufacturer.

 

o Serial Number (SNR):- 6 decimal places, assigned by the manufacturer.

 

o Spare (SP):- 1 decimal place. 

 

 International Mobile Subscriber Identity (IMSI): Each registered user is uniquely identified by

its international mobile subscriber identity (IMSI).

 

 It is stored in the subscriber identity module (SIM). A mobile station can only be operated if a

SIM with valid IMSI is inserted into equipment with a valid IMEI.

 

 The following are the parts of IMSI:-

 

o Mobile Country Code (MCC):- 3 decimal places, internationally standardized.

 

o Mobile Network Code (MNC):- 2 decimal places, for unique identification of mobile

network within the country.

 

o Mobile Subscriber Identification Number (MSIN):- Maximum 10 decimal places,

identification number of the subscriber in the home mobile network.

 

 MobileSubscriberISDNNumber(MSISDN):Therealtelephonenumberofamobilestationis

the mobile subscriber ISDN number (MSISDN).

 

 It is assigned to the subscriber, such that a mobile station set can have several MSISDNs

depending on theSIM.

 

 The MSISDN categories follow the international ISDN number plan and therefore have the

following structure:-

 

o Country Code (CC):- Up to 3 decimal places.

 

o National Destination Code (NDC):- Typically 2-3 decimal places.

 

oSubscriber Number (SN):- Maximum 10 decimal places. 

 

 

What is SMS? Explain the strengths of SMS.

 

 Short message service-SMS is one of the most popular data bearer/service within GSM.

 

 More than one billionSMS messages interchanged every day with a growth of more than half a

billion every month on an average

 

 Runs on SS7 signaling channels, which are always present but mostly unused, be it during an

active user connection or in the idle state

 

 Each short message is upto160 characters in length when 7-bit English characters are used and

140 octets when 8-bit characters are used 

 

 

Strength of SMS

 

 Various characteristics of SMS make it as an attractive bearer for mobile computing.

 

 Omnibus nature of SMS: SMS uses SS7 signaling channel which is available throughout the

world.

 

 Stateless: SMS is session-less and stateless as every SMS message is unidirectional and

independentofanycontext.ThismakesSMSthebestbearerfornotifications,alertsandpaging.

 

 Asynchronous:SMSiscompletelyasynchronous.IncaseofSMS,eveniftherecipientisoutof

service, the transmission will not be abandoned and hence, SMS can be used as message

queues.

 

 SMS can be used as a transport bearer for both synchronous (transaction oriented) and

asynchronous (message queue and notification) information exchange.

 

 Self-configurable and last mile problem resistant: SMS is self-configurable and subscriber is

always connected to the SMS bearer irrespective of the home and visiting network

configurations.

 

 Non-repudiable: SMS message carries the Service Center (SC) and the source MSISDN as a part

of the message header through which any SMS can prove beyond doubt its origin.

 

 Always connected: As SMS uses the SS7 signaling channel for its data traffic, the bearer media

is always on. Users cannot switch OFF, BAR or DIVERT anySMS message. SMS message is

delivered to the Mobile Station (MS) without any interruption to the ongoing call.

 

 

Explain Operator-centric Pull and Operator-independent

Push. 

 

Operator Centric Pull

 

 Operators offer different information on demand and entertainment services through

connecting an Origin server to the SC via a SMS gateway.

 

 Such service providers are known as Mobile Virtual Network Operator(s) (MVNO).

 

 MVNOs develop different systems, servicesand applications toofferdata servicesusingSMS.

 

 Many enterprises use MVNOs to make their services available to mobile phone users.

 

 Let’s say few banks offer balance enquiry and other low security banking services over SMS and

customers need to register for the service.

 

 During the registration, the customer needs to mention the MSISDN of the phone which will be

used for a banking service.

 

 Once a user is registered for the service, he enters ‘BAL’ and sends the message to a service

number(like333)asaMOmessageandthenSCdeliversthisMOmessagetotheSMSgateway

(known as SME-Short Message Entity) connected to this service number.

 

 SMS gateway then forwards this message to the enterprise application and response from the

enterprise application is delivered to the MS as a MT message from the SME.

 

 Even if the subscriber is in some remote region of a foreign network within GSM coverage, he

can send the same SMS to the same service number in his home network and this makes the

home services available in the foreign network.

 

 Hence, operator-centric SMS pull service is completely ubiquitous.

 

 Connectivity between SME and Origin server could be anything like SOAP (Simple Object Access

Protocol), direct connection through TCP socket or through HTTP.

 

 There are applications where SMS is used in session oriented transactions as ‘SMS chat’ and

‘SMS contests’ need to remember the user context over multiple transactions. 

 

 

Operator Independent Pull

 Any push, which may be an alert, notification or even response from a pull message generated

by an application, can be serviced by any network and delivered to any GSM phone in any

network without any difficulty.

 

 If appropriate roaming tie-ups are inplace,an enterprise can use SMS to send business alert sor

proactive notifications to its customer anywhere, anytime on his phone. 

 

 

Figure 13: Basic Network Structure of the SMS Push

 

For a SMS message to be routed to some enterprise SME connected to external SC,SAT is used.

 

 SAT application running on the SIM card changes the SC number during the transmission of the

SMS and forces theSMS to recognize a different SC of a different network as its home SC.

 

 Here, too, SMS is sent to the SME connected to the home SC. If a SMS service isoperator

dependent, the cellular operator can use this to its advantage.

 

 Enterprises need operator independent pull as enterprises have customers around the world

subscribing to different GSM networks

 

 Above scenario can also be achieved through Intelligent Network. 

 

 

Challenges for SMS as Mobile computing bearer

 The major challenge for implementing ubiquitous service through SMS requires operator

independent SM MO messages or operator independent pull services.

 

 The SMS routing needs to work exactly in the same fashion as 1-800 services.

 

 

Explain SMS Architecture and differentiate between SM MT and

SM MO

 Two types of SMS - SM MT (Short Message Mobile Terminated Point-to-Point) and SM MO

(Short Message Mobile Originated Point-to-Point)

 

 SM MT is an incoming short message from the network and is terminated in the MS

 

 SM MO is an outgoing message originated in the MS and forwarded to the network for delivery

 

 For an outgoing message, the path is from MS to SCvia theVLR and the IWMSC(InterWorking

MSC) function of the serving MSC whereas for an incoming message the path is from SC to the

MS via HLR and the GMSC (Gateway MSC) function of the home MSC

 

 

Short Message Mobile Terminated (SMMT)

 SMMT is an incoming short message from the network and is terminated in the MS.

 

 Message is sent from SC to the MS.

 

 Forthe delivery of MTorincomingSMS messages, theSCof the serving network is never used

which implies that a SMS message can be sent from any SC in any network to a GSM phone

anywhere in theworld.

 

 

Figure 14: Interface in SMMT

 

Short Message Mobile Originated

 SMMO is an outgoing message originated in the MS and forwarded to the network for the

delivery. For a MO message, the MSC forwards the message to the home SC.

 

 MO message works in two asynchronous phases. In the first phase, the message is sent from

the MS to the home SC as a MO message.

 

 In the second phase, the message is sent from the homeSC to the MS as a MT message

 

 

Figure 15: Interface in SMMO

 

Explain call routing in GSM with block diagram.

 

 Human interface is analog. However, the advancement in digital technology makes it very

convenient to handle information in digital way.

 

 Digitizer and source coding: The user speech is digitized at 8 KHz sampling rate using Regular

Pulse Excited–Linear Predictive Coder (RPE–LPC) with a Long Term Predictor loop where

information from previous samples is used to predict the current sample.

 

 Each sample is then represented in signed 13-bit linear PCM value.

 

 This digitized data is passed to the coder with frames of 160 samples where encoder

compresses these 160 samples into 260-bits GSM frames resulting in one second of speech

compressed into 1625 bytes and achieving a rate of 13 Kbits/sec.

 

 Channel coding: This introduces redundancy into the data for error detection and possible error

correctionwhere thegross bitrate after channel coding is 22.8kbps (or 456 bits every 20 ms).

 

 These 456 bits are divided into eight 57-bit blocks and the result is interleaved amongst eight

successive time slot bursts for protection against burst transmission errors.

 

 Interleaving: This step rearranges a group of bits in a particular way to improve the

performance of the error-correction mechanisms.

 

 The interleaving decreases the possibility of losing whole bursts during the transmission by

dispersing the errors.

 

 Ciphering:This encrypts blocks of user data using asymmetric key shared by the mobilestation

and the BTS.

 

 Burst formatting: It adds some binary information to the ciphered block for use in

synchronization and equalization of the received data.

 

 Modulation:The modulation technique chosenfortheGSMsystem is theGaussian Minimum

ShiftKeying (GMSK) where binary data is converted back into analog signalto fit thefrequency

and time requirements for the multiple access rules.

 

 This signal is then radiated as radio wave over the air.

 

 Multipath and equalization: An equalizer is in charge of extracting the ‘right’ signal from the

received signal while estimating the channel impulse response of the GSM system and then it

constructs an inversefilter.

 

 The received signal is then passed through the inverse filter.

 

 Synchronization: For successful operation of a mobile radio system, time and frequency

synchronization are needed.

 

 Frequency synchronization is necessary so that the transmitter and receiver frequency match

(in FDMA) while Time synchronization is necessary to identify the frame boundary and the bits

within the frame (in TDMA).

 

 To avoid collisions of burst transmitted by MS with the adjacent timeslot such collisions, the

TimingAdvancetechniqueisusedwhereframeisadvancedintimesothatthisoffsetsthedelay

due to greaterdistance.

 

 

 

Figure 16: From speech to radio waves

 

 Using this technique and the triangulation of the intersection cell sites, the location of a mobile

station can be determined from within the network.

 

Example

 

 The MSISDN number of a subscriber in Bangalore associated with Airtel network is

+919845XYYYYY which is a unique number and understood from anywhere in the world.

 

 Here, + means prefix for international dialing, 91 is the country code for India and 45 is the

network operator’s code (Airtel in this case).

 

 X is the level number managed by the network operator ranging from 0 to 9 while YYYYY is the

subscriber code which, too, is managed by the operator.

 

 The call first goes to the local PSTN exchange where PSTN exchange looks at the routing table

and determines that it is a call to a mobile network.

 

 PSTN forwards the call to the Gateway MSC (GMSC) of the mobile network.

 

 MSC enquires the HLR to determine the status of the subscriber. It will decide whether the call

is to be routed or not. If MSC finds that the call can be processed, it will find out the address of

the VLR where the mobile is expected to be present.

 

 If VLR is that of a different PLMN, it will forward the call to the foreign PLMN through the

Gateway MSC. If theVLR is in the home network, it will determine the LocationArea (LA).

 

Figure 17: Call Routing for a mobile terminating call

 

 Within the LA, it will page and locate the phone and connect the call. 

 

 

Write Note on Signaling Protocol Structure in GSM

 

 Layer 1 is the physical layer which uses the channel structures over the air interface.

 

 Layer 2 is the data link layer and across the Um interface, the data link layer is a modified

version of the LAPD protocol used in ISDN or X.25, called LAPDm. 

 

Figure 18: Signaling protocol structure in GSM

 

 Across the A interface,the Message Transfer Part layer 2 of Signaling System Number 7 is used.

 

 Layer 3 of the GSM signaling protocol is itself divided into three sub-layers:

 

o Radio Resources Management: It controls the set-up, maintenance and termination of

radio and fixed channels, including handovers.

 

o Mobility Management: It manages the location updating and registration procedures as

well as security and authentication.

 

o Connection Management: It handles general call control and manages Supplementary

Services and the Short Message Service.

 

Explain different GSM Services.

 

 There are three types of services offered through GSM which are:

1. Telephony (also referred as tele-services) Services

2. Data (also referred as bearer services) Services

3. Supplementary Services

 

Teleservices or Telephony Services

 

 A teleservices utilizes the capabilities of a Bearer Service to transport data, defining which

capabilities are required and how they should setup.

 

o VoiceCalls: The most basic teleservices supported by GSMis telephony.This includes

fullrate speech at 13 Kbps and emergency calls, where the nearest emergency service

provider is notified by dialing three digits.

 

o Videotext and Facsimile: Another group of teleservices includes Videotext access,

Tele text transmission,and Facsimile alternate speech and facsimile Group3, automatic

facsimile Group 3etc.

 

o Short Text Messages: SMS service is a text messaging which allow you to send and

receive text messages on your GSM mobile phones. 

 

Bearer Services or Data Services

 Using your GSM phone to receive and send data is the essential building block leading to

widespread mobile Internet access and mobile and mobile data transfer.

 

 GSM currently has a data transfer rate of 9.6k.

 

 New development that will push up data transferrated for GSM users HSCSD are now available.

 

Supplementary Services

 Supplementary services are provided on top of teleservices or bearer services, andinclude

features such as caller identification, call forwarding, call waiting, multi-party conversation. A

brief description of supplementary services is given here:

 

o Multiparty Service or conferencing: The multiparty service allows a mobile subscriber to

establish multiparty conservations. That is, conservation between three or more subscribers to setup a conference calls. This service is only applicable to normal telephony.

 

o Call Waiting: This service allows a mobile subscriber to be notified of an incoming call

during a conversation.The subscriber can answer, reject or ignore the incoming call.Call waiting is applicable to all GSM telecommunications services using circuit switched connection.

 

o Call Hold: This service allows a mobile subscriber to put an incoming call on hold and then resume this call. The call hold service is only applicable to normal telephony.

 

o Call Forwarding: The call forwarding supplementary service is used to divert calls from the original recipient to another number, and is normally set up by the subscriber himself.

 

o It can be used by the subscriber to divert calls from the Mobile Station when the subscriber is not available, and so to ensure that calls are not lost.

 

o A typical scenario would be a salesperson turns off his mobile phone during a meeting with customer, but does not wish to lose potential sales leads while he is unavailable.

 

o Call Barring: The concept of barring certain type of calls might seem to be a supplementary disservice rather than service.

 

o However, there are times when the subscriber is not the actual user of the Mobile Station, and as a consequence may wish to limit its functionality, so as to limit charges incurred.

 

o If thesubscriber and users and one and same,the callbarring may be use fulto stopcalls being routed to international destinations when they are route.

 

o The reasons for this are because it is expected that are roaming subscriber will pay the charges incurred for international re-routing of calls.

 

o So, GSM devised some flexible services that enable the subscriber to conditionally bar calls.

 

 

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Introduction to Wireless LAN and IEEE

802.11 

 

A wireless LAN (WLAN or WiFi) is a data transmission system designed to provide location

independent network access between computing devices by using radio waves rather than a

cable infrastructure

 

In the corporate enterprise, wireless LANs are usually implemented as the final link between

the existing wired network and a group of client computers, giving these users wireless access

to the full resources and services of the corporate network across a building or campus

setting.

 

The widespread acceptance of WLANs depends on industry standardization to ensure product

compatibility and reliability among the various manufacturers.

 

The 802.11 specification [IEEE Std 802.11 (ISO/IEC 8802-11: 1999)] as a standard for

wireless LANS was ratified by the Institute of Electrical and Electronics Engineers (IEEE) in

the year 1997. This version of 802.11 provides for 1 Mbps and 2 Mbps data rates and a set of

fundamental signaling methods and other services. Like all IEEE 802 standards, the 802.11

standards focus on the bottom two levels the ISO model, the physical layer and link layer (see

figure below). Any LAN application, network operating system, protocol, including TCP/IP and

Novell NetWare, will run on an 802.11-compliant WLAN as easily as they run over Ethernet.

 

Fig 1: "IEEE 802.11 and the ISO Model"

The major motivation and benefit from Wireless LANs is increased mobility. Untethered from

conventional network connections, network users can move about almost without restriction

and access LANs from nearly anywhere.

 

The other advantages for WLAN include cost-effective network setup for hard-to-wire locations

such as older buildings and solid-wall structures and reduced cost of ownership-particularly in

dynamic environments requiring frequent modifications, thanks to minimal wiring and

installation costs per device and user. WLANs liberate users from dependence on hard-wired

access to the network backbone, giving them anytime, anywhere network access. This

freedom to roam offers numerous user benefits for a variety of work environments, such as:

 

 Immediate bedside access to patient information for doctors and hospital staff

 Easy, real-time network access for on-site consultants or auditors

 Improved database access for roving supervisors such as production line managers,

warehouse auditors, or construction engineers

 Simplified network configuration with minimal MIS involvement for temporary setups such

as trade shows or conference rooms

 Faster access to customer information for service vendors and retailers, resulting in

better service and improved customer satisfaction

 Location-independent access for network administrators, for easier on-site

troubleshooting and support

 Real-time access to study group meetings and research links for students

 

IEEE 802.11 Architecture

The difference between a portable and mobile station is that a portable station moves from

point to point but is only used at a fixed point. Mobile stations access the LAN during

movement.

 

When two or more stations come together to communicate with each other, they form a Basic

Service Set (BSS). The minimum BSS consists of two stations. 802.11 LANs use the BSS as

the standard building block.

 

A BSS that stands alone and is not connected to a base is called an Independent Basic

Service Set (IBSS) or is referred to as an Ad-Hoc Network. An ad-hoc network is a network

where stations communicate only peer to peer. There is no base and no one gives permission

to talk. Mostly these networks are spontaneous and can be set up rapidly. Ad-Hoc or IBSS

networks are characteristically limited both temporally and spatially

 

Fig 1: "Adhoc Mode"

 

When BSS's are interconnected the network becomes one with infrastructure. 802.11

infrastructure has several elements. Two or more BSS's are interconnected using a

Distribution System or DS. This concept of DS increases network coverage. Each BSS

becomes a component of an extended, larger network. Entry to the DS is accomplished with

the use of Access Points (AP). An access point is a station, thus addressable. So, data moves

between the BSS and the DS with the help of these access points.

 

Creating large and complex networks using BSS's and DS's leads us to the next level of

hierarchy, the Extended Service Set or ESS. The beauty of the ESS is the entire network

looks like an independent basic service set to the Logical Link Control layer (LLC). This means

that stations within the ESS can communicate or even move between BSS′s transparently to

the LLC.

 

Fig 2: Infrastructure Mode

 

 

One of the requirements of IEEE 802.11 is that it can be used with existing wired networks.

802.11 solved this challenge with the use of a Portal. A portal is the logical integration

between wired LANs and 802.11. It also can serve as the access point to the DS. All data

going to an 802.11 LAN from an 802.X LAN must pass through a portal. It thus functions as

bridge between wired and wireless.

 

The implementation of the DS is not specified by 802.11. Therefore, a distribution system may

be created from existing or new technologies. A point-to-point bridge connecting LANs in two

separate buildings could become a DS.

 

While the implementation for the DS is not specified, 802.11 does specify the services, which

the DS must support. Services are divided into two sections

 

1. Station Services (SS)

2. Distribution System Services (DSS).

 

There are five services provided by the DSS

1. Association

2. Reassociation

3. Disassociation

4. Distribution

5. Integration

 

The first three services deal with station mobility. If a station is moving within its own BSS or is

not moving, the stations mobility is termed No-transition. If a station moves between BSS's

within the same ESS, its mobility is termed BSS-transition. If the station moves between BSS's

of differing ESS's it is ESS transition. A station must affiliate itself with the BSS infrastructure if

it wants to use the LAN. This is done by Associating itself with an access point. Associations

are dynamic in nature because stations move, turn on or turn off. A station can only be

associated with one AP. This ensures that the DS always knows where the station is.

 

Association supports no-transition mobility but is not enough to support BSS-transition. Enter

Reassociation. This service allows the station to switch its association from one AP to another.

 

Both association and reassociation are initiated by the station. Disassociation is when the

association between the station and the AP is terminated. This can be initiated by either party.

 

A disassociated station cannot send or receive data. ESS-transition are not supported. A

station can move to a new ESS but will have to reinitiate connections.

 

Distribution and Integration are the remaining DSS's. Distribution is simply getting the data

from the sender to the intended receiver. The message is sent to the local AP (input AP), then

distributed through the DS to the AP (output AP) that the recipient is associated with. If the

sender and receiver are in the same BSS, the input and out AP's are the same. So the

distribution service is logically invoked whether the data is going through the DS or not.

Integration is when the output AP is a portal. Thus, 802.x LANs are integrated into the 802.11

DS.

 

Station services are:

1. Authentication

2. Deauthentication

3. Privacy

4. MAC Service Data Unit (MSDU) Delivery.

 

With a wireless system, the medium is not exactly bounded as with a wired system. In order to

control access to the network, stations must first establish their identity. This is much like

trying to enter a radio net in the military.

 

Before you are acknowledged and allowed to converse, you must first pass a series of tests to

ensure that you are who you say you are. That is really all authentication is. Once a station

has been authenticated, it may then associate itself. The authentication relationship may be

between two stations inside an IBSS or to the AP of the BSS. Authentication outside of the

BSS does not take place.

 

There are two types of authentication services offered by 802.11. The first is Open System

Authentication. This means that anyone who attempts to authenticate will receive

authentication. The second type is Shared Key Authentication. In order to become

authenticated the users must be in possession of a shared secret. The shared secret is

implemented with the use of the Wired Equivalent Privacy (WEP) privacy algorithm. The

shared secret is delivered to all stations ahead of time in some secure method (such as

someone walking around and loading the secret onto each station).

 

Deauthentication is when either the station or AP wishes to terminate a stations

authentication. When this happens the station is automatically disassociated. Privacy is an

encryption algorithm, which is used so that other 802.11 users cannot eavesdrop on your LAN

traffic. IEEE 802.11 specifies Wired Equivalent Privacy (WEP) as an optional algorithm to

satisfy privacy. If WEP is not used then stations are "in the clear" or "in the red", meaning that

their traffic is not encrypted. Data transmitted in the clear are called plaintext. Data

transmissions, which are encrypted, are called ciphertext. All stations start "in the red" until

they are authenticated. MSDU delivery ensures that the information in the MAC service data

unit is delivered between the medium access control service access points.

