dmc 1630 - mobile computing

7/28/2019 Dmc 1630 - Mobile Computing

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MCA(DISTANCE MODE)

DMC 1630

MOBILE COMPUTING

COURSE MATERIAL

Centre for Distance EducationAnna University Chennai

Chennai 600 025


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Author

DrDrDrDrDr. P. P. P. P. P. Nar. Nar. Nar. Nar. NaraaaaayyyyyanasamyanasamyanasamyanasamyanasamyProfessor and Head,

Department of Computer Science & EngineeringAnna University

Chennai 600025

Reviewer

GGGGG. K. K. K. K. KousalyousalyousalyousalyousalyaaaaaProfessor

Department of Computer Science & EngineeringSri Krishna College of Engineering and Technology

Coimbatore - 641 008A

DrDrDrDrDr.T.T.T.T.T.V.V.V.V.V.Geetha.Geetha.Geetha.Geetha.GeethaProfessor

Department of Computer Science and EngineeringAnna University Chennai

Chennai - 600 025

DrDrDrDrDr.H.P.H.P.H.P.H.P.H.Pee reeree reereeru Mohamedu Mohamedu Mohamedu Mohamedu MohamedProfessor

Department of Management StudiesAnna University Chennai

Chennai - 600 025

DrDrDrDrDr.C.C.C.C.C. Chella. Chella. Chella. Chella. ChellappanppanppanppanppanProfessor


Chennai - 600 025

DrDrDrDrDr.A.K.A.K.A.K.A.K.A.KannanannanannanannanannanProfessor


Chennai - 600 025

Copyrights Reserved(For Private Circulation only)

Editorial Board


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ACKNOWLEDGEMENT

Next generation communication networks have been focusing on ubiquitous multimedia computing. It

aims towards seamless integration of different devices and network technologies together. Mobility, now-a-days,

becomes an integral part of any networking system. This study material is intended to prepare the students towards

such a design and practice of future communication networks. Students are required to have the basic knowledge

of communication networks and also the application domains of Internet.

This study material introduces the field of mobile computing and focuses on digital data transfer. It starts

with an overview of wireless technologies which cover signal processing, multiplexing and modulation. Media access

schemes that are adaptable for wireless communication are discussed in detail.

Mobile technologies, such as GSM, UMTS, GPRS, etc., are covered in depth. Wireless LANs such as

IEEE 802.11, HIPERLAN and Bluetooth are introduced. Several approaches for extending IP and TCP to adapt

mobile communication are also discussed. Finally, WAP which enables wireless and mobile devices to use World

Wide Web services are also covered.

As an author, I am thankful to Dr.B.N.Sankar, Director, CDE and Dr.T.V.Geetha, Deputy Director, CDE

who gave me an opportunity to design this study material on MOBILE COMPUTING for the students of

M.C.A. Also, I express my sincere thanks to my Research Scholars Mr.V.Mariappan and Ms.G.Kousalya who

supported me throughout in preparing this study material.

Dr. P .Narayanasamy

Author


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DMC 1630 MOBILE COMPUTING

1. INTRODUCTION

Medium Access Control : Motivation for Specialized MAC SDMA-FDMA- TDMA CDMA Comparison

of Access mechanisms Tele Communications :GSM DECT TETRA UMTS IMT 200 Satellite

Systems: Basics Routing Localization Handover Broadcast Systems: Overview Cyclic Repetition of

Data Digital Audio Broadcasting Digital Video Broadcasting

2. WIRELESS NETWORKS

Wireless LAN: Infrared - Radio Transmission Infrastructure Networks Ad hoc Networks IEEE

802.11 HIPERLAN Bluetooth Wireless ATM: Working Group Services Reference Model Functions

Radio Access Layer Handover Location Management Addressing Mobile Quality of Service Access

Point Control Protocol.

3. MOBILE NETWORK LAYER

Mobile IP: Goals Assumptions and Requirement Entities IP packet Delivery Agent Advertisement

and Discovery Registration Tunneling and Encapsulation Optimization Reverse Tunneling Ipv6

DHCP Ad hoc Networks.

4. MOBILE TRANSPORT LAYER

Traditional TCP Indirect TCP Snooping TCP Mobile TCP Fast retransmit/Fast Recovery

Transmission/Timeout Freezing Selective Retransmission Transaction Oriented TCP.

5. WAP

Architecture Datagram Protocol Transport Layer Security Transaction Protocol Session Protocol

Application Environment Wireless Telephony Application.