 

The bottom line is this, authentication is basically a network wide password. Privacy is whether

or not encryption is used. Wired Equivalent Privacy is used to protect authorized stations from

eavesdroppers. WEP is reasonably strong. The algorithm can be broken in time. The

relationship between breaking the algorithm is directly related to the length of time that a key is

in use. So, WEP allows for changing of the key to prevent brute force attack of the algorithm.

 

WEP can be implemented in hardware or in software. One reason that WEP is optional is

because encryption may not be exported from the United States. This allows 802.11 to be a

standard outside the U.S. albeit without the encryption.

 

Physical Layer

The three physical layers originally defined in 802.11 included two spread-spectrum radio

techniques and a diffuse infrared specification.

 

The radio-based standards operate within the 2.4 GHz ISM band. These frequency bands are

recognized by international regulatory agencies radio operations. As such, 802.11-based

products do not require user licensing or special training

 

Spread-spectrum techniques, in addition to satisfying regulatory requirements, increase

reliability, boost throughput, and allow many unrelated products to share the spectrum without

explicit cooperation and with minimal interference.

 

The original 802.11 wireless standard defines data rates of 1 Mbps and 2 Mbps via radio waves

using frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).

It is important to note that FHSS and DSSS are fundamentally different signaling mechanisms

and will not interoperate with one another.

 

Using the frequency hopping technique, the 2.4 GHz band is divided into 75 1-MHz subchannels.

The sender and receiver agree on a hopping pattern, and data is sent over a sequence of the

subchannels. Each conversation within the 802.11 network occurs over a different hopping

pattern, and the patterns are designed to minimize the chance of two senders using the same

subchannel simultaneously.

 

FHSS techniques allow for a relatively simple radio design, but are limited to speeds of no higher

than 2 Mbps. This limitation is driven primarily by FCC (Federal Communications Commission

USA) regulations that restrict subchannel bandwidth to 1 MHz. These regulations force FHSS

systems to spread their usage across the entire 2.4 GHz band, meaning they must hop often,

which leads to a high amount of hopping overhead.

 

In contrast, the direct sequence signaling technique divides the 2.4 GHz band into 14 22-MHz

channels. Adjacent channels overlap one another partially, with three of the 14 being completely

non-overlapping. Data is sent across one of these 22 MHz channels without hopping to other

channels.

 

To compensate for noise on a given channel, a technique called “chipping” is used. Each bit of

user data is converted into a series of redundant bit patterns called “chips.” The inherent

redundancy of each chip combined with spreading the signal across the 22 MHz channel

provides for a form of error checking and correction; even if part of the signal is damaged, it can

still be recovered in many cases, minimizing the need for retransmissions.

 

Data Link Layer

 

The data link layer within 802.11 consists of two sublayers: Logical Link Control (LLC) and Media

Access Control (MAC).

 

802.11 uses the same 802.2 LLC and 48-bit addressing as other 802 LANs, allowing for very

simple bridging from wireless to IEEE wired networks, but the MAC is unique to WLANs.

 

The 802.11 MAC is very similar in concept to 802.3, in that it is designed to support multiple

users on a shared medium by having the sender sense the medium before accessing it.

 

For 802.3 Ethernet LANs, the Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

protocol regulates how Ethernet stations establish access to the wire and how they detect and

handle collisions that occur when two or more devices try to simultaneously communicate over

the LAN. In an 802.11 WLAN, collision detection is not possible due to what is known as the

“near/far” problem: to detect a collision, a station must be able to transmit and listen at the same

time, but in radio systems the transmission drowns out the ability of the station to “hear” a

collision.

 

To account for this difference, 802.11 uses a slightly modified protocol known as Carrier Sense

Multiple Access with Collision Avoidance (CSMA/CA) or the Distributed Coordination Function

(DCF). CSMA/CA attempts to avoid collisions by using explicit packet acknowledgment (ACK),

which means an ACK packet is sent by the receiving station to confirm that the data packet

arrived intact.

 

CSMA/CA works as follows. A station wishing to transmit senses the air, and, if no activity is

detected, the station waits an additional, randomly selected period of time and then transmits if

the medium is still free. If the packet is received intact, the receiving station issues an ACK frame

that, once successfully received by the sender, completes the process. If the ACK frame is not

detected by the sending station, either because the original data packet was not received intact

or the ACK was not received intact, a collision is assumed to have occurred and the data packet

is transmitted again after waiting another random amount of time.

 

CSMA/CA thus provides a way of sharing access over the air. This explicit ACK mechanism also

handles interference and other radio related problems very effectively. However, it does add

some overhead to 802.11 that 802.3 does not have, so that an 802.11 LAN will always have

slower performance than an equivalent Ethernet LAN.

 

Another MAC-layer problem specific to wireless is the “hidden node” issue, in which two stations

on opposite sides of an access point can both “hear” activity from an access point, but not from

each other, usually due to distance or an obstruction. 

Fig 1: RTS/CTS Procedure eliminates the “Hidden Node” Problem

 

To solve this problem, 802.11 specifies an optional Request to Send/Clear to Send (RTS/CTS)

protocol at the MAC layer. When this feature is in use, a sending station transmits an RTS and

waits for the access point to reply with a CTS. Since all stations in the network can hear the

access point, the CTS causes them to delay any intended transmissions, allowing the sending

station to transmit and receive a packet acknowledgment without any chance of collision.

 

Since RTS/CTS adds additional overhead to the network by temporarily reserving the medium, it

is typically used only on the largest-sized packets, for which retransmission would be expensive

from a bandwidth standpoint.

 

Finally, the 802.11 MAC layer provides for two other robustness features: CRC checksum and

packet fragmentation. Each packet has a CRC checksum calculated and attached to ensure that

the data was not corrupted in transit. This is different from Ethernet, where higher-level protocols

such as TCP handle error checking. Packet fragmentation allows large packets to be broken into

smaller units when sent over the air, which is useful in very congested environments or when

interference is a factor, since larger packets have a better chance of being corrupted. This

technique reduces the need for retransmission in many cases and thus improves overall wireless

network performance. The MAC layer is responsible for reassembling fragments received,

rendering the process transparent to higher level protocols.

 

Support for Time-Bounded Data

Time-bounded data such as voice and video is supported in the 802.11 MAC specification

through the Point Coordination Function (PCF). As opposed to the DCF, where control is

distributed to all stations, in PCF mode a single access point controls access to the media. If a

BSS is set up with PCF enabled, time is spliced between the system being in PCF mode and in

DCF (CSMA/CA) mode. During the periods when the system is in PCF mode, the access point

will poll each station for data, and after a given time move on to the next station. No station is

allowed to transmit unless it is polled, and stations receive data from the access point only when

they are polled. Since PCF gives every station a turn to transmit in a predetermined fashion, a

maximum latency is guaranteed. A downside to PCF is that it is not particularly scalable, in that a

single point needs to have control of media access and must poll all stations, which can be

ineffective in large networks.

 

IEEE 802.11 Standards

The most critical issue affecting WLAN demand has been limited throughput.

 

The data rates supported by the original 802.11 standards are too slow to support most

general business requirements and slowed the adoption of WLANs.

 

Recognizing the critical need to support higher data-transmission rates, the IEEE ratified the

802.11b standard (also known as 802.11 High Rate) for transmissions of up to 11 Mbps.

 

After 802.11b one more standard 802.11a has been ratified and in January 2002 the draft

specification of another 802.11g has been approved. 802.11g is expected to be ratified till

early 2003.

 

The letters after the number "802.11" tell us the order in which the standards were first

proposed . This means that the "new" 802.11a is actually older than the currently used

802.11b, which just happened to be ready first because it was based on relatively simple

technology-Direct Sequence Spread Spectrum (DSSS), as opposed to 802.11a's Orthogonal

Frequency Division Multiplexing (OFDM). The more complex technology provides a higher

data rate: 802.11b can reach 11Mbits/sec, while 802.11a can reach 54Mbits/sec.

 

IEEE 802.11b

With 802.11b WLANs, mobile users can get Ethernet levels of performance, throughput, and

availability.

 

The basic architecture, features, and services of 802.11b are defined by the original 802.11

standard. The 802.11b specification affects only the physical layer, adding higher data rates and

more robust connectivity.

 

The key contribution of the 802.11b addition to the wireless LAN standard was to standardize the

physical layer support of two new speeds,5.5 Mbps and 11 Mbps.

 

To accomplish this, DSSS had to be selected as the sole physical layer technique for the

standard since, as frequency hopping cannot support the higher speeds without violating current

FCC regulations. The implication is that 802.11b systems will interoperate with 1 Mbps and 2

Mbps 802.11 DSSS systems, but will not work with 1 Mbps and 2 Mbps 802.11 FHSS systems.

 

The original 802.11 DSSS standard specifies an 11-bit chipping?called a Barker sequence?to

encode all data sent over the air. Each 11-chip sequence represents a single data bit (1 or 0),

and is converted to a waveform, called a symbol, that can be sent over the air.

 

These symbols are transmitted at a 1 MSps (1 million symbols per second) symbol rate using

technique called Binary Phase Shift Keying BPSK). In the case of 2 Mbps, a more sophisticated

implementation called Quadrature Phase Shift Keying (QPSK) is used; it doubles the data rate

available in BPSK, via improved efficiency in the use of the radio bandwidth. To increase the

data rate in the 802.11b standard, advanced coding techniques are employed.

 

Rather than the two 11-bit Barker sequences, 802.11b specifies Complementary Code Keying

(CCK), which consists of a set of 64 8-bit code words. As a set, these code words have unique

mathematical properties that allow them to be correctly distinguished from one another by a

receiver even in the presence of substantial noise and multipath interference (e.g., interference

caused by receiving multiple radio reflections within a building).

 

The 5.5 Mbps rate uses CCK to encode 4 bits per carrier, while the 11 Mbps rate encodes 8 bits

per carrier. Both speeds use QPSK as the modulation technique and signal at 1.375 MSps. This

is how the higher data rates are obtained. To support very noisy environments as well as

extended range, 802.11b WLANs use dynamic rate shifting, allowing data rates to be

automatically adjusted to compensate for the changing nature of the radio channel. Ideally, users

connect at the full 11 Mbps rate.

 

However when devices move beyond the optimal range for 11 Mbps operation, or if substantial

interference is present, 802.11b devices will transmit at lower speeds, falling back to 5.5, 2, and

1 Mbps. Likewise, if the device moves back within the range of a higher-speed transmission, the

connection will automatically speed up again. Rate shifting is a physical layer mechanism

transparent to the user and the upper layers of the protocol stack.

 

One of the more significant disadvantages of 802.11b is that the frequency band is crowded, and

subject to interference from other networking technologies, microwave ovens, 2.4GHz cordless

phones (a huge market), and Bluetooth. There are drawbacks to 802.11b, including lack of

interoperability with voice devices, and no QoS provisions for multimedia content. Interference

and other limitations aside, 802.11b is the clear leader in business and institutional wireless

networking and is gaining share for home applications as well.

 

IEEE 802.11a

802.11a, is much faster than 802.11b, with a 54Mbps maximum data rate operates in the 5GHz

frequency range and allows eight simultaneous channels

 

802.11a uses Orthogonal Frequency Division Multiplexing (OFDM), a new encoding scheme that

offers benefits over spread spectrum in channel availability and data rate.

 

Channel availability is significant because the more independent channels that are available, the

more scalable the wireless network becomes. 802.11a uses OFDM to define a total of 8 non

overlapping 20 MHz channels across the 2 lower bands. By comparison, 802.11b uses 3 non

overlapping channels.

 

All wireless LANs use unlicensed spectrum; therefore they're prone to interference and

transmission errors. To reduce errors, both types of 802.11 automatically reduce the Physical

layer data rate. IEEE 802.11b has three lower data rates (5.5, 2, and 1Mbit/sec), and 802.11a

has seven (48, 36, 24, 18, 12, 9, and 6Mbits/sec). Higher (and more) data rates aren't 802.11a's

only advantage. It also uses a higher frequency band, 5GHz, which is both wider and less

crowded than the 2.4GHz band that 802.11b shares with cordless phones, microwave ovens,

and Bluetooth devices.

 

The wider band means that more radio channels can coexist without interference. Each radio

channel corresponds to a separate network, or a switched segment on the same network. One

big disadvantage is that it is not directly compatible with 802.11b, and requires new bridging

products that can support both types of networks. Other clear disadvantages are that 802.11a is

only available in half the bandwidth in Japan (for a maximum of four channels), and it isn't

approved for use in Europe, where HiperLAN2 is the standard.

 

IEEE 802.11g

Though 5GHz has many advantages, it also has problems. The most important of these is

compatibility:

The different frequencies mean that 802.11a products aren't interoperable with the 802.11b

base. To get around this, the IEEE developed 802.11g, which should extend the speed and

range of 802.11b so that it's fully compatible with the older systems.

The standard operates entirely in the 2.4GHz frequency, but uses a minimum of two modes (both

mandatory) with two optional modes. The mandatory modulation/access modes are the same

CCK (Complementary Code Keying) mode used by 802.11b (hence the compatibility) and the

OFDM (Orthogonal Frequency Division Multiplexing) mode used by 802.11a (but in this case in

the 2.4GHz frequency band). The mandatory CCK mode supports 11Mbps and the OFDM mode

has a maximum of 54Mbps. There are also two modes that use different methods to attain a

22Mbps data rate--PBCC-22 (Packet Binary Convolutional Coding, rated for 6 to 54Mbps) and

CCK-OFDM mode (with a rated max of 33Mbps).

 

The obvious advantage of 802.11g is that it maintains compatibility with 802.11b (and 802.11b's

worldwide acceptance) and also offers faster data rates comparable with 802.11a. The number

of channels available, however, is not increased, since channels are a function of bandwidth, not

radio signal modulation - and on that score, 802.11a wins with its eight channels, compared to

the three channels available with either 802.11b or 802.11g. Another disadvantage of 802.11g is

that it also works in the 2.4 GHz band and so due to interference it will never be as fast as

802.11a.

 

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                        WIRELESS LANS AND PANS

 

1.1 INTRODUCTION

 

The field of computer networks has grown significantly in the last three decades. An interesting usage of computer networks is in offices and educational institutions, where tens (sometimes hundreds) of personal computers (PCs) are interconnected, to share resources (e.g., printers) and exchange information, using a high-bandwidth communication medium (such as the Ethernet). These privately-owned networks are known as local area networks (LANs) which come under the category of small-scale

networks(networks within a single building or campus with a size of a few kilometres). To do away with the wiring associated with the interconnection of PCs in LANs, researchers have explored the possible usage of radio waves and infrared light for interconnection. This has resulted in the emergence of wireless LANs (WLANs), where wireless transmission is used at the physical layer of the network. Wireless personal area networks (WPANs) are the next step down from WLANs, covering smaller areas with low power transmission, for networking of portable and mobile computing devices such as PCs, personal digital assistants (PDAs), which are essentially very small computers designed to consume as little power as possible so as to increase the lifetime of their batteries, cell phones, printers, speakers, microphones, and other consumer electronics. 

 

 

1.2 FUNDAMENTALS OF WLANS

The terms "node," "station," and "terminal" are used interchangeably. While both portable terminals and mobile terminals can move from one place to another, portable terminals are accessed only when they are stationary. Mobile terminals (MTs), on the other hand, are more powerful, and can be accessed when they are in motion. WLANs aim to support truly mobile work stations.

 

1.2.1 Technical Issues  

The differences between wireless and wired networks, the use of WLANs, and the design goals for WLANs. 

 

Differences Between Wireless and Wired Transmission

 

• Address is not equivalent to physical location: In a wireless network, address refers to a particular station and this station need not be stationary.

Therefore, address may not always refer to a particular geographical location. 

 

• Dynamic topology and restricted connectivity: The mobile nodes may often

go out of reach of each other. This means that network connectivity is partial at times. 

 

• Medium boundaries are not well-defined: The exact reach of wireless signals cannot be determined accurately. It depends on various factors such as signal strength and noise levels. This means that the precise boundaries of the medium cannot be determined easily. 

 

• Error-prone medium: Transmissions by a node in the wireless channel are affected by simultaneous transmissions by neighboring nodes that are located within the direct transmission range of the transmitting node. This means that the error rates are significantly higher in the wireless medium. We need to build a reliable network on top of an inherently unreliable channel. This is realized in practice by having reliable protocols at the MAC layer, which hide the unreliability that is present in the physical layer. 

 

Uses of WLANs 

 

Wireless computer networks are capable of offering versatile functionalities.

WLANs are very flexible and can be configured in a variety of topologies based on the application. Some possible uses of WLANs are mentioned below. 

 

• Users would be able to surf the Internet, check e-mail, and receive Instant Messages on the move.

 

• In areas affected by earthquakes or other such disasters, no suitable infrastructure may be available on the site. WLANs are handy in such locations to set up networks on the fly. 

 

• There are many historic buildings where there has been a need to set up computer networks. In such places, wiring may not be permitted or the building design may not be conducive to efficient wiring. WLANs are very good solutions in such places.

 

 

Design Goals

 

The following are some of the goals which have to be achieved while designing

WLANs:

 

• Operational simplicity: Design of wireless LANs must incorporate features to enable a mobile user to quickly set up and access network services in a simple and efficient manner. 

 

• Power-efficient operation: The power-constrained nature of mobile computing devices such as laptops and PDAs necessitates the important requirement of WLANs operating with minimal power consumption. Therefore, the design of WLAN must incorporate power-saving features and use appropriate technologies and protocols to achieve this. 

 

• License-free operation: One of the major factors that affects the cost of wireless access is the license fee for the spectrum in which a particular wireless access technology operates. Low cost of access is an important aspect for popularizing a WLAN technology. Hence the design of WLAN should consider the parts of the frequency spectrum (e.g., ISM band) for its operation which do not require an explicit licensing.

 

• Tolerance to interference: The proliferation of different wireless networking

technologies both for civilian and military applications and the use of the microwave frequency spectrum for non-communication purposes(e.g.,microwave ovens) have led to a significant increase in the interference level across the radio spectrum. The WLAN design should account for this and take appropriate measures by way of selecting technologies and protocols to operate in the presence of interference. 

 

• Global usability: The design of the WLAN, the choice of technology, and the

selection of the operating frequency spectrum should take into account the

prevailing spectrum restrictions in countries across the world. This ensures the

acceptability of the technology across the world.

 

• Security: The inherent broadcast nature of wireless medium adds to the requirement of security features to be included in the design of WLAN technology. 

 

• Safety requirements: The design of WLAN technology should follow the safety requirements that can be classified into the following: (i) interference to medical and other instrumentation devices and (ii) increased power level of transmitters that can lead to health hazards. A well-designed WLAN should follow the power emission restrictions that are applicable in the given frequency spectrum. 

 

• Quality of service requirements: Quality of service (QoS) refers to the

provisioning of designated levels of performance for multimedia traffic. The design of WLAN should take into consideration the possibility of supporting a wide variety of traffic, including multimedia traffic. 

 

• Compatibility with other technologies and applications: The interoperability among the different LANs (wired or wireless) is important for efficient communication between hosts operating with different LAN technologies. In addition to this, interoperability with existing WAN protocols such as TCP/IP of the Internet is essential to provide a seamless communication across the WANs.

1.2.2 Network Architecture  

This section lists the types of WLANs, the components of a typical WLAN, and the services offered by a WLAN. 

 

Infrastructure Based Versus Ad Hoc LANs

WLANs can be broadly classified into two types, infrastructure networks and

adhoc LANs, based on the underlying architecture. Infrastructure networks contain special nodes called access points (APs), which are connected via existing networks. APs are special in the sense that they can interact with wireless nodes as well as with the existing wired network. The other wireless nodes, also known as mobile stations, communicate via APs. The APs also act as bridges with other networks. Ad hoc LANs do not need any fixed infrastructure. These networks can be setup on the fly at any place. Nodes communicate directly with each other or forward messages through other nodes that are directly accessible. 

 

 

Components in a Typical IEEE 802.11 Network

 

IEEE 802.11 is the most popular WLAN standard that defines the specification

for the physical and MAC layers. The success of this standard can be understood from the fact that the revenue from the products based on this standard touched $730 million in the second quarter of the year 2003. The basic components in a typical IEEE 802.11 WLAN are listed. The set of stations that can remain in contact (i.e., are associated) with a given AP is called a basic service set (BSS). The coverage area of an AP within which member stations (STAs or MTs) may remain in communication is called the basic service area (BSA). The stations that are a part of a BSS need to be located within the BSA of the corresponding AP. A BSS is the basic building block of the network.

BSSs are connected by means of a distribution system(DS) to form an extended

network.DS refers to an existing network infrastructure. The implementation of the DS is not specified by the IEEE 802.11 standard. The services of the DS, however, are specified rigidly. This gives a lot of flexibility in the design of the DS. The APs are connected by means of the DS. Portals are logical points through which non-IEEE 802.11 packets (wired LAN packets) enter the system.

They are necessary for integrating wireless networks with the existing wired networks. Just as an AP interacts with the DS as well as the wireless nodes, the

portal interacts with the wired network as well as with the DS. The BSSs, DS,

and the portals together with the stations they connect constitute the extended

service set (ESS). An ad hoc LAN has only one BSS. Therefore, ad hoc LANs

are also known as independent basic service sets (IBSSs). It may be noted that

the ESS and IBSS appear identical to the logical link control (LLC).

Figure 1.1 gives a schematic picture of what a typical ESS looks like 

 

 

 

Figure 1.1. Extended Service Set.

 

Services Offered by a Typical IEEE 802.11 Network

The services offered by a typical IEEE 802.11 network can be broadly divided

into two categories: AP services and STA services. The following are the AP

services, which are provided by the DS:

 

• Association: The identity of an STA and its address should be known to the

AP before the STA can transmit or receive frames on the WLAN. This is done

during association, and the information is used by the AP to facilitate routing of

frames. 

 

• Reassociation: The established association is transferred from one AP to

another using reassociation. This allows STAs to move from one BSS to

another. 

 

• Disassociation: When an existing association is terminated, a notification is

issued by the STA or the AP. This is called disassociation, and is done when

nodes leave the BSS or when nodes shut down. 

 

• Distribution: Distribution takes care of routing frames. If the destination is in

the same BSS, the frame is transmitted directly to the destination, otherwise the

frame is sent via the DS. 

 

• Integration: To send frames through non-IEEE 802.11 networks, which may

have different addressing schemes or frame formats, the integration service is

invoked. 

 

The following are the STA services, which are provided by every

station, including APs: 

 

• Authentication: Authentication is done in order to establish the identity of

stations to each other. The authentication schemes range from relatively

insecure handshaking to public-key encryption schemes. 

 

• Deauthentication: Deauthentication is invoked to terminate existing

authentication. 

 

• Privacy: The contents of messages may be encrypted (say, by using the WEP

algorithm) to prevent eavesdroppers from reading the messages. 

 

• Data delivery: IEEE 802.11 naturally provides a way to transmit and receive

data. However, like Ethernet, the transmission is not guaranteed to be completely reliable. 

 

 

1.3 IEEE 802.11 STANDARD

IEEE 802.11 is a prominent standard for WLANs, which is adopted by many

vendors of WLAN products. A later version of this standard is the IEEE

802.11b, commercially known as Wi-Fi (wireless fidelity). The IEEE 802.11

standard, which deals with the physical and MAC layers in WLANs, was

brought out in 1997. It may be observed that IEEE 802.11 was the first WLAN

standard that faced the challenge of organizing a systematic approach for

defining a standard for wireless wideband local access (small-scale networks

capable of transmitting data at high rates). Wireless standards need to have

provisions to support mobility of nodes. The IEEE802.11 working group had to

examine connection management, link reliability management, and power

management — none of which was a concern for other standards in IEEE 802.

In addition, provision for security had to be introduced. For all these reasons

and because of several competing proposals, it took nearly ten years for the

development of IEEE 802.11, which was much longer compared to the time

taken for the development of other 802 standards for the wired media. Once the

overall picture and the ideas became clear, it took only a reasonable duration of

time to develop the IEEE 802.11a and IEEE 802.11b enhancements. Under the

IEEE 802.11 standard, MTs can operate in two modes: (i) infrastructure mode,

in which MTs can communicate with one or more APs which are connected to a

WLAN, and (ii) ad hoc mode, in which MTs can communicate directly with

each other without using an AP.

 

1.3.1 Physical Layer

IEEE 802.11 supports three options for the medium to be used at the physical

level — one is based on infrared and the other two are based on radio

transmission. The physical layer is subdivided conceptually into two parts —

Physical Medium Dependent sub layer (PMD) and Physical Layer Convergence

Protocol (PLCP). PMD handles encoding, decoding, and modulation of signals

and thus deals with the idiosyncrasies of the particular medium. The PLCP

abstracts the functionality that the physical layer has to offer to the MAC layer.

PLCP offers a Service Access Point (SAP) that is independent of the

transmission technology, and a Clear Channel Assessment (CCA) carrier sense

signal to the MAC layer. The SAP abstracts the channel which can offer up to 1

or 2 Mbps data transmission bandwidth. The CCA is used by the MAC layer to

implement the CSMA/CA mechanism. 

 

The three choices for the physical layer in the original 802.11 standard are as

follows:

(i) Frequency Hopping Spread Spectrum (FHSS) operating in the license-free

2.4 GHz industrial, scientific, and medical (ISM) band, at data rates of 1 Mbps

[using 2-level Gaussian frequency shift keying (GFSK) modulation scheme] and

2 Mbps(using 4-level GFSK); 

 

(ii) Direct Sequence Spread Spectrum (DSSS) operating in the 2.4 GHz ISM

band, at data rates of 1 Mbps [using Differential Binary Phase Shift Keying

(DBPSK) modulation scheme] and 2 Mbps [using Differential Quadrature Phase

Shift Keying (DQPSK)]; 

 

(iii) Infrared operating at wavelengths in 850-950 nm range, at data rates of 1

Mbps and 2 Mbps using Pulse Position Modulation (PPM) scheme. 

 

Carrier Sensing Mechanisms 

 

In IEEE 802.3, sensing the channel is very simple. The receiver reads the peak

voltage on the cable and compares it against a threshold. In contrast, the

mechanism employed in IEEE 802.11 is relatively more complex. It is performed either physically or virtually. As mentioned earlier, the physical layer sensing is through the Clear Channel Assessment (CCA) signal provided by the PLCP in the physical layer of the IEEE 802.11. The CCA is generated based on sensing of the air interface either by sensing the detected bits in the air or by checking the Received Signal Strength (RSS) of the carrier against a threshold. 

 

Decisions based on the detected bits are made somewhat more slowly, but they

are more reliable. Decisions based on the RSS can potentially create a false

alarm caused by measuring the level of interference.

 

1.3.2 Basic MAC Layer Mechanisms as specified by the IEEE 802.11

standard:

 

The primary function of this layer is to arbitrate and statistically multiplex the

transmission requests of various wireless stations that are operating in an area.

 

This assumes importance because wireless transmissions are inherently

broadcast in nature and contentions to access the shared channel need to be

resolved prudently in order to avoid collisions, or at least to reduce the number

of collisions. The MAC layer also supports many auxiliary functionalities such

as offering support for roaming, authentication, and taking care of power

conservation. The basic services supported are the mandatory asynchronous data

service and an optional real-time service. The asynchronous data service is

supported for unicast packets as well as for multicast packets. The real-time

service is supported only in infrastructure-based networks where APs control

access to the shared medium.

 

Distributed Foundation Wireless Medium Access Control (DFWMAC)

The primary access method of IEEE 802.11 is by means of a distributed

coordination function (DCF). This mandatory basic function is based on a

version of carrier sense with multiple access and collision avoidance

(CSMA/CA). To avoid the hidden terminal problem, an optional RTS-CTS

mechanism is implemented. There is a second method called the Point

Coordination Function (PCF) that is implemented to provide real-time services.

When the PCF is in operation, the AP controls medium access and avoids

simultaneous transmissions by the nodes.

 

Inter-Frame Spacing (IFS)

Inter-Frame Spacing refers to the time interval between the transmission

of two successive frames by any station. There are four types of IFS: SIFS,

PIFS, DIFS, and EIFS, in order from shortest to longest. They denote priority

levels of access to the medium. Shorter IFS denotes a higher priority to access

the medium, because the wait time to access the medium is lower. The exact

values of the IFS are obtained from the attributes specified in the Physical Layer

Management Information Base (PHYMIB) and are independent of the station

bit rate. 

 

• Short Inter-Frame Spacing (SIFS) is the shortest of all the IFSs and denotes

highest priority to access the medium. It is defined for short control messages

such as acknowledgments for data packets and polling responses. The

transmission of any packet should begin only after the channel is sensed to be

idle for a minimum time period of at least SIFS. 

 

• PCF Inter-Frame Spacing (PIFS) is the waiting time whose value lies

between SIFS and DIFS. This is used for real-time services. 

 

• DCF Inter-Frame Spacing (DIFS) is used by stations that are operating

under the DCF mode to transmit packets. This is for asynchronous data transfer

within the contention period. 

 

• Extended Inter-Frame Spacing (EIFS) is the longest of all the IFSs and

denotes the least priority to access the medium. EIFS is used for

resynchronization whenever physical layer detects incorrect MAC frame

reception. 

 

1.3.3 CSMA/CA Mechanism

Carrier Sense With Multiple Access And Collision Avoidance

(CSMA/CA) is the MAC layer mechanism used by IEEE 802.11 WLANs.

Carrier Sense With Multiple Access And Collision Detection (CSMA/CD) is a

well-studied technique in IEEE 802.x wired LANs. This technique cannot be

used in the context of WLANs effectively because the error rate in WLANs is

much higher and allowing collisions will lead to a drastic reduction in

throughput. Moreover, detecting collisions in the wireless medium is not always

possible. The technique adopted here is therefore one of collision avoidance.

 

The Medium Access Mechanism

The basic channel access mechanism of IEEE 802.11 is shown in Figure 1.2 (a).

If the medium is sensed to be idle for a duration of DIFS, the node accesses the

medium for transmission. Thus the channel access delay at very light loads is

equal to the DIFS.

 

Figure 1.2. IEEE 802.11 DCF and RTS-CTS mechanism

 

 

If the medium is busy, the node backs off, in which the station defers channel

access by a random amount of time chosen within a contention window(CW).

The value of CW can vary between CWmin and CWmax . The time intervals are all

integral multiples of slot times, which are chosen judiciously using propagation

delay, delay in the transmitter, and other physical layer dependent parameters.

As soon as the back-off counter reaches zero and expires, the station can access

the medium. During the back-off process, if anode detects a busy channel, it

freezes the back-off counter and the process is resumed once the channel

becomes idle for a period of DIFS. Each station executes the back-off procedure

at least once between every successive transmission.

 

In the scheme discussed so far, each station has the same chances for

transmitting data next time, independent of the overall waiting time for

transmission. Such a system is clearly unfair. Ideally, one would like to give

stations that wait longer a higher priority service in order to ensure that they are

not starved. The back-off timer incorporated into the above mechanism tries to

make it fair. Longer waiting stations, instead of choosing another random

interval from the contention window, wait only for a residual amount of time

that is specified by the back-off timer.

 

Contention Window Size

The size of the Contention Window (CW) is another important parameter. If the

CW is small in size, then the random values will be close together and there is a

high probability of packet collision. On the other hand, if the size of CW is very

large, there will be some unnecessary delay because of large back-off values.

Ideally, one would like the system to adapt to the current number of stations that

are contending for channel access. To effect this, the truncated binary

exponential back-off technique is used here, which is similar to the technique

used in IEEE 802.3. The initial contention window is set to a random value

between (0, CWmin) and each time a collision occurs, the CW doubles its size

up to a maximum of CWmax. So at high load, the CW size is high and therefore

the resolution power of the system is high. At low loads, small CW ensures low

access delay. The specified values of CWmin andCWmax for different physical

layer specifications are given in Table 1.1.

 

Table 2.1. IEEE 802.11 parameters

 

 

 

Acknowledgments

Acknowledgments (ACKs) must be sent for data packets in order to ensure their

correct delivery. For unicast packets, the receiver accesses the medium after

waiting for a SIFS and sends an ACK. Other stations have to wait for DIFS plus

their backoff time. This reduces the probability of a collision. Thus higher

priority is given for sending an ACK for the previously received data packet

than for starting a new data packet transmission. ACK ensures the correct

reception of the MAC layer frame by using cyclic redundancy checksum

(CRC)technique. If no ACK is received by the sender, then a retransmission

takesplace. The number of retransmissions is limited, and failure is reported to

the higher layer after the retransmission count exceeds this limit.

 

RTS-CTS Mechanism

The hidden terminal problem is a major problem that is observed in wireless

networks. This is a classic example of problems arising due to incomplete

topology information in wireless networks that was mentioned initially. It also

highlights the non-transitive nature of wireless transmission. In some situations,

one node can receive from two other nodes, which cannot hear each other. In

such cases, the receiver may be bombarded by both the senders, resulting in

collisions and reduced throughput. But the senders, unaware of this, may get the

impression that the receiver can clearly listen to them without interference from

anyone else. This is called the hidden terminal problem. To alleviate this

problem, the RTS-CTS mechanism has been devised as shown in Figure 1.2 (b).

 

 

How RTS-CTS Works

The sender sends a request to send (RTS) packet to the receiver. The packet

includes the receiver of the next data packet to be transmitted and the expected

duration of the whole data transmission. This packet is received by all stations

that can hear the sender. Every station that receives this packet will set its

Network Allocation Vector (NAV) accordingly. The NAV of a station specifies

the earliest time when the station is permitted to attempt transmission. After

waiting for SIFS, the intended receiver of the data packet answers with a clear

to send (CTS) packet if it is ready to accept the data packet. The CTS packet

contains the duration field, and all stations receiving the CTS packet also set

their NAVs. These stations are within the transmission range of the receiver.

 

The set of stations receiving the CTS packet may be different from the set of

stations that received the RTS packet, which indicates the presence of some

hidden terminals. Once the RTS packet has been sent and CTS packet has been

received successfully, all nodes within receiving distance from the sender and

from the receiver are informed that the medium is reserved for one sender

exclusively. The sender then starts data packet transmission after waiting for

SIFS. The receiver, after receiving the packet, waits for another SIFS and sends

the ACK.As soon as the transmission is over, the NAV in each node marks the

medium as free (unless the node has meanwhile heard some other RTS/CTS)

and the process can repeat again. The RTS packet is like any other packet and

collisions can occur only at the beginning when RTS or CTS is being sent. Once

the RTS and CTS packets are transmitted successfully, nodes that listen to the

RTS or the CTS refrain from causing collision to the ensuing data transmission,

because of their NAVs which will be set. The usage of RTS-CTS dialog before

data packet transmission is a form of virtual carrier sensing.

 

Overhead Involved in RTS-CTS

It can be observed that the above mechanism is akin to reserving the medium

prior to a particular data transfer sequence in order to avoid collisions during

this transfer. But transmission of RTS-CTS can result in non-negligible

overhead. Therefore, the RTS-CTS mechanism is used judiciously. An RTS

threshold is used to determine whether to start the RTSCTS mechanism or not.

Typically, if the frame size is more than the RTS threshold, the RTS-CTS

mechanism is activated and a four-way handshake (i.e., RTS-CTS-DATA

ACK) follows. If the frame size is below the RTS threshold, the nodes resort to

a two-way handshake (DATA-ACK).

 

MAC as a State Machine

Figure 1.3 diagrammatically shows what has been discussed so far. It models

the MAC layer as a finite state-machine, and shows the permissible transitions.

It must be noted that the state-machine is simplistic and is given only to ease the

understanding of the fundamental mechanisms at the MAC layer. The

functioning of the finite state-machine is explained in what follows.

 

Figure 1.3. MAC state transition diagram. 

 

 

If a node has a packet to send and is in the IDLE state, it goes into the

WAIT_FOR_NAV state. After the on-going transmissions (if any) in the

neighborhood are over, the node goes to the WAIT_FOR_DIFS state. After

waiting for DIFS amount of time, if the medium continues to be idle, the station

enters the BACKING_OFF state. Otherwise, the station sets its back-off

counter(if the counter value is zero) and goes back to the IDLE state. During

back-off, if the node senses a busy channel, the node saves the back-off counter

and goes back to the IDLE state. Otherwise, it goes into one of three states. If

the packet type is broadcast, the node enters the TRANSMITTING_BCAST

state where it transmits the broadcast packet. If the packet type is unicast and

the packet size is less than the RTS threshold, the node enters the

TRANSMITTING_UNICAST state and starts transmitting data. If the packet

size is greater than the RTS threshold, the node enters the

TRANSMITTING_RTS state and starts transmitting the RTS packet. After the

RTS transmission is over, the node enters the WAITING_FOR_CTS state. If the

CTS packet is not received within a specified time, the node times out and goes

back to the IDLE state, and increases the CW value exponentially up to a

maximum of CWmax

 

. If the CTS packet is received, the node enters the TRANSMITTING_UNICAST state and starts transmitting data. After the unicast packet is transmitted, the node enters the WAITING_FOR_ACK state.

When the node receives the ACK, it goes back to the IDLE state and reduces the

CW value to CWmin . If a node receives an RTS packet when in IDLE state and if

the NAV of the node indicates that no other on-going transmissions exist, the

node enters the TRANSMITTING_CTS state and starts transmitting the CTS

packet. After the CTS packet is transmitted, the node enters the

WAITING_FOR_DATA state and waits for the data packet from the sender. On

receiving the data packet, the node enters the TRANSMITTING_ACK state and

starts transmitting the ACK for the data packet. When the ACK has been

transmitted, the node goes back to the IDLE state. If the data packet is not

received, the receiver returns to the IDLE state. 

 

 

Fragmentation

Bit error rates in the wireless medium are much higher than in other media. The

bit error rate in fiber optics is only about 10-9 , whereas in wireless, it is as large

as 10-4. One way of decreasing the frame error rate is by using shorter frames.

IEEE 802.11 specifies a fragmentation mode where user data packets are split

into several smaller parts transparent to the user. This will lead to shorter 

frames, and frame error will result in retransmission of a shorter frame. The

RTS and CTS messages carry duration values for the current fragment and

estimated time for the next fragment. The medium gets reserved for the

successive frames until the last fragment is sent. The length of each fragment is

the same for all the fragments except the last fragment. The fragments contain

information to allow the complete MAC Protocol Data Unit (MPDU, informally

referred to as packet) to be reassembled from the fragments that constitute it.

The frame type, sender address, destination address, sequence control field, and

indicator for more fragments to come are all present in the fragment header. The

destination constructs the complete packet by reassembling the fragments in the

order of the sequence number field. The receiving station ensures that all

duplicate fragments are discarded and only one copy of each fragment is

integrated. Acknowledgments for the duplicates may, however, be sent. 

 

1.3.4 Other MAC Layer Functionalities

There are several other functionalities that the MAC layer provides in IEEE

802.11 WLANs. The functionalities are the Point Coordination Function (PCF)

which is used for QoS guarantees, Timing Synchronization, Power

Management, and Support For Roaming. 

 

Point Coordination Function

The objective of the point coordination function (PCF) is to provide guarantees

on the maximum access delay, minimum transmission bandwidth, and other

QoS parameters. Unlike the DCF, where the medium contention is resolved in a

distributed manner, the PCF works by effecting a centralized contention

resolution scheme, and is applicable only in networks where an AP polls the

nodes in its BSS. A point coordinator (PC) at the AP splits the access time into

super frame periods. The super frame period consists of alternating contention

free periods (CFPs) and contention periods (CPs). The PC will determine which

station has the right to transmit at any point of time. The PCF is essentially a

polled service with the PC playing the role of the polling master. The operation

of the PCF may require additional coordination to perform efficient operation in

cases where multiple PCs are operating simultaneously such that their

transmission ranges overlap. The IFS used by the PCF is smaller than the IFS of

the frames transmitted by the DCF. This means that point-coordinated traffic

will have higher priority access to the medium if DCF and PCF are concurrently

in action. The PC controls frame transmissions so that contentions are 

eliminated over a limited period of time, that is, the CFP.

 

 

Synchronization

Synchronization of clocks of all the wireless stations is an important function to

be performed by the MAC layer. Each node has an internal clock, and clocks are

all synchronized by a Timing Synchronization Function (TSF). Synchronized

clocks are required for power management, PCF coordination, and Frequency

Hopping Spread Spectrum (FHSS) hopping sequence synchronization. Without

synchronization, clocks of the various wireless nodes in the network may not

have a consistent view of the global time. Within a BSS, quasi periodic beacon

frames are transmitted by the AP, that is, one beacon frame is sent every Target

Beacon Transmission Time (TBTT) and the transmission of a beacon is deferred

if the medium is busy. A beacon contains a time-stamp that is used by the node

to adjust its clock. The beacon also contains some management information for

power optimization and roaming. Not all beacons need to be heard for achieving

synchronization.

 

Power Management

Usage of power cords restricts the mobility that wireless nodes can potentially

offer. The usage of battery-operated devices calls for power management

because battery power is expensive. Stations that are always ready to receive

data consume more power (the receiver current may be as high as 100 mA). The

transceiver must be switched off whenever carrier sensing is not needed. But

this has to be done in a manner that is transparent to the existing protocols. It is

for this reason that power management is an important functionality in the MAC

layer. Therefore, two states of the station are defined: sleep and awake. The

sleep state refers to the state where the transceiver cannot receive or send

wireless signals. Longer periods in the sleep state mean that the average

throughput will be low. On the other hand, shorter periods in the sleep state consume a lot of battery power and are likely to reduce battery life. If a sender

wants to communicate with a sleeping station, it has to buffer the data it wishes

to send. It will have to wait until the sleeping station wakes up, and then send

the data. Sleeping stations wake up periodically, when senders can announce the

destinations of their buffered data frames. If any node is a destination, then that

node has to stay awake until the corresponding transmission takes place.

 

 

Roaming

Each AP may have a range of up to a few hundred meters where its transmission

will be heard well. The user may, however, walk around so that he goes from

the BSS of one AP to the BSS of another AP. Roaming refers to providing

uninterrupted service when the user walks around with a wireless station. When

the station realizes that the quality of the current link is poor, it starts scanning

for another AP. This scanning can be done in two ways: active scanning and

passive scanning. Active scanning refers to sending a probe on each channel and

waiting for a response. Passive scanning refers to listening into the medium to

find other networks. The information necessary for joining the new BSS can be

obtained from the beacon and probe frames.

 

1.3.5 Other Issues

Improvements in the IEEE 802.11 standard have been proposed to support

higher data rates for voice and video traffic. Also, QoS provisioning and

security issues have been addressed in extended versions of the standard.