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CONTENTS

UNIT I

INTRODUCTION

CHAPTER 1

MEDIA ACCESS CONTROL

1.1 INTRODUCTION 1

1.2 ACCESS MECHANISMS 1

1.2.1 SDMA 1

1.2.2 FDMA 21.2.3 TDMA 3

1.2.4 CDMA 4

1.2.5 Comparison of Access Mechanism 5

1.3 QUESTIONS 5

CHAPTER 2

CELLULAR NETWORKS

2.1 INTRODUCTION 6

2.2 HISTORY OF CELLULAR SERVICE 6

2.3 THE EVOLUTION OF CELLULAR SERVICE 6

2.3.1 Digital Cellular Service 7

2.3.2 Personal Communications Services 7

2.4 BASIC NETWORK OPERATIONS 8

2.4.1 Terminal 8

2.4.2 Base Station 8

2.4.3 Cell Organization 9

2.5 CHALLENGES OF CELLULAR COMMUNICATION 10

2.5.1 Network Routing 11

2.5.2 Signal Corruption 12

2.6 HANDOFF STRATEGIES 14

2.7 QUESTIONS 17

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CHAPTER 3

TELECOMMUNICATIONS SYSTEMS3.1 INTRODUCTION 38

3.2 CELLULAR COMMUNICATION STANDARDS 19

3.2.1 AMPS 19

3.2.2 IS-54/136 20

3.2.3 GSM 21

3.2.4 IS-95 22

3.3 GSM 22

3.3.1 GSM Specifications 23

3.3.2 GSM Services 24

3.3.3 The GSM Network 26

3.3.4 GSM Network Areas 28

3.4 QUESTIONS 29

CHAPTER 4

SATELLITE SYSTEMS

4.1 INTRODUCTION 30

4.1.1 Purpose 30

4.1.2 Communications satellites 30

4.1.3 Other applications 30

4.2 GLOBAL SATELLITE SYSTEMS 314.3 TYPES OF SATELLITE SYSTEMS 32

4.3.1 GEO 32

4.3.2 LEO 33

4.3.3 MEO 35

4.3.4 HEO 36

4.4 QUESTIONS 36

CHAPTER 5

BROADCAST SYSTEMS

5.1 INTRODUCTION 37

5.2 BROADCAST DISK 38

5.3 DAB 38

5.3.1 DAB Systems 38

5.3.2 Benefits of DAB 40

5.4 DMB 41

5.4.1 DMB System 41

5.4.2 Benefits of DMB 42

5.5 QUESTIONS 42

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UNIT II

WIRELESS NETWORKS

CHAPTER 6

WIRELESS LAN

6.1 INTRODUCTION 43

6.2 IEEE 802 11 ARCHITECTURE 43

6.2.1 Architecture Components 43

6.2.2 IEEE 802.11 Layers Description 44

6.2.3 The MAC Layer 44

6.3 HOW DOES A STATION JOIN AN EXISTING CELL (BSS) 49

6.4 THE AUTHENTICATION PROCESS 49

6.5 THE ASSOCIATION PROCESS 496.6 ROAMING 49

6.6.1 Synchronization 50

6.7 SECURITY 50

6.7.1 Preventing Access to Network Resources 50

6.8 POWER SAVING 51

6.9 QUESTIONS 51

CHAPTER 7

BLUETOOTH

7.1 INTRODUCTION 52

7.2 BLUETOOTH CONCEPTS 52

7.2.1 Bluetooth Connections 53

7.2.2 Reliable and Secure Transmission 53

7.2.3 Low Power Architecture 54

7.2.4 Interoperability, standards, and specifications 54

7.3 BLUETOOTH TERMINOLOGY 55

7.3.1 Bluetooth Stack 55

7.3.2 Links and Channels 56

7.3.3 Protocols 57

7.3.4 Bluetooth Networking 58

7.4 QUESTIONS 59

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CHAPTER 8

WIRELESS ATM

8.1 INTRODUCTION 60

8.2 WIRELESS ATM REFERENCE MODELS 60

8.2.1 Fixed Wireless Components 60

8.2.2 Mobile End Users 61

8.2.3 Mobile Switches with Fixed End Users 61

8.2.4 Mobile Switches with Mobile End Users 61

8.2.5 Inter working with PCS 61

8.2.6 Wireless Ad Hoc Networks 61

8.3 WATM DESIGN ISSUES 62

8.3.1 Wireless ATM Protocol Architecture 62

8.3.2 Radio Access Layer 62

8.3.3 Mobile ATM 64

8.4 SUMMARY 65

8.5 QUESTIONS 65

UNIT III

MOBILE NETWORK LAYERCHAPTER 9

MOBILE NETWORK LAYER

9.1 INTRODUCTION 67

9.2 COMPONENTS OF MOBILE IP 67

9.3 HOW MOBILE IP WORKS 68

9.3.1 Agent Discovery 68

9.3.2 Registration 69

9.3.3 Tunneling 70

9.4 SECURITY 72

9.5 SOLUTION TO NETWORK MOBILITY 72

9.6 OVERVIEW OF AD-HOC NETWORKING 73

9.6.1 Routing in Ad Hoc Networks 73

9.7 QUESTIONS 74

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UNIT IV

MOBILE TRANSPORT LAYER

CHAPTER 10


10.1 OVERVIEW 75

10.1.1 Slow Start and Congestion Avoidance 75