 

 

Newer Standards

The original standards for IEEE 802.11 came out in 1997 and promised a data

rate of 1-2 Mbps in the license-free 2.4 GHz ISM band. Since then, several

improvements in technology have called for newer and better standards that

offer higher data rates. This has manifested in the form of IEEE802.11aand

IEEE 802.11b standards, both of which came out in 1999. IEEE 802.11b, an

extension of IEEE 802.11 DSSS scheme, defines operation in the 2.4GHz ISM

band at data rates of 5.5 Mbps and 11 Mbps, and is trademarked commercially

by the Wireless Ethernet Compatibility Alliance (WECA) as Wi-Fi. It achieves

high data rates due to the use of Complimentary Code Keying(CCK). IEEE

802.11a operates in the 5 GHz band (unlicensed national information

infrastructure band), and uses orthogonal frequency division multiplexing

(OFDM) at the physical layer. IEEE 802.11a supports data rates up to 54 Mbps

and is the fast Ethernet analogue to IEEE 802.11b.Other IEEE 802.11 (c, d, and

h) task groups are working on special regulatory and networking issues. IEEE

802.11e deals with the requirements of time sensitive applications such as voice

and video. IEEE802.11f deals with inter-AP communication to handle roaming.

IEEE 802.11g aims at providing the high speed of IEEE 802.11a in the ISM

band. IEEE 802.11i deals with advanced encryption standards to support better

privacy.

 

 

QoS for Voice and Video Packets

In order to offer QoS, delay-sensitive packets (such as voice and video packets)

are to be given a higher priority to get ahead of less time-critical (e.g., file

transfer) traffic. Several mechanisms have been proposed to offer weighted

priority. Hybrid Coordination Function (HCF) can be used where the AP polls

the stations in a weighted way in order to offer QoS. Extended DCF is another

mechanism which has been proposed where the higher priority stations will

choose the random back-off interval from a smaller CW. Performance of

WLANs where voice and data services are integrated.

 

 

Wired Equivalent Privacy

Security is a very important issue in the design of WLANs. In order to provide a

modest level of physical security, the Wired Equivalent Privacy (WEP)

mechanism was devised. The name WEP implies that this mechanism is aimed

at providing the level of privacy that is equivalent to that of a wired LAN. Data

integrity, access control, and confidentiality are the three aims of WEP. It

assumes the existence of an external key management service that distributes

the key sequence used by the sender. This mechanism relies on the fact that the

secret key cannot be determined by brute force. However, WEP has been proven

to be vulnerable if more sophisticated mechanisms are used to crack the key. It

uses the pseudo-random number key generated by RSA RC4 algorithm which

has been efficiently implemented in hardware as well as in software. This

mechanism makes use of the fact that if we take the plain text, XOR (bit-by-bit

exclusive OR) it with a pseudo-random key sequence, and then XOR the result

with the same key sequence, we get back the plain text.

 

 

1.4 HIPERLAN STANDARD

The European counterparts to the IEEE 802.11 standards are the high

performance radio LAN(HIPERLAN) standards defined by the European

Telecommunications Standards Institute (ETSI). It is to be noted that while the

IEEE 802.11 standards can use either radio access or infrared access, the

HIPERLAN standards are based on radio access only. The standards have been

defined as part of the ETSI Broadband Radio Access Networks (BRAN) project.

In general, broadband systems are those in which user data rates are greater than

2Mbps (and can go up to 100s of Mbps). Four standards have been defined for

wireless networks by the ETSI.

 

• HIPERLAN/1 is a wireless radio LAN (RLAN) without a wired infrastructure,

based on one-to-one and one-to-many broadcasts. It can be used as an extension

to a wired infrastructure, thus making it suited to both ad hoc and infrastructure

based networks. It employs the 5.15 GHz and the 17.1 GHz frequency bands

and provides a maximum data rate of 23.5 Mbps.

 

• The HIPERLAN/2 standard intends to provide short-range (up to 200

m)wireless access to Internet Protocol (IP), Asynchronous Transfer Mode

(ATM),and other infrastructure-based networks and, more importantly, to

integrate WLANs into cellular systems. It employs the 5 GHz frequency band

and offers a wide range of data rates from 6 Mbps to 54 Mbps. HIPERLAN/2

has been designed to meet the requirements of future wireless multimedia

services.

 

• ATM networks are connection-oriented and require a connection to set up prior

to transfer of information from a source to a destination. All information to be

transmitted — voice, data, image, and video — is first fragmented into small,

fixed-size packets known as cells. These cells are then switched and routed using

packet switching principles.

 

• HIPERACCESS (originally called HIPERLAN/3) covers "the last mile" to the

customer; it enables establishment of outdoor high-speed radio access networks,

providing fixed radio connections to customer premises. HIPERACCESS

provides a data rate of 25 Mbps. It can be used to connect

HIPERLAN/2deployments that are located far apart (up to 5 Km away). It

offers point-to multi point communication.

 

•The HIPERLINK (originally called HIPERLAN/4) standard provides high

speed radio links for point-to-point static interconnections. This is used to

connect different HIPERLAN access points or HIPERACCESS networks with

high-speed links over short distances of up to 150 m. For example, the

HIPERLINK can be employed to provide links between different rooms or

floors within a large building. HIPERLINK operates on the 17 GHz frequency

range. Figure 1.4 shows a typical deployment of the ETSI standards. The

standards excluding HIPERLAN/1 are grouped under the BRAN project. The

scope of the BRAN has been to standardize the radio access network and the

functions that serve as the interface to the infrastructural networks.

 

Figure 2.10. The ETSI-BRAN systems.

 

 

1.4.1 HIPERLAN/1

 HIPERLAN/1 is a RLAN standard that was introduced by

the ETSI in 1995.The standard allows nodes to be deployed either in a pre

arranged or in an adhoc fashion. Apart from supporting node mobility,

HIPERLAN/1 provides forwarding mechanisms (multi-hop routing). Thus,

coverage is not limited to just the neighboring nodes. Using a clever framing

scheme as explained later in this section, HIPERLAN/1 provides a data rate of

around 23.5 Mbps without utilizing much power, thus having the capability to

support multimedia data and asynchronous data effectively. This data rate is

significantly higher than that provided by IEEE 802.11. The HIPERLAN/1

protocol stack is restricted to the two lower-most layers in the OSI reference

model: the data link layer (DLL)and the physical layer. The DLL is further

divided into the medium access control (MAC) sublayer and the channel access

control (CAC) sublayer. The sections that follow describe the standard.

 

 

The Physical Layer

The tasks of the physical layer are modulation and demodulation of a radio

carrier with a bit stream, forward error-correction mechanisms, signal strength

measurement, and synchronization between the sender and the receiver. The

standard uses the CCA scheme (similar to IEEE 802.11) to sense whether the

channel is idle or busy.

 

 

The MAC Sublayer

The HIPERLAN/1 MAC (HM) sublayer is responsible for processing the

packets from the higher layers and scheduling the packets according to the QoS

requests from the higher layers specified by the HMQoS parameters. The MAC

sublayer is also responsible for forwarding mechanisms, power conservation

schemes, and communication confidentiality through encryption–decryption

mechanisms. Because of the absence of an infrastructure, the forwarding

mechanism is needed to allow the physical extension of HIPERLAN/1 to go

beyond the radio range of a single station. Topology-related data are exchanged

between the nodes periodically with the help of special packets, for the purpose

of forwarding. In order to guarantee a time-bound service, the HM protocol data

unit (HMPDU)selected for channel access has to reflect the user priority and the

residual lifetime of the packet (the time remaining for the packet to expire).The

MAC layer computes the channel access priority for each of the PDUs

following a mapping from the MAC priority to the channel access

mechanism(CAM) priority. One among those PDUs which has the highest

CAM priority and the least residual time will be selected for access to the

channel.

 

 

The CAC Sublayer

The CAC sublayer offers a connectionless data service to the MAC sublayer.

The MAC layer uses this service to specify a priority (called the CAM

priority)which is the QoS parameter for the CAC layer. This is crucial in the

resolution of contention in the CAM.EY-NPMA After a packet with an associated

CAM priority has been chosen in the CAC sublayer for transmission, the next

phase is to compete with packets of other nodes for channel access. The channel

access mechanism is a dynamic, listen-and-then-talk protocol that is very

similar to the CSMA/CA used in 802.11 and is called the elimination yield non

pre-emptive multiple access (EYNPMA) mechanism. Figure 1.5 shows the

operation of the EY-NPMA mechanism in which the nodes 1, 2, 3, and 4 have

packets to be sent to the AP. The CAM priority for nodes 2 and 4 is higher with

priority 2 followed by node 3with priority 3, and node 1 with the least priority

of 4. The prioritization phase will have k slots where k (can vary from 1 to 5

with k − 1 having higher priority than k) refers to the number of priority levels.

 

Figure 1.5. The operation of EY-NPMA.

 

 

The entire process of channel access occurs in the form of channel access

cycles. A synchronization interval occurs after the end of every such cycle. This

access cycle is comprised of three phases: prioritization, contention, and

transmission.

 

1. Prioritization: This phase culls out nodes with packets of the highest CAM

priority and lets them participate in the next phase. The prioritization phase

consists of two events, namely, priority detection and priority assertion. During

the priority detection period, a node listens to the channel for a number of time

slots proportional to the CAM priority assigned to the packet that the node

wants to send. In Figure 1.5, the nodes 2 and 4 wait for one slot and assert their

priority in the second slot as they hold packets with higher priority, and nodes 3

and 1 wait for slots equal to their priority level. By listening to the channel,

nodes 3 and 1detect the existence of other nodes with higher priority and hence

leave the prioritization phase. If a low-priority node has succeeded in waiting up

to this slot, it enters the priority assertion period during which it sends a burst,

signaling its selection to the next stage. In this process, the node(s) with the

highest CAM priority will finish the prioritization phase first and hence will be

selected for the next phase.

 

2. Contention: This phase is to eliminate as many nodes as possible, in order to

minimize the collision rate during transmission. This phase extends to a

maximum of 13 slots, each of the same width as that of the slots in the

prioritization phase. In this phase, the nodes that transmitted a burst in the

previous phase, resolve access to the channel by contention. This phase consists

of two sub-phases, namely, the elimination phase and the yield phase. Nodes in

this phase (nodes 2 and 4 in Figure 1.5) get to transmit a burst for a

geometrically distributed number of time slots the probability of a node's

transmission extending to a slot length of k slots(where k < 12 slots) is 0.5

k+1] which is then followed by a sensing period of 1 slot. During this period, if a

node detects another node's burst, it stops the contention process (node 2 in

Figure 1.5). This period during which each contending node will have to listen

to the channel for a slot duration is called the elimination survival identification

interval. If the channel is sensed idle during this interval, the node reaches the

yield phase. This period is also called elimination survival verification. This

ensures that the node(s) which sent the elimination burst for the maximum

number of slots will be chosen for the next phase. The next phase is the yield

phase which complements the elimination phase; it involves each node listening

to the channel for a number of time slots (up to a maximum of 15 slots, each

with duration of the slot duration in the prioritization phase).This is in fact

similar to the back-off state in which the probability of backing off for k slots is

0.1 × 0.9k

 

. If the channel is sensed to be idle during these slots, the node is said

to be eligible for transmission. The node that waits for the shorter number of

slots initiates transmission and other nodes defer their access to the next cycle to

begin the process a fresh.

 

3. Transmission: This is the final stage in the channel access where the

transmission of the selected packet takes place. During this phase, the successful

delivery of a data packet is acknowledged with an ACK packet. The

performance of EY-NPMA protocol suffers from major factors such as packet

length, number of nodes, and the presence of hidden terminals. The efficiency

of this access scheme varies from 8% to 83% with variation of packet sizes from

50 bytes to 2 Kbytes. The above-described channel access takes place during

what is known as the channel synchronization condition. The other two

conditions during which channel access can take place are

 

(a) the channel free condition, when the node senses the channel free for some

amount of time and then gains access, and

 

(b) the hidden terminal condition, when a node is eliminated from contention,

but still does not sense any data transmission, indicating the presence of a

hidden node.

 

 

Power Conservation Issues

The HIPERLAN/1 standard has suggested power conservation schemes at both

the MAC and the physical layers. At the MAC level, the standard suggests

awake/sleep modes similar to the DFWMAC in IEEE 802.11. Two roles defined

for the nodes are the p-savers (nodes that want to implement the function) and

the p-supporters (neighbors to the p-saver that are deputized to aid the latter's

power conservation). The psaver can receive packets only at predetermined time

intervals and is active only during those intervals, in the process saving power.

 

At the physical level, a framing scheme has been adopted to conserve power.

The physical burst is divided into High Bit Rate (HBR) and Low Bit Rate

(LBR)bursts. The difference between the two bursts lies in the keying

mechanisms employed for them – the HBR burst is based on Gaussian

Minimum Shift Keying(GMSK) that yields a higher bit rate, but consumes more

power than Frequency Shift Keying (FSK) used for the LBR bursts. The LBR

burst contains the destination address of the frame and precedes the HBR burst.

 

Any node receiving a packet, first reads the LBR burst. The node will read the

HBR burst only if it is the destination for that frame. Otherwise, the burst is

simply ignored, thereby saving the power needed to read the HBR burst.

 

Failure of HIPERLAN/1

In spite of the high data rate that it promised, HIPERLAN/1 standard has always

been considered unsuccessful. This is because IEEE Ethernet had been

prevalent and hence, for its wireless counterpart too, everybody turned toward

IEEE, which came out with its IEEE 802.11 standard. As a result, hardly any

manufacturer adopted the HIPERLAN/1 standard for product development.

 

However, the standard is still studied for the stability it provides and for the fact

that many of the principles followed have been adopted in the otherstandards.

 

 

1.4.2 HIPERLAN/2 

 The IEEE 802.11 standard offers data rates of 1 Mbps

while the newer standard IEEE802.11a offers rates up to 54 Mbps. However,

there was a necessity to support QoS, handoff (the process of transferring an

MT from one channel/AP to another), and data integrity in order to satisfy the

requirements of wireless LANs. This demand was the motivation behind the

emergence of HIPERLAN/2. The standard has become very popular owing to

the significant support it has received from cellular manufacturers such as Nokia

and Ericsson. The HIPERLAN/2 tries to integrate WLANs into the next

generation cellular systems. It aims at converging IP and ATM type services at a

high data rate of 54 Mbps for indoor and outdoor applications. The

HIPERLAN/2, an ATM compatible WLAN, is a connection-oriented system,

which uses fixed size packets and enables QoS applications easy to implement.

 

The HIPERLAN/2 network has a typical topology as shown in Figure 1.6. The

figure shows MTs being centrally controlled by the APs which are in turn

connected to the core network (infrastructure-based network). It is to be noted

that, unlike the IEEE standards, the core network for HIPERLAN/2 is not just

restricted to Ethernet. Also, the AP used in HIPERLAN/2 consists of one or

many transceivers called Access Point Transceivers (APTs) which are

controlled by a single Access Point Controller (APC).

 

Figure 1.6. A typical deployment of HIPERLAN/2. 

 

There are two modes of communication in a HIPERLAN/2 network, which are

described by the following two environments:

 

• Business environment: The ad hoc architecture of HIPERLAN/1 has been

extended to support a centralized mode of communication using APs. This

topology corresponds to business environments. Accordingly, each AP serves a

number of MTs.

 

• Home environment: The home environment enables a direct mode of

communication between the MTs. This corresponds to an ad hoc architecture

that can be operated in a plug-and-play manner. The direct mode of

communication is, however, managed by a central control entity elected from

among the nodes called the central controller (CC).There are several features of

HIPERLAN/2 that have attracted many a cellular manufacturer. These features

are part of the discussion on the protocol stack of HIPERLAN/2 below. The

HIPERLAN/2 protocol stack consists of the physical layer, convergence layer

(CL), and the data link control (DLC) layer

 

 

The Physical Layer

 

The physical layer is responsible for the conversion of the PDU train from the

DLC layer to physical bursts that are suitable for radio transmission.

HIPERLAN/2, like IEEE 802.11a, uses OFDM for transmission. The

HIPERLAN/2 allows bit rates from 6 Mbps to 54 Mbps using a scheme called

link adaptation. This scheme allows the selection of a suitable modulation

method for the required bit rate. This scheme is unique to HIPERLAN/2 and is

not available in the IEEE standards and HIPERLAN/1. More details on the

physical layer can be found in.

 

 

The CL

The topmost layer in the HIPERLAN/2 protocol stack is the CL. The functions

of the layer are to adapt the requirements of the different higher layers of the

core network with the services provided by the lower layers of HIPERLAN/2,

and to convert the higher layer packets into ones of fixed size that can be used

by the lower layers. A CL is defined for every type of core network supported.

In short, this layer is responsible for the network-independent feature of

HIPERLAN/2. The CL is classified into two types, namely, the packet-based

CL and the cell based CL. The packet-based CL processes variable-length

packets (such as IEEE 802.3, IP, and IEEE 1394). The cell-based CL processes

fixed sized ATM cells. The CL has two sublayers, namely, the common part

(CP) and the service-specific convergence sublayer (SSCS). The CP is

independent of the core network. It allows parallel segmentation and reassembly

of packets. The CP comprises of two sublayers, namely, the common part

convergence sublayer (CPCS) and the segmentation and reassembly (SAR)

sublayer. The CPCS processes the packets from the higher layer and adds

padding and additional information, so as to be segmented in the SAR. For

further information on the CP, readers are referred to. The SSCS consists of

functions that are specific to the core network. For example, the Ethernet SSCS

has been standardized in for Ethernet core networks. The SSCS adapts the

different data formats to the HIPERLAN/2 DLC format. It is also responsible

for mapping the QoS requests of the higher layers to the QoS parameters of

HIPERLAN/2 such as data rate, delay, and jitter.

 

The DLC Layer

 

The DLC layer constitutes the logical link between the AP and the MTs. This

ensures a connection-oriented communication in a HIPERLAN/2 network, in

contrast to the connectionless service offered by the IEEE standards. The DLC

layer is organized into three functional units, namely, the Radio Link Control

(RLC) sublayer on the control plane, the error control (EC) sublayer on the user

plane, and the MAC sublayer.

 

 

The RLC Sublayer

The RLC sublayer takes care of most of the control procedures on the DLC

layer. The tasks of the RLC can be summarized as follows.

 

• Association Control Function (ACF): The ACF handles the registration and

the authentication functions of an MT with an AP within a radio cell. Only after

the ACF procedure has been carried out can the MT ever communicate with the

AP.

 

• DLC user Connection Control (DCC): The DCC function is used to control

DLC user connections. It can set up new connections, modify existing

connections, and terminate connections.

 

• Radio Resource Control (RRC): The RRC is responsible for the surveillance

and efficient utilization of the available frequency resources.

It performs the following tasks:

 

Dynamic frequency selection: This function is not available in IEEE 802.11,

IEEE 802.11a, IEEE802.11b, and HIPERLAN/1, and is thus unique to

HIPERLAN/2. It allows the AP to select a channel (frequency) for

communication with the MTs depending on the interferences in each channel,

thereby aiding in the efficient utilization of the available frequencies.

 

Handoff: HIPERLAN/2 supports three types of handoff, namely, sector

handoff(moving to another sector of the same antenna of an APT), radio

handoff(handoff between two APTs under the same APC), and network

handoff(handoff between two APs in the same network).

Power saving: Power-saving schemes much similar to those in

HIPERLAN/1and IEEE 802.11 have been implemented.

Error Control (EC) 

Selective repeat (where only the specific damaged or lost frame is

retransmitted)protocol is used for controlling the errors across the medium. To

support QoS for stringent and delay-critical applications, a discard mechanism

can be provided by specifying a maximum delay.

 

The MAC Sublayer

The MAC protocol is used for access to the medium, resulting in the

transmission of data through that channel. However, unlike the IEEE standards

and the HIPERLAN/1 in which channel access is made by sensing it, the MAC

protocol follows a dynamic time division multiple access/time division

duplexing (TDMA/TDD) scheme with centralized control. The protocol

supports both AP-MT unicast and multicast transfer, and at the same time MT

MT peer-to-peer communication. The centralized AP scheduling provides QoS

support and collision-free transmission. The MAC protocol provides a

connection-oriented communication between the AP and the MT (or between

MTs).

 

 

Security Issues

Elaborate security mechanisms exist in the HIPERLAN/2 system. The

encryption procedure is optional and can be selected by the MT during

association. Two strong encryption algorithms are offered, namely, the Data

Encryption Standard (DES) and the triple-DES algorithms

 

 

1.5 BLUETOOTH

WLAN technology enables device connectivity to infrastructure-based services

through a wireless carrier provider. However, the need for personal devices to

communicate wirelessly with one another, without an established infrastructure,

has led to the emergence of Personal Area Networks (PANs). The first attempt to

define a standard for PANs dates back to Ericsson's Bluetooth projectin 1994 to

enable communication between mobile phones using low power and low-cost

radio interfaces. In May 1998, several companies such as Intel, IBM, Nokia, and

Toshiba joined Ericsson to form the Bluetooth Special Interest Group (SIG)

whose aim was to develop a de facto standard for PANs. Recently, IEEE has

approved a Bluetooth-based standard (IEEE 802.15.1) for wireless personal area

networks (WPANs). The standard covers only the MAC and the physical layers

while the Bluetooth specification details the whole protocol stack. Bluetooth

employs radio frequency (RF) technology for communication. It makes use of

frequency modulation to generate radio waves in the ISM band. Low power

consumption of Bluetooth technology and an offered range of up to ten meters

has paved the way for several usage models. One can have an interactive

conference by establishing an ad hoc network of laptops. Cordless computer,

instant postcard [sending digital photographs instantly (a camera is cordlessly

connected to a mobile phone)], and three-in-one phone [the same phone

functions as an intercom (at the office, no telephone charge), cordless phone (at

home, a fixed-line charge), and mobile phone (on the move, a cellular charge)]

are other indicative usage models.

 

 

1.5.1 Bluetooth Specifications

The Bluetooth specification consists of two parts: core and profiles. The core

provides a common data link and physical layer to application protocols, and

maximizes reusability of existing higher layer protocols. The profiles

specifications classify Bluetooth applications into thirteen types. The protocol

stack of Bluetooth performs the functions of locating devices, connecting other

devices, and exchanging data. It is logically partitioned into three layers,

namely, the transport protocol group, the middleware protocol group, and the

application group. The transport protocol group consists of the radio layer,

baseband layer, link manager layer, logical link control and adaptation layer,

and the host controller interface. The middleware protocol group comprises of

RFCOMM, SDP, and IrDA (IrOBEX and IrMC). The application group consists

of applications (profiles) using Bluetooth wireless links, such as the modem

dialer and the Web-browsing client.

 

Figure1.7 shows the protocol stack of Bluetooth.

 

Figure 1.7. Bluetooth protocol stack.

 

1.5.2 Transport Protocol Group This group is composed of the protocols

designed to allow Bluetooth devices to locate each other and to create,

configure, and manage the wireless links. Design of various protocols and

techniques used in Bluetooth communications has been done with the target of

low power consumption and ease of operation. This shall become evident in the

design choice of FHSS and the master–slave architecture. The following

sections study the various protocols in this group, their purpose, their modes of

operation, and other specifications.

 

 

Radio (Physical) Layer

The radio part of the specification deals with the characteristics of the

transceivers and design specifications such as frequency accuracy, channel

interference, and modulation characteristics. The Bluetooth system operates in

the globally available ISM frequency band and the frequency modulation is

GFSK. It supports 64 Kbps voice channels and asynchronous data channels with

a peak rate of 1 Mbps. The data channels are either asymmetric (in one

direction) or symmetric (in both directions). The Bluetooth transceiver is a

FHSS system operating over a set of m channels each of width 1 MHz. In most

of the countries, the value of m is 79. Frequency hopping is used and hopsare

made at a rapid rate across the possible 79 hops in the band, starting at 2.4GHz

and stopping at 2.480 GHz. The choice of frequency hopping has been made to

provide protection against interference. The Bluetooth air interface is based on a

nominal antenna power of 0 dBm (1mW) with extensions for operating at up to

20 dBm (100 mW) worldwide. The nominal link range is from 10 centimetres to

10 meters, but can be extended to more than 100 meters by increasing the

transmit power (using the 20 dBm option). It should be noted here that a WLAN

cannot use an antenna power of less than 0 dBm (1 mW) and hence an 802.11

solution might not be apt for power-constrained devices.

 

 

Baseband Layer

The key functions of this layer are frequency hop selection, connection creation,

and medium access control. Bluetooth communication takes place by adhoc

creation of a network called a piconet. The address and the clock associated

with each Bluetooth device are the two fundamental elements governing the

formation of a piconet. Every device is assigned a single 48-bit address which is

similar to the addresses of IEEE 802.xx LAN devices. The address field is

partitioned into three parts and the lower address part (LAP) is used in several

baseband operations such as piconet identification, error checking, and security

checks. The remaining two parts are proprietary addresses of the manufacturing

organizations. LAP is assigned internally by each organization. Every device

also has a 28-bit clock (called the native clock) that ticks 3,200 times per second

or once every 312.5 μs. It should be noted that this is twice the normal hopping

rate of 1,600 hops per second.

 

Piconet

The initiator for the formation of the network assumes the role of the master (of

the piconet). All the other members are termed as slaves of the piconet. A

piconet can have up to seven active slaves at any instant. For the purpose of

identification, each active slave of the piconet is assigned a locally unique active

member address AM_ADDR. Other devices could also be part of the piconet by

being in the parked mode (explained later). A Bluetooth device not associated

with any piconet is said to be in standby mode. Figure 1.8 shows a piconet with

several devices.

 

Figure 1.8. A typical piconet.

 

 

Operational States

Figure 1.9 shows the state diagram of Bluetooth communications. Initially, all

the devices would be in the standby mode. Then some device (called the

master)could begin the inquiry and get to know the nearby devices and, if

needed, join them into its piconet. After the inquiry, the device could formally

be joined by paging, which is a packet-exchange process between the master

and a prospective slave to inform the slave of the master's clock. If the device

was already inquired, the master could get into the page state bypassing the

inquiry state. Once the device finishes getting paged, it enters the connected

state. This state has three power-conserving sub-states – hold, sniff, and park

(described later in this section). A device in the connected state can participate

in the data transmission.

 

Figure 1.9. Operational states.

 

 

Frequency Hopping Sequences

It is evident (in any wireless communication) that the sender and the receiver

should use the same frequency for communication to take place. A Frequency

Selection Module (FSM) is present in each device to select the next frequency

to be used under various circumstances. In the connected state, the clock and the

address of the device (master) completely determine the hopping sequence.

Different combination of inputs (clock, address) are used depending on the

operational state. During the inquiry operation, the address input to FSM is a

common inquiry address. This common address is needed because at the time of

inquiry no device has information about the hopping sequence being followed.

The address of the paged device is fed as input to the FSM for the paging state.

 

Communication Channel

The channel is divided into time slots, each 625 μs in length. The time slots are

numbered according to the Bluetooth clock of the piconet master. A time

division duplex (TDD) scheme is used where master and slave alternately

transmit. The master starts its transmission in even-numbered time slots only,

and the slave starts its transmission in odd-numbered time slots only. This is

clearly illustrated in Figure 1.10 (a). The packet start shall be aligned with the

slot start. A Bluetooth device would determine slot parity by looking at the least

significant bit (LSB) in the bit representation of its clock. IfLSB is set to 1, it is

the possible transmission slot for the slave. A slave in normal circumstances is

allowed to transmit only if in the preceding slot it has received a packet from the

master. A slave should know the master's clock and address to determine the

next frequency (from the FSM). This information is exchanged during paging.

 

Figure 1.10. Transmission of packets over a channel.

 

Packet-Based Communication

 

Bluetooth uses packet-based communication where the data to be transmitted is

fragmented into packets. Only a single packet can be transmitted in each slot. A

typical packet used in these communications has three components: access code,

header, and payload. The main component of the access code is the address of

the piconet master. All packets exchanged on the channel are identified by the

master's identity. The packet will be accepted by the recipient only if the access

code matches the access code corresponding to the piconet master. This also

helps in resolving conflicts in the case where two piconets are operating

currently on the same frequency. A slave receiving two packets in the same slot

can identify its packet by examining the access code. The packet header

contains many fields such as a three-bit active slave address, a one-bit

ACK/NACK for ARQ scheme [Automatic Repeat reQuest — anytime an error

is detected, a negative acknowledgment (NACK) is returned and the specified

frames are retransmitted], a four-bit packet type to distinguish payload types,

and an eight-bit header error check code to detect errors in the header.

Depending on the payload size, one, three, or five slots may be used for the

packet transmission. The hop frequency which is used for the first slot is used

for the remainder of the packet. While transmitting packets in multiple slots, it

is important that the frequencies used in the following time slots are those that

are assigned to those slots, and that they do not follow the frequency sequence

that should have normally applied. This is illustrated in Figure 1.10 (b). When a

device uses five slots for packet transmission, the next packet transmission is

allowed in F(k+6) and not in F(k+2). Also note that the receiving time slot

becomes F(k+5) as opposed to F(k+1). On this slotted channel, both

synchronous and asynchronous links are supported. Between a master and a

slave there is a single asynchronous connectionless link(ACL) supported. This

is the default link that would exist once a link is established between a master

and a slave. Whenever a master would like to communicate, it would, and then

the slave would respond. Optionally, a piconet may also support synchronous

connection oriented (SCO) links. SCO link is symmetric between master and

slave with reserved bandwidth and regular periodic exchange of data in the form

of reserved slots. These links are essential and useful for high-priority and time

bound information such as audio and video.

 

 

Inquiry State

As shown in Figure 1.9, a device which is initially in the standby state enters the

inquiry state. As its name suggests, the sole purpose of this state is to collect

information about other Bluetooth devices in its vicinity. This information

includes the Bluetooth address and the clock value, as these form the crux of the

communication between the devices. This state is classified into three substates:

inquiry, inquiry scan, and inquiry response. A potential master sends an inquiry

packet in the inquiry state on the inquiry hop sequence of frequencies. This

sequence is determined by feeding a common address as one of the inputs to the

FSM. A device (slave) that wants to be discovered will periodically enter the

inquiry scan state and listen for these inquiry packets. When an inquiry message

is received in the inquiry scan state, a response packet called the Frequency

Hopping Sequence (FHS) containing the responding device address must be

sent. Devices respond after a random jitter to reduce the chances of collisions.

 

 

Page State

A device enters this state to invite other devices to join its piconet. A device

could invite only the devices known to itself. So normally the inquiry operation

would precede this state. This state also is classified into three sub-states: page,

page scan, and page response. In the page mode, the master estimates the slave's

clock based on the information received during the inquiry state, to determine

where in the hop sequence the slave might be listening in the page scan mode.

 

In order to account for inaccuracies in estimation, the master also transmits the

 page message through frequencies immediately preceding and succeeding the

estimated one. On receiving the page message, the slave enters the slave page

response substate. It sends back a page response consisting of its ID packet

which contains its Device Access Code (DAC). Finally, the master (after

receiving the response from a slave) enters the page response state and informs

the slave about its clock and address so that the slave can go ahead and

participate in the piconet. The slave now calculates an offset to synchronize

with the master clock, and uses that to determine the hopping sequence for

communication in the piconet.

 

 

Scatternets and Issues

 

Piconets may overlap both spatially and temporally, that is, many piconets could

operate in the same area at the same time. Each piconet is characterized by a

unique master and hence the piconets hop independently, each with its own

channel hopping sequence as determined by the respective master. In addition,

the packets carried on the channels are preceded by different channel access

codes as determined by the addresses of the master devices. As more piconets

are added, the probability of collisions increases, and a degradation in

performance results, as is common in FHSS systems. In this scenario, a device

can participate in two or more overlaying piconets by the process of time

sharing. To participate on the proper channel, it should use the associated master

device address and proper clock offset. A Bluetooth unit can act as a slave in

several piconets, but as a master in only a single piconet. A group of piconets in

which connections exist between different piconets is called a scatternet (Figure

1.11).

 

Figure 1.11. A typical scatternet.

 

 

When a device changes its role and takes part in different piconets, it is bound to

lead to a situation in which some slots remain unused (for synchronization).This

implies that complete utilization of the available bandwidth is not achieved. An

interesting proposition at this juncture would be to unite the timings of the

whole of the scatternet. But this may lead to an increase in the probability of

packets colliding. Another important issue is the timing that a device would be

missing by participating in more than one piconet. A master that is missing from

a piconet (by momentarily becoming a slave in another piconet) may miss

polling slaves and must ensure that it does not miss beacons from its slaves.

Similarly, a slave(by becoming a master or slave in another piconet) that is

missing from a piconet could appear to its master to have gone out of range or to

be connected through a poor-quality link.

 

 

Link Manager Protocol

Link manager protocol (LMP) is responsible for setting and maintaining the

properties of the Bluetooth link. Currently, the major functionality of this layer

is power management and security management. It also provides minimal QoS

support by allowing control over parameters such as delay and delay jitter.

Normally, a paging device is the default master of the piconet, but, depending

on the usage scenario, the roles of the master and a slave could be switched and

this is coordinated by exchange of LMP packets.

 

 

Power Management

The Bluetooth units can be in several modes of operation during the connection

state, namely, active mode, sniff mode, hold mode, and park mode. These

modes are now described.

 

• Active mode: In this mode, the Bluetooth unit actively participates in the

piconet. Various optimizations are provided to save power. For instance, if the

master informs the slave when it will be addressed, the slave may sleep until

then. The active slaves are polled by the master for transmissions.

 

• Sniff mode: This is a low-power mode in which the listening activity of the

slave is reduced. The LMP in the master issues a command to the slave to enter

the sniff mode, giving it a sniff interval, and the slave listens for transmissions

only at these fixed intervals.

 

• Hold mode: In this mode, the slave temporarily does not support ACL packets

on the channel (possible SCO links will still be supported). In this mode,

capacity is made available for performing other functions such as scanning,

paging, inquiring, or attending another piconet.

 

• Park mode: This is a very low-power mode. The slave gives up its active

member address and is given an eight-bit parked member address. The slave,

however, stays synchronized to the channel. Any messages to be sent to a

parked member are sent over the broadcast channel characterized by an active

member address of all zeros. Apart from saving power, the park mode helps the

master to have more than seven slaves (limited by the three-bit active member

address space) in the piconet.

 

 

Bluetooth Security

In Bluetooth communications, devices may be authenticated and links may be

encrypted. The authentication of devices is carried out by means of a challenge

response mechanism which is based on a commonly shared secret link key

generated through a user-provided personal identification number (PIN). The

authentication starts with the transmission of an LMP challenge packet and ends

with the verification of result returned by the claimant. Optionally, the link

between them could also be encrypted.

 

 

Logical Link Control and Adaptation Protocol (L2CAP)

This is the protocol with which most applications would interact unless a host

controller is used. L2CAP supports protocol multiplexing to give the abstraction

to each of the several applications running in the higher layers as if it alone is

being run. Since the data packets defined by the baseband protocol are limited in

size, L2CAP also segments large packets from higher layers such as RFCOMM

or SDP into multiple smaller packets prior to their transmission over the

channel. Similarly, multiple received baseband packets may be reassembled into

a single larger L2CAP packet. This protocol provides QoS on certain parameters

such as peak bandwidth, latency, and delay variation when the link is

established between two Bluetooth units.

 

 

Host Controller Interface

This is the optional interface layer, provided between the higher (above LMP)

and lower layers of the Bluetooth protocol stack, for accessing the Bluetooth

hardware capabilities. Whenever the higher layers are implemented on the

motherboard of a host device, this layer is needed. Such an approach could

prove beneficial as the spare capacity of the host device (say, a personal

computer) could be utilized. The specification defines details such as the

different packet types as seen by this layer. Command packets that are used by

the host to control the device, event packets that are used by the device to

inform the host of the changes, and data packets come under this category.

 

2.5.3 Middleware Protocol Group

 

The basic functionality of the middleware protocol group is to present to the

application layers a standard interface that may be used for communicating

across the transport layer, that is, the applications need not know the transport

layer's complexities, they can just use the application programming

interfaces(APIs) or higher level functions provided by the middleware

protocols. This group consists of the RFCOMM layer, service discovery

protocol (SDP), IrDA interoperability protocols, telephony control specification

(TCS), and audio. The RFCOMM layer presents a virtual serial port to

applications using the serial interface. Any application which is using the serial

port can work seamlessly on Bluetooth devices. RFCOMM uses an L2CAP

connection to establish a link between two devices. In the case of Bluetooth

devices, there is no device which will be static and hence services offered by the

other devices have to be discovered. This is achieved by using the service

discovery protocol (SDP) of the Bluetooth protocol stack. Service discovery

makes the device self configured without manual intervention. The IrDA

interoperability protocol is not for communication between Bluetooth devices

and Infrared devices. It is only for the existing IrDA applications to work on

Bluetooth devices without any changes. The main protocols in the IrDA set are

IrOBEX (IrDA object exchange) for exchanging objects between two devices

and IrMC (infrared mobile communications) for synchronization. Audio is the

distinguishing part of Bluetooth. Audio is given the highest priority and is

directly carried over the baseband at 64 Kbps so that a very good quality of

voice is provided. Another important point to note here is that audio is actually

not a layer of the protocol stack, but only a specific packet format that can be

transmitted directly over the SCO links of the baseband layer. Telephony

control is implemented using the telephony control specification –binary (TCS

BIN) protocol. TCS defines three major functional areas: call control, group

management, and connectionless TCS. Call control is used to setup calls which

can be subsequently used to carry voice and data traffic. TCS operates in both

point-to-point and point-to-multipoint configurations. One of the main concepts

of TCS is that of the wireless user group (WUG). Group management enables

multiple telephone extensions, call forwarding, and group calls. For example,

consider multiple handsets and a single base set. When a call comes in to the

base set, all the multiple handsets can receive this call. In a similar fashion, calls

can also be forwarded. The functionalities of TCS include configuration

distribution and fastintermember access. Configuration distribution is the

mechanism used to find the information about the other members in a group.

Fast inter member access is a method for two slaves to create a new piconet.

AWUG member uses the information from the configuration distribution and

determines another member which it wants to contact. Then it sends the device's

information to the master, which forwards it to this device. The contacted device

then responds with its device address and clock information and places itself in

a page scan state. Then the master contacts the device initiating the

communication. This device now pages the contacted device and forms a new

piconet. This explains how a new piconet is formed between two slaves with the

help of the master. In all the above cases, a connection-oriented channel is

established. To exchange simple information such as adjusting volume or

signaling information, establishing such a channel is overkill and hence

connectionless TCS has been provided for having a connectionless channel.

 

1.5.4 Bluetooth Profiles

 

These profiles have been developed to promote interoperability among the many

implementations of the Bluetooth protocol stack. Each Bluetooth profile

specification has been defined to provide a clear and transparent standard that

can be used to implement a specific user end function. Two Bluetooth devices

can achieve a common functionality only if both devices support identical

profiles. For example, a cellular phone and a headset both have to support the

Bluetooth headset profile for the headset to work with the phone. The Bluetooth

profiles spring up from the usage models. In all, 13 profiles have been listed and

these can be broadly classified into the following four categories:

 

1. Generic profiles: The Generic access profile, which is not really an

application, provides a way to establish and maintain secure links between the

master and the slaves. The service discovery profile enables users to access SDP

to find out which applications (Bluetooth services)are supported by a specific

device.

 

2. Telephony profiles: The cordless telephony profile is designed for three in

one phones. The Intercom profile supports two-way voice communication

between two Bluetooth devices within range of each other. The Headset profile

specifies how Bluetooth can provide a wireless connection to a headset (with

earphones/microphones) for use with a computer or a mobile phone.

 

3. Networking profiles: The LAN Access profile enables Bluetooth devices to

either connect to a LAN through APs or form a small wireless LAN among

themselves. The dial-up networking profile is designed to provide dial-up

connections via Bluetooth-enabled mobile phones. The FAX profile, very

similar to the dial-up networking profile, enables computers to send and receive

faxes via a Bluetooth-enabled mobile phone.

 

4. Serial and object exchange profiles: The serial port profile emulates a serial

line (RS232 and USB serial ports) for (legacy) applications that require a serial

line. The other profiles, generic object exchange, object push, file transfer, and

synchronization, are for exchanging objects between two wireless devices.

Bluetooth is the first wireless technology which has actually tried to attempt to

make all the household consumer electronics devices follow one particular

communication paradigm. It has been partially successful, but it does have its

limitations. Bluetooth communication currently does not provide support for

routing. It should be noted that some research efforts are under way to

accommodate this in the Bluetooth specification. Once the routing provision is

given, inter-piconet communication could be enhanced. The issues of handoffs

also have not yet been dealt with till now. Although master–slave architecture

has aided low cost, the master becomes the bottleneck for the whole piconet in

terms of performance, fault tolerance, and bandwidth utilization. Most

importantly, Bluetooth communication takes place in the same frequency band

as that of WLAN and hence robust coexistence solutions need to be developed

to avoid interference. The technology is still under development. Currently,

there are nearly 1,800 adopter companies which are contributing toward the

development of the technology.

 

1.6 HOME RF

Wireless home networking represents the use of the radio frequency

(RF)spectrum to transmit voice and data in confined areas such as homes and

small offices. One of the visionary concepts that home networking intends to

achieve is the establishment of communication between home appliances such

as computers, TVs, telephones, refrigerators, and air conditioners. Wireless

home networks have an edge over their wired counterparts because features

such as flexibility (enabling of file and drive sharing) and interoperability that

exist in the wired networks are coupled with those in the wireless domain,

namely, simplicity of installation and mobility. The HIPERLAN/2, as

mentioned earlier, has provisions for direct communication between the mobile

terminals (the home environment). The home environment enables election of a

central controller (CC) which coordinates the communication process. This

environment is helpful in setting up home networks. Apart from this, an industry

consortium known as the Home RF Working Group has developed a technology

that is termed HomeRF. This technology intends to integrate devices used in

homes into a single network and utilize RF links for communication. HomeRF

is a strong competitor to Bluetooth as it operates in the ISM band.

 

Technical Features The HomeRF provides data rates of 1.6 Mbps, a little

higher than the Bluetooth rate, supporting both infrastructure-based and ad hoc

communications. It provides a guaranteed QoS delivery to voice-only devices

and best-effort delivery for data-only devices. The devices need to be plug-and

play enabled; this needs automatic device discovery and identification in the

network. Atypical HomeRF network consists of resource providers (through

which communication to various resources such as the cable modem and phone

lines is effected), and the devices connected to them (such as the cordless

phone, printers, and file servers). The HomeRF technology follows a protocol

called the shared wireless access protocol (SWAP). The protocol is used to set

up a network that provides access to a public network telephone, the Internet

(data), entertainment networks (cable television, digital audio, and video),

transfer and sharing of data resources (such as disks and printers), and home

control and automation. The SWAP has been derived from the IEEE 802.11 and

the European digitally enhanced cordless telephony (DECT) standards. It

employs a hybrid TDMA/CSMA scheme for channel access. While TDMA

handles isochronous transmission (similar to synchronous transmission,

isochronous transmission is also used for multimedia communication where

both the schemes have stringent timing constraints, but isochronous

transmission is not as rigid as synchronous transmission in which data streams

are delivered only at specific intervals), CSMA supports asynchronous

transmission (in a manner similar to that of the IEEE 802.11 standard), thereby

making the actual framing structure more complex. The SWAP, however,

differs from the IEEE 802.11specification by not having the RTS-CTS

handshake since it is more economical to do away with the expensive

handshake; moreover, the hidden terminal problem does not pose a serious

threat in the case of small-scale networks such as the home networks. The

SWAP can support up to 127 devices, each identified uniquely by a 48-

bitnetwork identifier. The supported devices can fall into one (or more) of the

following four basic types:

 

• Connection point that provides a gateway to the public switched telephone

network (PSTN), hence supporting voice and data services.