10.1.2 Fast Retransmit and Fast Recovery 77

10.1.3 TCP Options 77

10.1.4 Other Mechanisms 78

10.2 PROBLEMS WITH TCP IN WIRELESS NETWORKS 79

10.3 OPTIMIZATIONS 79

10.3.1 Link Layer 80

10.3.2 Snoop 80

10.3.3 Split Connection 81

10.4 COMPARISON OF DIFFERENT APPROACHES 82

10.5 QUESTIONS 82

UNIT V

WIRELESS APPLICATION PROTOCOL

CHAPTER 11

WIRELESS APPLICATION PROTOCOL

11.1 INTRODUCTION 83

11.1.1 History 84

11.1.2 Benefits 85

11.2 ARCHITECTURE OVERVIEW 86

11.2.1 WWW Model 86

11.2.2 WAP Model 87

11.2.3 Example WAP network 88

11.2.4 WAP Network Elements 89

11.2.5 Device Architecture 90

11.2.6 Security Model 90

11.3 COMPONENTS OF THE WAP ARCHITECTURE 91

11.3.1 Bearer Layer 91

11.3.2 Transport Layer Wireless Datagram Protocol (WDP) 92

11.3.3 Transfer Services 92

11.3.4 Security Layer Wireless Transport Layer Security (WTLS) 92

11.3.5 Transaction Layer Wireless Transaction Protocol (WTP) 93v


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11.3.6 Session Layer Wireless Session Protocol (WSP) 93

11.3.7 Application Layer Wireless Application Environment (WAE) 94

11.3.8 Security Services 94

11.3.9 Service Discovery 95

11.3.10 Other Service and Applications 95

11.4 SUMMARY 96

11.5 QUESTIONS 96

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NOTES

1 ANNA UNIVERSITY CHENNAI

UNIT I

INTRODUCTION

CHAPTER I

MEDIA ACCESS CONTROL

1.1 INTRODUCTION

The Media Access Control (MAC) data communication protocol sub-layer, also

known as the Medium Access Control, is a sub layer of the Data Link Layer specified in

the seven-layer OSI model (layer 2). It provides addressing and channel access control

mechanisms that make it possible for several terminals or network nodes to communicate

within a multipoint network, typically a local area network (LAN) or metropolitan area

network (MAN).

The MAC sub-layer acts as an interface between the Logical Link Control (LLC)

sub layer and the networks physical layer. The MAC layer emulates a full-duplex logicalcommunication channel in a multipoint network. This channel may provide unicast, multicast

or broadcast communication service.

1.2 ACCESS MECHANISMS

A limited amount of bandwidth is allocated for wireless services. A wireless system is

required to accommodate as many users as possible by effectively sharing the limited

bandwidth. Therefore, in the field of communications, the term multiple access could be

defined as a means of allowing multiple users to simultaneously share the finite bandwidth

with least possible degradation in the performance of the system. There are several techniques

how multiple accessing can be achieved. There are four basic schemes. Space Division Multiple Access (SDMA)

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

1.2.1 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 re-used in

different cells in a cellular wireless network. However for limited co-channel interference it


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that separate neighboring channels. This leads to a waste of bandwidth. When continuous

transmission is not required, bandwidth goes wasted since it is not being utilized for a

portion of the time. In wireless communications, FDMA achieves simultaneous transmission

and reception by using Frequency Division Duplexing (FDD). In order for both thetransmitter and the receiver to operate at the same time, FDD requires duplexers. The

requirement of duplexers in the FDMA system makes it expensive.