 

• Asynchronous data node that uses the CSMA/CA mechanism to communicate

with other nodes.

• Voice node that uses TDMA for communication.

• Voice and data node that can use both CSMA/CA and TDMA for channel

access. Home networking also needs strong security measures to safeguard

against potential eavesdroppers. That is the reason why SWAP uses strong

algorithms such as Blowfish encryption. HomeRF also includes support for

optional packet compression which provides a trade-off between bandwidth and

power consumption. Because of its complex (hybrid) MAC and higher

capability physical layer, the cost of HomeRF devices is higher than that of

Bluetooth devices. HomeRF Version 2.0, released recently, offers higher data

rates (up to 10 Mbps by using wider channels in the ISM band through FHSS).

 

 

Infrared The infrared technology (IrDA) uses the infrared region of the light for

communication . Some of the characteristics of these communications are as

follows:

• The infrared rays can be blocked by obstacles, such as walls and buildings.

• The effective range of infrared communications is about one meter. But when

high power is used, it is possible to achieve better ranges.

• The power consumed by infrared devices is extremely low.

• Data rates of 4 Mbps are easily achievable using infrared communications.

• The cost of infrared devices is very low compared to that of Bluetooth devices.

Although the restriction of line of sight (LoS) is there on the infrared devices,

they are extremely popular because they are cheap and consume less power. The

infrared technology has been prevalent for a longer time than Bluetooth wireless

communications. So it has more widespread usage than Bluetooth. Table1.2

compares the technical features of Bluetooth, HomeRF, and IrDAtechnologies.

 

Table 1.2. Illustrative comparison among Bluetooth, HomeRF, and IrDA

technologies

 

 

 

 **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** **** ****

 

 

        MULTIPLE ACCESS TECHNIQUES 

 

In wireless communication systems, it is often desirable to allow the subscriber to

send information simultaneously from the mobile station to the base station while

receiving information from the base station to the mobile station.

 

A cellular system divides any given area into cells where a mobile unit in each cell

communicates with a base station. The main aim in the cellular system design is to

be able to increase the capacity of the channel, i.e., to handle as many calls as

possible in a given bandwidth with a sufficient level of quality of service.

 

There are several different ways to allow access to the channel. These includes

mainly the following –

 Frequency division multiple-access (FDMA)

Time division multiple-access (TDMA)

Code division multiple-access (CDMA)

Space division multiple access (SDMA)

 

Depending on how the available bandwidth is allocated to the users, these

techniques can be classified as narrowband and wideband systems.

 

    

 Narrowband Systems

Systems operating with channels substantially narrower than the coherence

bandwidth are called as Narrow band systems. Narrow band TDMA allows users to

use the same channel but allocates a unique time slot to each user on the channel,

thus separating a small number of users in time on a single channel.

 

 

Wideband Systems

In wideband systems, the transmission bandwidth of a single channel is much larger

than the coherence bandwidth of the channel. Thus, multipath fading doesn’t greatly

affect the received signal within a wideband channel, and frequency selective fades

occur only in a small fraction of the signal bandwidth.

 

 

FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)

FDMA is the basic technology for advanced mobile phone services. The features of

 

FDMA are as follows.

 FDMA allots a different sub-band of frequency to each different user to access the

network.

 If FDMA is not in use, the channel is left idle instead of allotting to the other users.

 FDMA is implemented in Narrowband systems and it is less complex than TDMA.

 Tight filtering is done here to reduce adjacent channel interference.

 The base station BS and mobile station MS, transmit and receive simultaneously

and continuously in FDMA.

 

 

 

 FDMA is different from frequency division duplexing (FDD). 

While FDMA permits multiple users to simultaneously access a transmission system, FDD describes the way the radio channel is shared between the downlink and uplink. 

 

FDMA is also different from Frequency-division multiplexing (FDM). FDM refers to a

physical layer method that blends and transmits low-bandwidth channels via a high

bandwidth channel. FDMA, in contrast, is a channel access technique in the data link

layer. 

 

 

TIME DIVISION MULTIPLE ACCESS (TDMA) 

 

In the cases where continuous transmission is not required, there TDMA is used

instead of FDMA. The features of TDMA include the following. 

 

 TDMA shares a single carrier frequency with several users where each users

makes use of non overlapping time slots.

 Data transmission in TDMA is not continuous, but occurs in bursts. Hence handsoff

process is simpler.

 TDMA uses different time slots for transmission and reception thus duplexers are

not required.

 TDMA has an advantage that is possible to allocate different numbers of time slots

per frame to different users.

 Bandwidth can be supplied on demand to different users by concatenating or

reassigning time slot based on priority.Time Division Multiple Access (TDMA) is a digital cellular telephone communication technology. It facilitates many users to share the same frequency without interference. Its technology divides a signal into different timeslots, and increases the data carrying capacity. 

 

 

 

Time Division Multiple Access (TDMA) is a complex technology, because it requires

an accurate synchronization between the transmitter and the receiver. TDMA is used

in digital mobile radio systems. The individual mobile stations cyclically assign a

frequency for the exclusive use of a time interval. 

 

In most of the cases, the entire system bandwidth for an interval of time is not

assigned to a station. However, the frequency of the system is divided into sub

bands, and TDMA is used for the multiple access in each sub-band. Sub-bands are

known as carrier frequencies. The mobile system that uses this technique is referred

as the multi-carrier systems. 

 

 

CODE DIVISION MULTIPLE ACCESS (CDMA) 

 

Code division multiple access technique is an example of multiple access where

several transmitters use a single channel to send information simultaneously. Its

features are as follows. 

 

 In CDMA every user uses the full available spectrum instead of getting allotted by

separate frequency.

 

 CDMA is much recommended for voice and data communications.

 

 While multiple codes occupy the same channel in CDMA, the users having same

code can communicate with each other.

 

 CDMA offers more air-space capacity than TDMA.

 

 The hands-off between base stations is very well handled by CDMA.Code Division Multiple Access (CDMA) is a sort of multiplexing that facilitates

various signals to occupy a single transmission channel. It optimizes the use of

available bandwidth. The technology is commonly used in ultra-high-frequency

(UHF) cellular telephone systems, bands ranging between the 800-MHz and 1.9-GHz. 

 

 

SPACE DIVISION MULTIPLE ACCESS (SDMA) 

 

Space division multiple access or spatial division multiple access is a technique

which is MIMO (multiple-input multiple-output) architecture and used mostly in

wireless and satellite communication. It has the following features.

 

 All users can communicate at the same time using the same channel.

 

 SDMA is completely free from interference.

 

 A single satellite can communicate with more satellites receivers of the same

frequency. 

 

 The directional spot-beam antennas are used and hence the base station in SDMA,

can track a moving user.

 

 Controls the radiated energy for each user in space.

 

 

Space-division multiple access (SDMA) is a channel access method based on

creating parallel spatial pipes (focused signal beams) using advanced antenna

technology next to higher capacity pipes through spatial multiplexing and/or diversity,

by which it is able to offer superior performance in radio multiple access

communication systems (where multiple users may need to use the communication

media simultaneously)

 

In traditional mobile cellular network systems, the base station has no information on

the position of the mobile units within the cell and radiates the signal in all directions

within the cell in order to provide radio coverage. This method results in wasting

power on transmissions when there are no mobile units to reach, in addition to

causing interference for adjacent cells using the same frequency, so called co

channel cells. Likewise, in reception, the antenna receives signals coming from all

directions including noise and interference signals. 

 

By using smart antenna technology and differing spatial locations of mobile units within the cell, space division multiple access techniques offer attractive performance enhancements. The radiation pattern of the base station, both in transmission and reception, is adapted

to each user to obtain highest gain in the direction of that user. This is often done

using phased array techniques.

 

 

 ********************************************************************

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                                    SPREAD SPECTRUM MODULATION 

 

Introduction:

 

Initially developed for military applications during II world war, that was less sensitive

to intentional interference or jamming by third parties. Spread spectrum technology has

blossomed into one of the fundamental building blocks in current and next-generation

wireless systems.

 

 

Problem of radio transmission

 

Narrow band can be wiped out due to interference. To disrupt the communication,

the adversary needs to do two things,

(a) to detect that a transmission is taking place and

(b) to transmit a jamming signal which is designed to confuse the receiver.

 

Solution

 

A spread spectrum system is therefore designed to make these tasks as difficult

as possible.

        Firstly, the transmitted signal should be difficult to detect by an adversary/jammer,

i.e., the signal should have a low probabilityof intercept (LPI).

        Secondly, the signal should be difficult to disturb with a jamming signal, i.e., the

transmitted signal should possess an anti-jamming (AJ) property

 

 

Remedy

 

spread the narrow band signal into a broad band to protect against

interference

 

In a digital communication system the primary resources are Bandwidth and

Power. The study of digital communication system deals with efficient utilization of

these two resources, but there are situations where it is necessary to sacrifice their

efficient utilization in order to meet certain other design objectives.

 

For example to provide a form of secure communication (i.e. the transmitted

signal is not easily detected or recognized by unwanted listeners) the bandwidth of the

transmitted signal is increased in excess of the minimum bandwidth necessary to

transmit it. This requirement is catered by a technique known as “Spread Spectrum

Modulation”.

 

The primary advantage of a Spread – Spectrum communication system is its

ability to reject ‘Interference’ whether it be the unintentional or the intentional

interference. 

 

The definition of Spread – Spectrum modulation may be stated in two parts.

1. Spread Spectrum is a mean of transmission in which the data sequence

occupies a BW (Bandwidth) in excess of the minimum BW necessary to transmit it.

2. The Spectrum Spreading is accomplished before transmission through the use of

a code that is independent of the data sequence. The Same code is used in the receiver to despread the received signal so that the original data sequence may be recovered. 

 

 

Fig. Block diagram for spread spectrum communication 

 

 

Fig: Spread spectrum technique. 

 

b(t) = Data Sequence to be transmitted (Narrow Band);

c(t) = Wide Band code ;

s(t) = c(t) * b(t) – (wide Band) 

 

 

Fig: Spectrum of signal before & after spreading 

 

 

PSUEDO-NOISE SEQUENCE:

 

Generation of PN sequence:

 

 

Fig: Maximum-length sequence generator for n=3 

 

A feedback shift register is said to be Linear when the feedback logic consists of

entirely mod-2-address (Ex-or gates). In such a case, the zero state is not permitted.

The period of a PN sequence produced by a linear feedback shift register with ‘n’ flip

flops cannot exceed 2^n-1. 

 

When the period is exactly 2^n-1, the PN sequence is called a ‘maximum length

sequence’ or ‘m-sequence’..

 

Example1: Consider the linear feedback shift register shown in above figure

 

Involve three flip-flops. The input so is equal to the mod-2 sum of S1 and S3. If

the initial state of the shift register is 100. Then the succession of states will be as

follows. 

 

100,110,011,011,101,010,001,100 . . . . . .

 

The output sequence (output S3) istherefore. 00111010..........Which repeats itself with

period 23–1 = 7 (n=3). Maximal length codes are commonly used PN codes In binary

shift register, the maximum length sequence is 

 

        N = 2m-1

chips, where m is the number of stages of flip-flops in the shift register.

 

At each clock pulse

• Contents of register shifts one bitright.

• Contents of required stages are modulo 2 added and fed back to input. 

 

Fig: Initial stages of Shift registers 1000 

 

Let initial status of shift register be 1000 

 

Properties of PN Sequence 

 

Randomness of PN sequence is tested byfollowing properties

1. Balance property

2. Run length property

3. Autocorrelation property 

 

 

1. Balance property

In each Period of the sequence , number of binary ones differ from binary zeros by

at most one digit.

Consider output of shift register 0 0 0 1 0 0 1 1 0 1 0 1 1 1 1

Seven zeros and eight ones -meets balance condition. 

 

2. Run length property

Among the runs of ones and zeros in each period, it is desirable that about one

half the runs of each type are of length 1, one- fourth are of length 2 and one-eighth

are of length 3 and so-on. 

 

Consider output of shift register

Number of runs =8

0 0 0         1                     0 0              1 1             0         1         0             1 1 1 1

3             1                          2             2                  1         1       1                 4 

 

 

3. Auto correlation property 

 

Auto correlation function of a maximal length sequence is periodic and binary

valued. Autocorrelation sequence of binary sequence in polar format is given by

                        1

=                                     −

                                    =1

 

Where N is length or the period of the sequence, k is the lag of auto correlation function.

            1 = 1

         = − 1 ≠ 1

 

Where 1 is any Integer. We can also state the auto correlation function is

                                        1    

                                     =

{ No. of agreements – No. of disagreements in comparison of one full period } 

 

Consider output of shift register for l=1 

 

 

 

                = 1/15   *   7 -  8   =  -1/15 

 

 

Yields PN autocorrelation as 

 

 

Range of PN Sequence Lengths 

 

 

Length 0f Shift Register, m                 PN Sequence Length,

7                                                                     127

8                                                                    255

9                                                                    511

10                                                                 1023

11                                                                    2047

12                                                                    4095

13                                                                    8191

17                                                                     131071

19                                                                    524287

 

 

Notion of Spread Spectrum:

An important attribute of Spread Spectrum modulation is that it can provide

protection against externally generated interfacing signals with finite power. Protection

against jamming (interfacing) waveforms is provided by purposely making the

information – bearing signal occupy a BW far in excess of the minimum BW necessary

to transmit it. This has the effect of making the transmitted signal a noise like

appearance so as to blend into the background. Therefore Spread Spectrum is a

method of ‘camouflaging’ the information – bearing signal.

 

 

 Let { bK} denotes a binary data sequence.

{ cK } denotes a PN sequence.

 

b(t) and c(t) denotes their NRZ polar representation respectively.

 

The desired modulation is achieved by applying the data signal b(t) and PN signal

c(t) to a product modulator or multiplier. If the message signal b(t) is narrowband and

the PN sequence signal c(t) is wide band, the product signal m(t) is also wide band. The

PN sequence performs the role of a ‘SpreadingCode”. 

 

 

For base band transmission, the product signal m(t) represents the transmitted

signal. Therefore   m(t) = c(t).b(t) 

 

The received signal r(t) consists of the transmitted signal m(t) plus an additive

interference noise n(t), Hence

 

r(t) = m(t) + n(t)

 

= c(t).b(t) + n(t)

 

 

 

 

 

 To recover the original message signal b(t), the received signal r(t) is applied to a

demodulator that consists of a multiplier followed by an integrator and a decision device.

The multiplier is supplied with a locally generated PN sequence that is exact replica of

that used in the transmitter. The multiplier output is given by 

 

Z(t) = r(t).c(t)

 

= [b(t) * c(t) + n(t)] c(t) = c2 (t).b(t) + c(t).n(t)

 

The data signal b(t) is multiplied twice by the PN signal c(t), where as unwanted

signal n(t) is multiplied only once. But c2 (t) = 1, hence the above equation reduces to

 

Z(t) = b(t) + c(t).n(t)

 

Now the data component b(t) is narrowband, where as the spurious component

c(t)n(t) is wide band. Hence by applying the multiplier output to a base band (low pass)

filter most of the power in the spurious component c(t)n(t) is filtered out. Thus the effect

of the interference n(t) is thus significantly reduced at the receiver output. 

 

The integration is carried out for the bit interval 0 ≤ t ≤ Tb to provide the sample

value V. Finally, a decision is made by the receiver. 

 

If V > Threshold Value ‘0’, say binary symbol ‘1’ If V < Threshold Value ‘0’, say

binary symbol ‘0’ 

 

 

Direct – Sequence Spread Spectrum with coherent binary Phase shift 

 

Keying:-  

 

 

To provide band pass transmission, the base band data sequence is multiplied

by a Carrier by means of shift keying. Normally binary phase shift keying (PSK) is used

because of its advantages. The transmitter first converts the incoming binary data

sequence {bk} into an NRZ waveform b(t), which is followed by two stages of

modulation. 

 

The first stage consists of a multiplier with data signal b(t) and the PN signal c(t)

as inputs. The output of multiplier is m(t) is a wideband signal. Thus a narrow – band

data sequence is transformed into a noise like wide band signal. 

 

The second stage consists of a binary Phase Shift Keying (PSK) modulator.

Which converts base band signal m(t) into band pass signal x(t). The transmitted signal

x(t) is thus a direct – sequence spread binary PSK signal. The phase modulation θ(t) of

x(t) has one of the two values ‘0’ and ‘π’ (180o ) depending upon the polarity of the

message signal b(t) and PN signal c(t) at time t. 

 

Polarity of PN & Polarity of PN signal both +, + or - - Phase ‘0’

Polarity of PN & Polarity of PN signal both +, - or - + Phase ‘π’ 

 

 

The receiver consists of two stages of demodulation. 

 

In the first stage the received signal y(t) and a locally generated carrier are

applied to a coherent detector (a product modulator followed by a low pass filter), Which

converts band pass signal into base band signal. 

 

The second stage of demodulation performs Spectrum despreading by

multiplying the output of low-pass filter by a locally generated replica of the PN signal

c(t), followed by integration over a bit interval Tb and finally a decision device is used to

get binary sequence. 

 

 

Fig : Direct Sequence Spread Spectrum Example

 

 

Fig : Direct Sequence Spread Spectrum Using BPSK Example 

 

 

Signal Space Dimensionality and Processing Gain 

 

Fundamental issue in SS systems is how much protection spreading can

provide against interference.

 

SS technique distribute low dimensional signal into large dimensional signal

space (hide the signal).

 

Jammer has only one option; to jam the entire space with fixed total power or

to jam portion of signal space with large power. 

 

Consider set of orthonormal basis functions;

=            2 cos 2 ≤   ≤ + 1

0

=        2 sin 2 ≤   ≤ + 1

0

= 0,1… … … … … − 1 

 

Where Tc is chip duration, N is number of chips per bit. 

 

Transmitted signal x(t) for the interval of an information bit is

= ( ) ( )

= ± 2 c(t)cos 2

N−1

= ± ck  φk (t)       0 ≤  ≤

                k=0

where, Eb is signal energy per bit. 

 

PN Code sequence { c0, c1, ……cN-1} with ck= + 1, Transmitted signal x(t) is

therefore N dimensional and requires N orthonormal functions to represent it. j(t)

represent interfering signal (jammer). As said jammer tries to places all its available

energy in exactly same N dimension signal space. But jammer has no knowledge

of signal phase. Hence tries to place equal energy in two phase coordinates that is

cosine and sine. As per that jammer can be represented as

 

=     −1      +  -1                    0 ≤   ≤

    =0                =0

Where

=0                                        = 0,1, …?…? − 1

= 0                                                      0,1, …?…? −1

Thus j(t) is 2N dimensional, twice the dimension as that of x(t). 

 

Average interference power of j(t)

=   1 _   2          = 1 _      -1            2 +  1__  -1    2

            0                         =0                               =0

as jammer places equal energy in two phase coordinates , hence

-1                  -1

2        =            2

=0            =0

 

           -1

=2___                2

                =0

To evaluate system performance we calculate SNR at input and output of DS/BPSK

receiver. The coherent receiver input is u(t) =s(t) + c(t)j(t) and using this u(t), output at

coherent receiver

2

=

Tb

u(t) cos 2 = +

0 

 

Where vs is despread component of BPSK and vcj of spread interference.

= 2

Tb

s(t) cos 2

02

=

Tb

c t j(t) cos 2

0

 

Consider despread BPSK signal s(t)

( ) = ±

2

cos 2 0 ≤? ≤

 

Where + sign is for symbol 1

- sign for symbol 0.

 

If carrier frequency is integer multiple of 1 / Tb , we have = ±

 

Consider spread interference component vcj, here c(t) is considered in sequence form

{ c0, c1, ……cN-1}

=

N

−1

C

=

N

−1

C

k

k=0

0

k

k=0

 

With Ck treated as independent identical random variables with both symbols having

equal probabilities

1

= 1 =

= −1 = 2

 

Expected value of Random variable vcj is zero, for fixed k we have

And Variance

| = = 1 − = −1 =

1

2

−

1

2 = 0

−

1

| =1

2 =

 

Spread factor N = Tb/Tc

 

Output signal to noise ratio is

2

=0

( ) = 2The average signal power at receiver input is Eb/Tb hence input SNR

(

) =

( )0 =

2

(

)

 

Expressing SNR in decibels

10 10 ( )0 = 10

10 (

) + 3 + 10 10 ,

Where = 

 

3db term on right side accounts for gain in SNR due to coherent detection. Last term

accounts for gain in SNR by use of spread spectrum. PG is called Processing Gain. 

 

 

 

1. Bit rate of binary data entering the transmitter input is = 1_

2. The bandwidth of PN sequence c(t) , of main lobe is Wc = 1_

                                                                                            =---

Probability of error

To calculate probability of error, we consider output component v of coherent

detector as sample value of random variable

= ±         + 

 

Eb is signal energy per bit and Vcj is noise component

 

Decision rule is, if detector output exceeds a threshold of zero volts; received bit is

symbol 1 else decision is favored for zero.

 

• Average probability of error Pe is nothing but conditional probability which

depends on random variable Vcj.

• As a result receiver makes decision in favor of symbol 1 when symbol 0

transmitted and vice versa

• Random variable Vcj is sum of N such random variables. Hence for

Large N it can assume Gaussian distribution.

•  As mean and variance has already been discussed , zero mean and variance

JTc/2

 

Probability of error can be calculated from simple formula for DS/BPSK system

            ___

 ≅    1/2  __

 

 

Antijam Characteristics

Consider error probability of BPSK

=

Comparing both probabilities;

Since bit energy Eb =PTb , P= average signal power.

 

We can express bit energy to noise density ratio as0 

 

Or

 

The ratio J/P is termed jamming margin. Jamming Margin is expressed in decibels as

=

 

Where    ~0 is minimum bit energy to noise ration needed to support a prescribed

average probability of error. 

 

 

Example1 

 

A pseudo random sequence is generated using a feed back shift register of

length m=4. The chip rate is 107 chips per second. Find the following

a) PN sequence length b) Chip duration of PN sequence c) PN sequence

period 

 

Solution

a) Length of PN sequence N = 2^m-1= 2^4-1 =15

b) Chip duration Tc = 1/chip rate =1/107 = 0.1µsec

c) PN sequence period T = NTc

= 15 x 0.1µ sec = 1.5µ sec 

 

 

Example2 

 

A direct sequence spread binary phase shift keying system uses a feedback

shift register of length 19 for the generation of PN sequence. Calculate the

processing gain of the system. 

 

Solution

Given length of shift register = m =19

Therefore length of PN sequence N = 2^m - 1

= 2^19 - 1

Processing gain PG = Tb/Tc =N in db =10log10N = 10 log10 (2 ^19) = 57db

 

 Example3 

 

A Spread spectrum communication system has the following parameters.

Information bit duration Tb = 1.024 msecs and PN chip duration of 1µsecs. The

average probability of error of system is not to exceed 10-5 . calculate a) Length of

shift register b) Processing gain c) jamming margin 

 

Solution

Processing gain PG =N= Tb/Tc =1024

corresponding length of shift register m = 10

In case of coherent BPSK For Probability of error 10-5. [Referring to error function table]

Eb/N0 =10.8

Therefore jamming margin

=

− 10 10

0

= 10 10 − 10

10

0

= 10 101024 − 10 1010.8

= 30.10 − 10.33 = 19.8 

 

 

 

Frequency – Hop Spread Spectrum: 

 

In a frequency – hop Spread – Spectrum technique, the spectrum of data

modulated carrier is widened by changing the carrier frequency in a pseudo – random

manner. The type of spread – spectrum in which the carrier hops randomly form one

frequency to another is called Frequency – Hop (FH) Spread Spectrum. 

 

Since frequency hopping does not covers the entire spread spectrum

instantaneously. We are led to consider the rate at which the hop occurs. Depending

upon this we have two types of frequency hop. 

 

1. Slow frequency hopping:- In which the symbol rate Rs of the MFSK signal is an

integer multiple of the hop rate Rh. That is several symbols are transmitted on

each frequency hop.

 

2. Fast – Frequency hopping:- In which the hop rate Rh is an integral multiple of the

MFSK symbol rate Rs. That is the carrier frequency will hoop several times

during the transmission of one symbol. A common modulation format for

frequency hopping system is that of M- ary frequency – shift – keying (MFSK).

 

 

Slow frequency hopping:-

Fig Shows the block diagram of an FH / MFSK transmitter, which involves

frequency modulation followed by mixing. 

 

The incoming binary data are applied to an M-ary FSK modulator. The resulting

modulated wave and the output from a digital frequency synthesizer are then applied to

a mixer that consists of a multiplier followed by a band – pass filter. The filter is

designed to select the sum frequency component resulting from the multiplication

process as the transmitted signal. An ‘k’ bit segments of a PN sequence drive

the frequency synthesizer, which enables the carrier frequency to hop over 2 ^n

distinct values. Since frequency synthesizers are unable to maintain phase

coherence over successive hops, most frequency hops spread spectrum

communication system use non coherent M-ary modulation system. 

 

 

Fig :- Frequency hop spread transmitter

 

 

 Fig :- Frequency hop spread receiver 

 

In the receiver the frequency hopping is first removed by mixing the received

signal with the output of a local frequency synthesizer that is synchronized with the

transmitter. The resulting output is then band pass filtered and subsequently processed

by a non coherent M-ary FSK demodulator. To implement this M-ary detector, a bank of

M non coherent matched filters, each of which is matched to one of the MFSK tones is

used. By selecting the largest filtered output, the original transmitted signal is estimated.

An individual FH / MFSK tone of shortest duration is referred as a chip. The chip

rate Rc for an FH / MFSK system is defined by 

 

Rc = Max(Rh,Rs)

Where Rh is the hop rate and Rs is Symbol Rate 

 

In a slow rate frequency hopping multiple symbols are transmitted per hop.

Hence each symbol of a slow FH / MFSK signal is a chip. The bit rate Rb of the

incoming binary data. The symbol rate Rs of the MFSK signal, the chip rate Rc and the

hop rate Rn are related by

Rc = Rs = Rb /k ≥ Rh

where k= log2M 

 

 

Fast frequency hopping:-

A fast FH / MFSK system differs from a slow FH / MFSK system in that

there are multiple hops per m-ary symbol. Hence in a fast FH / MFSK system each hop

is a chip. 

 

Fast Frequency Hopping                                         Slow Frequency Hopping

Several frequency hops Per modulation         Several modulation symbols per hop

 

Shortest uninterrupted waveform                    Shortest uninterrupted waveformin    

in the system is that of hop                                 the system is that of data symbol

Chip duration =hop duration                                 Chip duration=bit duration.

The following figure illustrates the variation of the frequency of a slow FH/MFSK

signal with time for one complete period of the PN sequence. The period of the PN

sequence is 24-1 = 15. 

 

The FH/MFSK signal has the following parameters:

Number of bits per MFSK symbol K = 2. Number of MFSK tones M = 2 ^K = 4

Length of PN segment per hop k = 3; Total number of frequency hops 2 ^k = 8 

 

 

Fig. Slow frequency hopping 

 

The following figure illustrates the variation of the transmitted frequency of a fast

FH/MFSK signal with time.

The signal has the following parameters:

Number of bits per MFSK symbol K = 2. Number of MFSK tones M = 2 K = 4

Length of PN segment per hop k = 3; Total number of frequency hops

2 k = 8 

 

 

Fig. Fast frequency hopping 

 

FHSS Performance Considerations:

• Typically large number of frequencies used

– Improved resistance to jamming 

 

Code Division Multiple Access (CDMA):

• Multiplexing Technique used with spread spectrum

• Start with data signal rate D

– Called bit data rate

• Break each bit into k chips according to fixed pattern specific to each user

– User’s code

• New channel has chip data rate kD chips per second

• E.g. k=6, three users (A,B,C) communicating with base receiver R

• Code for A = <1,-1,-1,1,-1,1>

• Code for B = <1,1,-1,-1,1,1>

• Code for C = <1,1,-1,1,1,-1> 

 

CDMA Example:

 

 

 

•  Consider A communicating with base

• Base knows A’s code

• Assume communication already synchronized

• A wants to send a 1

– Send chip pattern <1,-1,-1,1,-1,1>

• A’s code

• A wants to send 0

– Send chip[ pattern <-1,1,1,-1,1,-1>

• Complement of A’s code

• Decoder ignores other sources when using A’s code to decode

– Orthogonal codes

–

 

 

CDMA for DSSS:

• n users each using different orthogonal PN sequence

• Modulate each users data stream

– Using BPSK

•  Multiply by spreading code of user 

 

 

CDMA in a DSSS Environment: 

 

 

 

QUESTIONS FOR PRACTISE

Part A 

 

1. Define constraint length in convolutional codes?

2. What is pseudo noise sequence?

3. What is direct sequence spread spectrum modulation

4. What is frequency hap spread spectrum modulation?

5. What is processing gain?

6. What is jamming margin ?

7. When is the PN sequence called as maximal length sequence?

8. What is meant by processing gain of DS spread spectrum system?

9. What is the period of the maximal length sequence generated using 3 bit shift

register.

10. Define frequency hopping.

11. What are the Advantages of DS-SS system

12. What are the Disadvantages of DS-SS system.

13. What are the Advantages of FH-SS System

14. What are the Disadvantages of FH-SS System15. Define synchronization in Spread Spectrum Systems

16. Comparison between DS-SS and FH-SS

17. What are the Application of Direct Sequence Spread Spectrum

18. State the balance property of random binary sequence.

19. Mention about the run property.

20. What is called jamming effect.

21. What is Anti jamming ?

22. What is slow and fast frequency hopping.

23. What is called multipath Interference? 

 

 

PART B 

 

1. What is Spread Spectrum Techniques Explain in detail about Direct

Sequence Spread Spectrum Techniques with necessary diagrams?

i. Concept of Spread Spectrum Techniques

ii. Block Diagram Representation.

iii.Waveform at all stages of the system.

iv. Derivation of processing Gain. 

 

2. What is Frequency Hopping? Explain the different types of frequency hopping

with necessary diagrams.

i. Concept of frequency hopping.

ii. Explanation of slow frequency hopping

iii.Explanation of Fast frequency hopping

iv. Block Diagrams and waveform

 

 

 ******************************************************************************

 ******************************************************************************

 

 

Explain various Multiplexing Techniques.

 

 

Frequency Division Multiple Access (FDMA)

 It is one of the most common multiplexing procedures. FDMA is a channel access technique

found in multiple-access protocols as a channelization protocol.

 FDMA permits individual allocation of single or multiple frequency bands, or channels to the

users. 

 

 

 

Figure 12: Frequency Division Multiple Access

 

 FDMA permits multiple users to simultaneously access a transmission system.

 In FDMA, every user shares the frequency channel or satellite transpondersimultaneously;

however, every user transmits at single frequency 

 FDMA is compatible with both digital and analog signals.

 FDMA demands highly efficient filters in the radio hardware, contrary to CDMA and TDMA.

 FDMA is devoid of timing issues that exist in TDMA.

 As a result of the frequency filtering, FDMA is not prone to the near-far problem that exists in

CDMA.

 All users transmit and receive at different frequencies because every user receives an individual

frequency slot.

 One disadvantage of FDMA is crosstalk, which can cause interference between frequencies and

interrupt the transmission. 

 

Space Division Multiple Access (SDMA)

 

 SDMA utilizes the spatial separation of the users in order to optimize the use of the frequency

spectrum.

 A primitive form of SDMA is when the same frequency is reused in different cells in a cellular

wireless network.

 The radiated power of each user is controlled by Space division multiple access.

 SDMA serves different users by using spot beam antenna. These areas may be served by the

same frequency or different frequencies.

 However for limited co-channel interference it is required that the cells are sufficiently

separated. This limits the number of cells a region can be divided into and hence limits the

frequency re-use factor. A more advanced approach can further increase the capacity of the

network. This technique would enable frequency re-use within the cell. In a practical cellular

environment it is improbable to have just one transmitter fall within the receiver beam width.

 

Therefore it becomes imperative to use other multiple access techniques in conjunction with

SDMA.

 

 

Figure 1: Space Division Multiple Access

 

  When different areas are covered by the antenna beam, frequency can be re-used, in which

case TDMA or CDMA is employed, for different frequencies FDMA can be used.

 

 

Time Division Multiple Access (TDMA)

 It is a multiplexing technique where multiple channels are multiplexed over time.

 In TDMA, several users share the same frequency channel of higher bandwidth by dividing the

signal into different timeslots.

 Users transmit their data using their own respective time slots in rapid succession; to

synchronize, the transmitter and the receiver need to synchronize using a global clock.

 It is divided into two types:-

 

Fixed TDMA

 In this, connections between time slots in each frame and data streams assigned to a user

remain static and switched only when large variations in traffic are required.

 In this variant, the slot sizes are fixed atT/N (T is time in seconds and N is the number of users).

 

Dynamic TDMA

 

 

 

Figure 2: Time Division Multiple Access

 

 In this, a scheduling algorithm is used to dynamically reserve a variable number of time slots in

each frame to variable bit-rate data streams.

 This reservation algorithm is based on the traffic demand of each data stream.

 

Code Division Multiple Access (CDMA)

 Short for Code-Division Multiple Access, a digital cellular technology that uses spread-spectrum

techniques. It is a broadband system.

 CDMA uses spread spectrum technique where each subscriber uses the whole system

bandwidth.Figure 3: Code Division Multiple Access

 Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific

frequency to eachuser.

 Instead, every channel uses the full available spectrum. Individual conversations are encoded

with a pseudo-random digital sequence.

 CDMA consistently provides better capacity for voice and data communications than other

commercialmobiletechnologies,allowing moresubscriberstoconnectatanygiventime,andit

is the common platform on which 3G technologies are built.

 For example, CDMA is a military technology first used during World War II by English allies to

foil German attempts at jamming transmissions.

 Unlike the FDMA or TDMA where a frequency or time slot is assigned exclusively to a

subscriber,inCDMAall subscribers ina celluse thesamefrequencyband simultaneously.

 To separate the signals, each subscriber is assigned an orthogonal code called “chip”.

 

 

4. Define various mobile computing functions.

The mobile computing functions can be divided into the following major segments:-

 

User with Device

 This means that this could be a fixed device like a desktop computer in an office or a portable

device like mobile phone. Example: Laptop computers, desktop computers, fixed telephone,mobile phones, digital TV with set-top box, palmtop computers, pocket PCs, two-way pagers,

handheld terminals, etc.

 

Network

 

 

Figure 16: Mobile Computing Functions

 

 Whenever a user is using a mobile, he will use different networks at different locations at

different times. Example: GSM, CDMA, iMode, Ethernet,Wireless LAN,Bluetooth, etc.

 

Gateway

 This acts as an interface between different transports bearers. These gateways convert one

specific transport bearer toanother.

 Example, from a fixed phone we access a service by pressing different keys on the telephone.

These key generates DTMF (Dual Tone Multi Frequency).

 These analog signals are converted into digital data by the IVR (Interactive Voice Response)

gateway to interface with a computer application.

 

 

Middleware

 This is more of a function rather than a separate visible node. In the present context,

middleware handles the presentation and rendering of the content on a particular device.

 It may optionally also handle the security and personalization for different users.

 

Content

 This is the domain where the origin server and content is. This could be an application, system,

or even an aggregation of systems.

 The content can be mass market, personal or corporate content. The origin server will have

some means of accessing the database and storage devices.

 

 

Explain design consideration for mobile computing.

 Mobile computing is basically the use of portable devices that are capable of use wireless

network communication.

 It is divided into mobile devices and wireless communication.

 For designing mobile applications have altogether different challenges than designing desktop

application. It requires different mind-set. 

 On mobile platform everything is limited to make balance between design principles and

resources at hand such changes shall mean that content and behavior of applications should be

adapted to suit the current situation.

 Few of design consideration parameter for mobile computing: 

 

 

Native vs. Mobile Web

 If your application requires local processing, access to local resources and can work in

occasionally connected scenario or no connectivity consider designing a native application.

 A native application is hard to maintain, requires separate distribution and upgrade

infrastructure, are compatible only with target device/platform, requires more effort

(sometimes huge) to port on different devices.

 A mobile web application is compatible with all devices with internet connection and a browser. 

 

 

Target device

 Target device and platform (OS) play a key role throughout design decisionsmaking process.

 Design decisions are influenced by target device’s screen size, resolution, orientations, memory,

CPU performance characteristics, Operating systems capabilities, OEM (device vendor) specific

OS changes/limitations, device hardware, user input mechanism (touch/non-touch), sensors

(such as GPS or accelerometer) etc. 

 

User experience

 User experience, for mobile applications, needs utmost importance (may be more than

desktop).

 User interface should be rich, intuitive and responsive. While using mobile application user is

often distracted by external or internal (e.g. incoming call when user is in middle of a wizard)

events. 

 

 

Resource Constraint

 In design decision should take into account the limited CPU, memory and battery life.

 Reading and writing to memory, wireless connections, specialized hardware, and processor

speed all have an impact on the overall power usage.

 For example using notification or app directedSMSinstead of polling to monitor a value/flag on

server. 

 

Multiple Platform

  An application will target not only one platform or only one device.

 In near future, requirement like same code base should support iPhone and iPad or Android

Phone and Android tablet will arise.

 Design Architect should consider portability, technology agnostic with platform specific

implementation. To make design with reuse across the platforms.

 

 Security

 Devices are more vulnerable than desktop, primarily due to lack of awareness.

 It may device can be lost easily. It needs to secureddevice – server communication and server

accepts request only from authentic source (device).

 If you are storing any confidential application or configuration data locally, ensure that the data

is encrypted.

 

Network Communication

 Network communication on device is very significant parameter.

 To reduce network traffic by combining several commands in one request.

 For example, committing added, updated and deleted records in one request instead of firing

separate request on each add/update/delete.

 

Explain the differences between 1G, 2G, 2.5G and 3G mobile

communications.

1G

 It is the first generation cellular network that existed in 1990’s.

 It transfer data in analog wave, it has limitation because there are no encryption, the sound

quality is poor and the speed of transfer is only 9.6 kbps.

 

2G

 It is the second generation, improved by introducing the concept of digital modulation, which

means converting the voice into digital code and then into analog signals.

 Being over limitation 1G, such as it omits the radio power from handsets making life healthier,

and it has enhancedprivacy.

 

2.5G

 It is a transition of 2G and 3G.

 In 2.5G, the most popular services like SMS, GPRS, EDGE, high speed circuit switched data and

more had beenintroduced.

 

3G

 It is the current generation of mobile telecommunication standards.

 It allows use of speech and data services and offers data rates up to 2 mbps, which provide

services like video calls, mobile TV, mobile internet and downloading.

 There are bunch of technologies that fall 3G, likeWCDMA, EV-DO, and HSPA etc.

 

4G

 It is the fourth generation of cellular wireless standards. It is a successor to the 3G and 2G

families of standards.

 In 2008, the ITU-R organization specifies the IMT Advanced (International Mobile

Telecommunication Advanced) requirements for 4G standards, setting peak speed requirements for 4G service at 100 Mbit/s for high mobility communication and 1 Gbit/s for low mobility communication. 

 

4G system

 

 It is expected to provide a comprehensive and secure all-IP based mobile broadband solution to

laptop computer wireless modems, smart phones, and other mobile devices.

 

 

Gen.             Definition      Throughput/Speed      Technology             Features

1G                 Analog                14.4 Kbps\                AMPS,NMT,      During 1G Wireless phones are

(1970 to                                         (peak)                     TACS             used for voiceonly.

1980)

2G               Digital                   9.6/14.4             TDMA,CDMA    2G capabilities are achieved by

(1990 to     Narrow band           Kbps                                     allowing multiple users on a single

2000)         circuit data                                                                 channel via multiplexing

                                                                                           During 2G Cellular phones are used 

                                                                                        for data also along with voice.

.

2.5G         Packet Data        171.2                     GPRS       In 2.5G the internet becomes popular and

(2001 to                              Kbps(peak)                                data becomes more relevant.2.5G Multimedia

2004)                                  20-40 Kbps                   services and streaming starts to show growth

                                                                                 Phones start supporting web

                                                                                browsing through limited and very

                                                                               few phones have that.

.

3G                 Digital                 3.1 Mbps         CDMA 20     3G has Multimedia services support

(2004 to         Broadband         (peak)                     (1xRTT,    along with streaming are more

2005)             Packet Data        500-700 Kbps         EVDO)    popular.In 3G, Universal access and

                                                                                            UMTS, EDGE     portability across different device

                                                                                                                          types are made possible.

                                                                                                                          (Telephones, PDA’s, etc.)

3.5G             Packet Data         14.4 Mbps    HSPA                3.5G supports higher throughput

(2006 to                                   (peak)                              and speeds to support higher data

2010)                                       1-3 Mbps                          needs of the consumers

. 

 

4G                 Digital         100-300            WiMax    Speeds for 4G are further increased

(Now              Broadband   Mbps (peak)     LTE          to keep up with data access

(Read             Packet       3-5 Mbps         Wi-Fi         demand used by various services.

more on        All IP          100 Mbps                          High definition streaming is now

Transition        Very high     (Wi-Fi)                            supported in 4G. New phones with

ingto4G)         throughput                                         HD capabilities surface. It gets pretty cool.

                                                                                 In 4G, Portability is increased

                                                                                further. World-wide roaming is not

                                                                                 a distant dream. 

                                                                                  Facilities such as ultra-broadband Internet                                                                                 access, IP telephony, gaming services and

                                                                          streamed multimedia may be provided to users. 

 

 

PRE-4G 

 

 This technology such asmobileWiMax and Long term evolution (LTE) has been on

the market since 2006 and 2009 respectively, and are often branded as 4G.

 

 

 

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ****************************************************

. 

DECT DIGITAL ENHANCED CORDLESS TELECOMMUNICATION

BACKGROUND INFORMATION AND HISTORY

FUNCTIONALITY

USAGE IN SOCIETY

FUTURE OUTLOOK

*************************************************************************************

*************************************************************************************

 

 

 GSM ARCHITECTURE

CONTENT

 

WHAT IS GSM?