Figure1.2 Channel Usage by FDMA

1.2.3 TDMA

In digital systems, continuous transmission is not required because users do not use

the allotted bandwidth all the time. In such systems, TDMA is a complimentary accesstechnique to FDMA. Global Systems for Mobile communications (GSM) uses the TDMA

technique. In TDMA, the entire bandwidth is available to the user but only for a finite

period of time. In most cases the available bandwidth is divided into fewer channels compared

to FDMA and the users are allotted time slots during which they have the entire channel

bandwidth at their disposal. This is illustrated in Figure 1.3. TDMA requires careful time

synchronization since users share the bandwidth in the frequency domain. Since the number

of channels are less, inter channel interference is almost negligible, hence the guard time

between the channels is considerably smaller. Guard time is spacing in time between the

TDMA bursts. In cellular communications, when a user moves from one cell to anotherthere is a chance that user could experience a call loss if there are no free time slots

available. TDMA uses different time slots for transmission and reception. This type of

duplexing is referred to as Time Division Duplexing (TDD). TDD does not require duplexers.


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Figure 1.5 Walsh Code

1.2.5 Comparison of Access Mechanism

1.3 QUESTIONS

1. Define TDD & FDD

2. Explain & compare various access technologies.


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CHAPTER - 2

CELLULAR NETWORKS

2.1 INTRODUCTION

Cellular networks got their name because of the way they divide service areas into

cells. A cell is a relatively small area that is serviced by a single transmitter/receiver unit (cell

site). Mobile phones operating within this area use that cell site to communicate with the

rest of the cellular network and with the public phone network.

The basic premise of a cellular network is that user can have a communication device,

starting with car phones, which have evolved into hand-held phones. Over time, hand-heldphones have been getting smaller, gaining longer battery life, and getting new features like

paging. Network coverage and capacity have also increased to the point where cellular

service is available almost anywhere and at any time to those that want to use it. Cellular

service has seen tremendous acceptance, especially in the last few years, with millions of

new subscribers each year and the new subscriber rate growing.

2.2. HISTORY OF CELLULAR SERVICE

Cellular service was invented by Bell Laboratories and introduced around 1980,

based on radio-telephone systems that dated back to 1940s. The Bell Labs offeringbecame the basis for the Advanced Mobile Phone System (AMPS), which is the current

standard for U.S. cellular service. It is the least common denominator of U.S. cellular

service, and the FCC has mandated that all U.S. cellular phones must fall back to AMPS

service at least until the year 2002.

Many nations adopted variants of AMPS service, such as Nordic Mobile Telephone

(NMT), first introduced in Scandinavia, Total Access Communication System (TACS),

first introduced in Britain, and other systems introduced in France, Italy, and Germany.

The protocols and communications standards used by each of these varied slightly, so that

the various European analog systems were not compatible with each other.

2.3 THE EVOLUTION OF CELLULAR SERVICE

Most U.S. cellular service still uses the same AMPS analog technology that was used

in the earliest mobile telephones, although digital cellular service is rapidly gaining popularity.

The key motivator for this is that digital cellular networks can offer more subscriber channels

over the same radio bandwidth, although digital networks can offer additional services as

well.


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Unlike ISDN, Asynchronous Transfer Mode (ATM) and other innovations in regular

telephony, cellular systems have fewer infrastructures to be replaced when new ideas are

developed. Coupled with the rapid growth of cellular service, it is possible to make sweeping

changes in the basic nature of cellular service. In fact, it is perhaps too easy to change,

causing much debate over what the cellular network of the future should look like. A large

division is over whether to use TDMA methods over existing analog frequencies or to use

spread-spectrum CDMA methods.

2.3.1 Digital Cellular Service

Europe was the first to embrace digital service with the Global System for Mobile

communication (GSM). The incompatible existing analog system in the 1980s made it

impossible to use a single mobile phone in several European countries. With the European

Union and increased trade and commerce throughout Europe, a need was seen for a single

European standard. To choose one of the existing standards would have given an unfair

advantage to those that already provided that service, so it made sense to create an entirely

new service that could take advantage of technological advances since the advent of cellular

service. Thus, the Group Special Mobile, European Telecommunication Standard Institute

(ETSI) committee was formed. It established the GSM standard in the 1980s; GSM was

first implemented in 1992-1993. This all-digital standard became the least common

denominator of service in Europe, and is quickly replaced the analog systems currently in

place.