 

 

NETWORK STRUCTURE

 

 

MOBILE STATION

MOBILE EQUIPMENT

SUBSCRIBER IDENTITY MODULE

BASE STATION SUBSYSTEM 

BASE TRANSCEIVER STATION

BASE STATION CONTROLLER

NETWORK SWITCHING SUBSYSTEM

MOBILE SWITCHING CENTER

HOME LOCATION REGISTER

VISITORS LOCATION REGISTER

AUTHENTICATION CENTER

EQUIPMENT IDENTITY REGISTER

OPERATION AND MAINTENANCE CENTER 

************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************

                                GSM Architecture 

 

The GSM network comprises of many functional units, this function and interface are explained in this

part. The GSM network can be broadly divided into

1. Mobile Station (MS)

2. the Base Station Subsystem (BSS)

3. The network switching Subsystem(NSS)

4. Operation Support subsystem(OSS)

 

Mobile Station (MS)

the MS consist of Physical Equipment such as the Radio transiver, display, digital signal

processor and SIM Card . The MS Also provides access to the various data services avaliable in

GSM network. These data services include:

1. X.25 packet switching through a synchronous or Asynchronous dialog connection to the

PAD(Portable access device) at speed tipically at 9.6 kbps.

2. General Packet Radio Service(GPRS) using IP based data transfer method at the speed upto 115

kbps

3. High Speed Circuit Switch Data at speed upto 64kbps

 

Base Station SubSystem (BSS)

The BSS is composed of two parts

1. The base transiver station (BTS)

2. Base station Controller (BSC)

 

BTS

the BTS houses the radio transivers that defines a cell and handles the radio link protocol with the MS.

Each BTS serves as a single cell.

It also includes the following functinos

1. Encoding, Encrypting, Multiplexing, Modulating and Feeding the RF signal to the antina.

2. Transcoding and Data Adoptation

3. Time and Frequency Synchronizing4. Decoding, Decryption, Equilizing Recived Signal, Uplink Channel Messurement 

 

 

BSC

The BSC manages the radio resource for one or more BTS. It handle radio channel setup, Frquency

Hopping and handover. The BSC is the connection between the mobile and MSC. The Additional

Functions include:

1. Control of frequency Hopping

2. Performing traffic concentration to reduce the number of lines from the MSC

3. Providing an Interface to an operations and maintenance center for the BSS

4. Power Management

5. Time and Frequency Synchronization

*************************************************************************

**************************************************************************

  

                    CDMA TDMA FDMA

DIFFERENCE BETWEEN MULTIPLE

ACCESS AND MULTIPLEXING

 

 

FREQUENCY DIVISION MULTIPLE

ACCESS (FDMA)

 

HOW IT WORKS?

 

DIFFERENCE BETWEEN FDMA AND FDM

 

USES OF FDMA:

 

ADVANTAGES OF FDMA

 

DISADVANTAGES OF FDMA

 

EXAMPLE

 

TIME DIVISION MULTIPLE ACCESS

(TDMA)

 

HOW IT WORKS?

FDMA VS TDMA

 

CDMA VS TDMA

 

 

ADVANTAGES

 

 

DISADVANTAGES

 

CODE DIVISION MULTIPLE ACCESS

(CDMA)

ADVANTAGES

 

 

DISADVANTAGES

 

WORKING OF CDMA

 

APPLICATIONS OF CDMA TECHNOLOGY

 

 

****************************************

****************************************

 

                   Spread Spectrum 

 

Introduction to Spread Spectrum

 

 

Spread Spectrum Criteria

 

Spread Spectrum Background

 

Why Spread Spectrum?

 

 

How Spread Spectrum Works

 

Spreading Codes

 

 

PN Sequences

 

Spread Spectrum Classification

Direct Sequence Spread Spectrum

(DSSS)

 

 

Direct Sequence Spread Spectrum Example

Direct Sequence Spread Spectrum:

Transmission Technique

 

Direct Sequence Spread Spectrum

Transmitter

 

Direct Sequence Spread Spectrum

Receiver

 

 

Frequency Hopping Spread Spectrum

(FHSS)

 

 

 

******************************************************************************************

 

 

 

WIRELESS COMMUNICATION – SATELLITE 

 

 

A satellite is an object that revolves around another object. For example, earth is a

satellite of The Sun, and moon is a satellite of earth. 

 

A communication satellite is a microwave repeater station in a space that is used for

telecommunication, radio and television signals. A communication satellite processes the data

coming from one earth station and it converts the data into another form and send it to the

second earth station. 

 

Two stations on earth want to communicate through radio broadcast but are too far away

to use conventional means. The two stations can use a relay station for their communication. 

 

One earth station transmits the signal to the satellite. 

 

Uplink frequency is the frequency at which ground station is communicating with

satellite. The satellite transponder converts the signal and sends it down to the second earthstation, and this is called Downlink frequency. The second earth station also communicates with the

first one in the same way. 

 

 

 

ADVANTAGES OF SATELLITE

 

The advantages of Satellite Communications are as follows −

 The Coverage area is very high than that of terrestrial systems.

 The transmission cost is independent of the coverage area.

 Higher bandwidths are possible. 

 

 

 

DISADVANTAGES OF SATELLITE

 

The disadvantages of Satellite Communications are as follows −

 Launching satellites into orbits is a costly process.

 The bandwidths are gradually used up.

 High propagation delay for satellite systems than the conventional terrestrial systems. 

 

 

SATELLITE COMMUNICATION BASICS

 

The process of satellite communication begins at an earth station. Here an installation is

designed to transmit and receive signals from a satellite in orbit around the earth. Earth stations

send information to satellites in the form of high powered, high frequency (GHz range) signals.

 

The satellites receive and retransmit the signals back to earth where they are received

by other earth stations in the coverage area of the satellite. Satellite's footprint is the area

which receives a signal of useful strength from the satellite.

 

The transmission system from the earth station to the satellite through a channel is called

the uplink. The system from the satellite to the earth station through the channel is called

the downlink.

 

 

SATELLITE FREQUENCY BANDS

 

The satellite frequency bands which are commonly used for communication are

the Cband, Ku-band, and Ka-band. C-band and Ku-band are the commonly used frequency

spectrums by today's satellites.

 

It is important to note that there is an inverse relationship between frequency and

wavelength i.e. when frequency increases, wavelength decreases this helps to understand therelationship between antenna diameter and transmission frequency. Larger antennas

(satellite dishes) are necessary to gather the signal with increasing wavelength.

 

EARTH ORBITS

 

A satellite when launched into space, needs to be placed in certain orbit to provide a

particular way for its revolution, so as to maintain accessibility and serve its purpose whether

scientific, military or commercial. Such orbits which are assigned to satellites, with respect to

earth are called as Earth Orbits. The satellites in these orbits are Earth Orbit Satellites.

 

The important kinds of Earth Orbits are −

 Geo-synchronous Earth Orbit

 Geo-stationary Earth Orbit

 Medium Earth Orbit

 Low Earth Orbit 

 

 

Geo-synchronous Earth Orbit (GEO) Satellites

A Geo-synchronous Earth orbit Satellite is one which is placed at an altitude of 22,300

miles above the Earth. This orbit is synchronized with a side real day (i.e., 23hours 56minutes).

 

This orbit can have inclination and eccentricity. It may not be circular. This orbit can be tilted

at the poles of the earth. But it appears stationary when observed from the Earth.

 

The same geo-synchronous orbit, if it is circular and in the plane of equator, it is called

as geo-stationary orbit. These Satellites are placed at 35,900kms (same as geosynchronous)

above the Earth’s Equator and they keep on rotating with respect to earth’s direction (west to

east). These satellites are considered stationary with respect to earth and hence the name

implies.

 

Geo-Stationary Earth Orbit Satellites are used for weather forecasting, satellite TV,

satellite radio and other types of global communications.FIGURE -GEO-SYNCHRONOUS

 

The above figure shows the difference between Geo-synchronous and Geo- Stationary

orbits. The Axis of rotation indicates the movement of Earth.

 

The main point to note here is that every Geo-Stationary orbit is a Geo-Synchronous

orbit. But every Geo-Synchronous orbit is NOT a Geo-stationary orbit.

 

Medium Earth Orbit (MEO) Satellites 

 

Medium earth orbit (MEO) satellite networks will orbit at distances of about 8000 miles

from earth's surface. Signals transmitted from a MEO satellite travel a shorter distance. This

translates to improved signal strength at the receiving end. This shows that smaller, more

lightweight receiving terminals can be used at the receiving end.

 

Since the signal is travelling a shorter distance to and from the satellite, there is less

transmission delay. Transmission delay can be defined as the time it takes for a signal to travel

up to a satellite and back down to a receiving station.

 

For real-time communications, the shorter the transmission delay, the better will be the

communication system. As an example, if a GEO satellite requires 0.25 seconds for a round

trip, then MEO satellite requires less than 0.1 seconds to complete the same trip. MEOs

operates in the frequency range of 2 GHz and above.Low Earth Orbit (LEO) Satellites

 

The LEO satellites are mainly classified into three categories namely, little LEOs, big

LEOs, and Mega-LEOs. LEOs will orbit at a distance of 500 to 1000 miles above the earth's

surface.

 

This relatively short distance reduces transmission delay to only 0.05 seconds. This

further reduces the need for sensitive and bulky receiving equipment. Little LEOs will operate

in the 800 MHz (0.8 GHz) range. Big LEOs will operate in the 2 GHz or above range, and

Mega-LEOs operates in the 20-30 GHz range.

 

The higher frequencies associated with Mega-LEOs translates into more information

carrying capacity and yields to the capability of real-time, low delay video transmission scheme.

 

High Altitude Long Endurance (HALE) Platforms

Experimental HALE platforms are basically highly efficient and lightweight airplanes

carrying communications equipment. This will act as very low earth orbit geosynchronous

satellites.

 

These crafts will be powered by a combination of battery and solar power or high

efficiency turbine engines. HALE platforms will offer transmission delays of less than 0.001

seconds at an altitude of only 70,000 feet, and even better signal strength for very lightweight

hand-held receiving devices.

 

Orbital Slots

Here there may arise a question that with more than 200 satellites up there in

geosynchronous orbit, how do we keep them from running into each other or from attempting to

use the same location in space? To answer this problem, international regulatory bodies like the

International Telecommunications Union (ITU) and national government organizations like the

Federal Communications Commission (FCC) designate the locations on the geosynchronous

orbit where the communications satellites can be located.

 

These locations are specified in degrees of longitude and are called as orbital slots. The

FCC and ITU have progressively reduced the required spacing down to only 2 degrees for C

band and Ku-band satellites due to the huge demand for orbital slots.

 

 ************************************************************************************************************************************

 ************************************************************************************************************************************

                                 WIRELESS NETWORKS

 

Presentation Outline

• Wireless Technology overview

• The IEEE 802.11 WLAN Standards

• Secure Wireless LANs

• Migrating to Wireless LANs (Cutting the cord)

 

 Wireless?

• A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier.

• The last link with the users is wireless, to give a network connection to all users in a building or campus.

• The backbone network usually uses cables

 

 Common Topologies

The wireless LAN connects to a wired LAN 

 

• There is a need of an access point that bridges wireless LAN traffic into the

wired LAN.

• The access point (AP) can also act as a repeater for wireless nodes,

effectively doubling the maximum possible distance between nodes.

 

 

 

 Common Topologies 

 

Complete Wireless Networks

 

• The physical size of the network is determined by the maximum reliable propagation range of the radio signals.

• Referred to as ad hoc networks

• Are self-organizing networks without any centralized control

• Suited for temporary situations such as meetings and conferences. 

 

How do wireless LANs work?

Wireless LANs operate in almost the same way as wired LANs, using the same networking protocols and supporting the most of the same applications.  

 

How are WLANs Different?

• They use specialized physical and data link protocols

• They integrate into existing networks through access points which provide a bridging function

• They let you stay connected as you roam from one coverage area to another

• They have unique security considerations

• They have specific interoperability requirements

• They require different hardware

• They offer performance that differs from wired LANs. 

 

Physical and Data Link Layers

 

Physical Layer:

• The wireless NIC takes frames of data from the link layer, scrambles the data in a

predetermined way, then uses the modified data stream to modulate a radio carrier

signal. 

 

Data Link Layer:

• Uses Carriers-Sense-Multiple-Access with

Collision Avoidance (CSMA/CA).

 

 Integration With Existing Networks 

•  Wireless Access Points (APs) - a small device that bridges wireless traffic to your network.

• Most access points bridge wireless LANs into Ethernet networks, but Token-Ring options are available as well.

 

 

Roaming

• Users maintain a continuous connection as they roam from one physical area to another

• Mobile nodes automatically register with the new access point.

•Methods: DHCP, Mobile IP 

• IEEE 802.11 standard does not address roaming, you may need to purchase equipment from one vendor if your users need to roam from one access point to another. 

 

Security

• In theory, spread spectrum radio signals are inherently difficult to decipher without knowing the exact hopping sequences or direct sequence codes used

• The IEEE 802.11 standard specifies optional security called "Wired Equivalent Privacy"

whose goal is that a wireless LAN offer privacy equivalent to that offered by a wired LAN. The standard also specifies optional authentication measures. 

 

Interoperability

• Before the IEEE 802.11 interoperability was based on cooperation between vendors.

• IEEE 802.11 only standardizes the physical and medium access control layers.

• Vendors must still work with each other to ensure their IEEE 802.11 implementations interoperate

• Wireless Ethernet Compatibility Alliance (WECA) introduces the Wi-Fi Certification to ensure cross vendor interoperability of 802.11b solutions 

 

Hardware

• PC Card, either with integral antenna or with external antenna/RF module.

• ISA Card with external antenna connected by cable.

• Handheld terminals

• Access points

 

 Hardware 

 

 

 

Performance

• 802.11a offers speeds with a theoretically maximum rate of 54Mbps in the 5 GHz band

•802.11b offers speeds with a theoretically maximum rate of 11Mbps at in the 2.4 GHz

spectrum band

• 802.11g is a new standard for data rates of up to a theoretical maximum of 54 Mbps at 2.4 GHz.

 

 What is 802.11?

• A family of wireless LAN (WLAN) specifications developed by a working group at the Institute of Electrical and Electronic Engineers (IEEE)

• Defines standard for WLANs using the following four technologies

• Frequency Hopping Spread Spectrum (FHSS)

• Direct Sequence Spread Spectrum (DSSS)

• Infrared (IR)

• Orthogonal Frequency Division Multiplexing (OFDM)

• Versions: 802.11a, 802.11b, 802.11g, 802.11e, 802.11f, 802.11i

 

 802.11 - Transmission

• Most wireless LAN products operate in unlicensed radio bands

• 2.4 GHz is most popular

• Available in most parts of the world

• No need for user licensing

• Most wireless LANs use spread-spectrum radio

• Resistant to interference, secure

• Two popular methods

• Frequency Hopping (FH)

• Direct Sequence (DS)

 

 

 Frequency Hopping Vs. Direct Sequence

• FH systems use a radio carrier that “hops” from frequency to frequency in a pattern known to both transmitter and receiver 

• Easy to implement

• Resistance to noise

• Limited throughput (2-3 Mbps @ 2.4 GHz)

• DS systems use a carrier that remains fixed to a specific frequency band. The data signal is spread onto a much larger range of frequencies (at a much lower power level) using a

specific encoding scheme.

• Much higher throughput than FH (11 Mbps)

• Better range

• Less resistant to noise (made up for by redundancy – it transmits at

least 10 fully redundant copies of the original signal at the same time)

 

 

 802.11a

• Employs Orthogonal Frequency Division Multiplexing (OFDM)

• Offers higher bandwidth than that of 802.11b, DSSS (Direct Sequence Spread Spectrum)

• 802.11a MAC (Media Access Control) is same as802.11b

• Operates in the 5 GHz range

 

 

 802.11a Advantages

• Ultra-high spectrum efficiency

• 5 GHz band is 300 MHz (vs. 83.5 MHz @ 2.4 GHz)

• More data can travel over a smaller amount of bandwidth

• High speed

• Up to 54 Mbps

•  Less interference

• Fewer products using the frequency

• 2.4 GHz band shared by cordless phones, microwave ovens, Bluetooth, and WLANs 

 

802.11a Disadvantages

•Standards and Interoperability

• Standard not accepted worldwide

• No interoperability certification available for 802.11a products

• Not compatible or interoperable with 802.11b

• Legal issues

• License-free spectrum in 5 GHz band not available worldwide

•  Market

• Beyond LAN-LAN bridging, there is limited interest for5 GHz adoption 

 

802.11a Disadvantages

• Cost

• 2.4 GHz will still has >40% cost advantage

• Range

• At equivalent power, 5 GHz range will be ~50% of 2.4 GHz

• Power consumption

• Higher data rates and increased signal require more power

• OFDM is less power-efficient then DSSS 

 

802.11a Applications

• Building-to-building connections

• Video, audio conferencing/streaming video,and audio

• Large file transfers, such as engineeringCAD drawings 

• Faster Web acess and browsing

• High worker density or high throughput scenarios

• Numerous PCs running graphics-intensive applications 

 

802.11a Vs. 802.11b 

 

802.11a vs.                   802.11a                      802.11b

802.11b

Raw data rates         Up to 54 Mbps                      Up to 11 Mbps

                               (54, 48, 36, 24,18, 12            (11, 5.5, 2, and       

                               and 6 Mbps)                        1 Mbps)

Range                    50 Meters                            100 Meters

 

Bandwidth                 UNII and ISM                    ISM (2.4000—

                                (5 GHz range)                2.4835 GHz range)

 

Modulation               OFDM technology              DSSS technology                                

802.11g

• 802.11g is a high-speed extension to 802.11b

• Compatible with 802.11b

• High speed up to 54 Mbps

• 2.4 GHz (vs. 802.11a, 5 GHz)

• Using ODFM for backward compatibility

• Adaptive Rate Shifting

 

 

 802.11g Advantages

• Provides higher speeds and higher capacity requirements for applications

• Wireless Public Access

• Compatible with existing 802.11b standard

• Leverages Worldwide spectrum availability in 2.4 GHz

• Likely to be less costly than 5 GHz alternatives

• Provides easy migration for current users of 802.11b WLANs 

• Delivers backward support for existing 802.11b products

• Provides path to even higher speeds in the future 

 

802.11e Introduces Quality of Service

• Also know as P802.11 TGe

• Purpose:

    • To enhance the 802.11 Medium Access Control (MAC) to improve and manage Quality of Service (QoS)

• Cannot be supported in current chip design

• Requires new radio chips

• Can do basic QoS in MAC layer 

 

802.11f – Inter Access Point Protocol

• Also know as P802.11 TGf

• Purpose:

• To develop a set of requirements for Inter-Access Point Protocol (IAPP), including operational and management aspects 

 

802.11b Security Features

• Wired Equivalent Privacy (WEP) – A protocol to protect link-level data during wireless transmission between clients and access points.

• Services:

• Authentication: provides access control to the network bydenying access to client stations that fail to authenticate properly.

• Confidentiality: intends to prevent information compromise from casual eavesdropping

• Integrity: prevents messages from being modified while in

transit between the wireless client and the access point. 

 

Authentication

Means:

• Based on cryptography

• Non-cryptographic

• Both are identity-based verification mechanisms (devices request access based on the SSID –Service Set Identifier of the wireless network). 

 

Authentication

• Authentication techniques 

 

Privacy

• Cryptographic techniques

• WEP Uses RC4 symmetric key, stream cipher  algorithm to generate a pseudo random data sequence. The stream is XORed with the data to be transmitted

• Key sizes: 40bits to 128bits

• Unfortunately, recent attacks have shown that the WEP approach for privacy is vulnerable to certain attack regardless of key size 

 

Data Integrity

• Data integrity is ensured by a simple encrypted version of CRC (Cyclic Redundant Check)

• Also vulnerable to some attacks 

 

WLAN Migration – Cutting The Cord

• Essential Questions

• Choosing the Right Technology

• Data Rates

• Access Point Placement and Power

• Antenna Selection and Placement

• COnnecting to the Wired LAN

• The Site Survey 

 

Essential Questions

• Why is the organization considering wireless?

Allows to clearly define requirements of the WLAN -> development plan

• How many users require mobility?

• What are the applications that will run over the WLAN? Helps to determine bandwidth

requirements, a criteria to choose between available technologies. Wireless is a shared

medium, not switched!!! 

 

Choose the right technology

• Usually IEEE 802.11b or 802.11a

• 802.11b offers interoperability (WECA Wi-Fi Certification Program)

• 802.11a offers higher data rates (up to 54 mbps) -> higher throughput per user. Limite 

interoperability. 

 

Data rates 

• Data rates affect range

• 802.11b 1 to 11 Mbps in 4 increments

• 802.11a 6 to 54 Mbps in 7 increments

• The minimum data rate must be determined at design time

• Selecting only the highest data rate will require a greater number of APs to cover a specific area

• Compromise between data rates and overall system cost 

 

Access Point Placement and Power

• Typically – mounted at ceiling height.

• Between 15 and 25 feet (4.5m to 8m)

• The greater the height, the greater the difficulty to get power to the unit. Solution:

consider devices that can be powered using CAT5 Ethernet cable (CISCO Aironet 1200

Series).

• Access points have internal or external antennas 

 

Antenna Selection and Placement

• Permanently attached.

• Remote antennas connected using an antenna cable.

•Coax cable used for RF has a high signal loss, should  not be mounted more than a 1 or 2 meters away from the device.

• Placement: consider building construction, ceiling height, obstacles, and aesthetics. Different materials (cement, steel) have different radio propagation characteristics.

 

Connecting to the Wired LAN

• Consider user mobility

• If users move between subnets, there are challenges to consider.

• OSes like Windows XP and 2000, Linux support DHCP to obtain the new IP address for the subnet. Certain applications such as VPN will fail.

 • Solution: access points in a roaming area areon the same segment. 

 

 

 

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