U.S. digital systems have also recently emerged, with IS-54 (also called D-AMPS or

U.S. TDMA) systems already in place, and replaced by IS-136 systems (the successor to

IS-54). One more major cellular service provider putting a competing system in place

called IS-95 (or U.S. CDMA). Other two important Japanese digital standards are: Personal

Hand phone System (PHS) and Personal Digital Cellular (PDC), which both use TDMA

like IS-54/136 and GSM.

2.3.2 Personal Communications Services

There is a great deal of talk about Personal Communications Services (PCS). FCC

auctioned off 160 MHz of radio spectrum for PCS services, which defined as a broad

range of radio communication services freeing users from the limitations of wired phone

networks. These are essentially cellular phone systems, although the intent of PCS systems

is not to supplement the existing phone system but rather to become an integral part of it.

The first specification for a Personal Communication Network was actually made in

1990 based on the GSM cellular standard at the request of the United Kingdom. It became

part of the GSM standard, which includes GSM-900 (the cellular standard) and DCS-

1800 (the PCN standard). A variant, PCS-1900, is one of the contending standards for

PCS service in the United States.


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2.4 BASIC NETWORK OPERATIONS

Traditional mobile phone service has used only terrestrial radio. In other words, it

relies on ground-based cell sites, which are usually small towers with three antennae arranged

in a triangle. Satellite implementations are possible, although they cannot use radio bandwidth

as efficiently. Thus, they are used commercially primarily for pager, broadcast, and some

specific site-to-site links.

A cellular network is designed to connect to the existing phone system (Public Switched

Telephone Network (PSTN)) or potentially to a data network (Public Data Network

(PDN)). The connection to the PSTN is not much different than the connection to other

telephone switching equipment such as a Public Branch Exchange (PBX).

Cellular networks are comprised of terminals and base stations. Terminals are the

end-user equipment (Mobile Stations). Everything else in a cellular network is consideredto be base station equipment.

2.4.1 Terminal

There are three types of cellular terminals. Each type has different output power

restrictions, based on how near the antenna is to people when it is in operation. They are

summarized in Table 2.1.

Table 2.1 Type of Terminals

Fixed installations might be used in dwellings that cannot be reached via landlines or

where landlines would be impractical. These are not too power-constrained, although the

vast majority of terminals face strict power constraints. In addition, portable phones are

usually running on very limited battery power.

Most terminals are extremely cost-constrained as well. The sheer number of consumers

and competitors in the market is the primary reason for this. When the basic requirements

for the GSM standard were written in 1985, there was even a specific requirement thatthe system parameters shall be chosen with a view to limit the cost of the complete system,

in particular the mobile units.

2.4.2 Base Station

There are three components of base stations.

Base Transceiver Station (BTS): BTS communicates directly with the end-user terminals and also called as cell site.

Base Station Controller (BSC): BSC controls the base transceiver stations

either over land links (typically) or over radio links.

Type Output Power

Portable (hand-held) Less than 0.6 WattsMobile (car or bag phones) Less than 3 WattsFixed No fixed limit


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Mobile Switching Center (MSC): MSC controls the base station controllers,usually over land links and also called as mobile telephone switching office.

There is no fixed ratio of BTS to BSC to MSC, although there are typically about five

to ten BTSs per BSC and anywhere from one to ten BSCs per MSC, depending on the

capacity needs and geographic distribution of an area. In fact, base station functions may

be combined into a single site, especially in the more remote areas where a single site might

serve as BTS, BSC, and MSC.

Figure 2.1 demonstrates six cell sites communicating over radio links to two BSCs,

which in turn communicate with a single MSC, which is connected to PSTN.

Figure 2.1 Base Station Organization

Cell sites have one antenna for upload (transmitting to the terminals) and two download

antennas (receiving from the terminals). The download antennas allow to work as a bigger

antenna and help counter multipath effects (which are described later). They generally

operate around 900MHz, though other frequencies are also used (especially for PCS).

BSC-BTS communication uses a low-speed link, (may be a radio link). MSC-BSC and

PSTN-MSC communication is at a much higher rate (land-lines).

As can be seen by examining the type of data carried between them, the three types

of base stations each perform different types of operations. Rather than performing all

cellular operations in a single unit, base stations divide the operations and perform them

where they make the most sense. The cell sites perform operations that need to be doneindependently on each channel while the base station controllers and mobile switching

centers can perform certain operations on multiple channels simultaneously.

2.4.3 Cell Organization

Cellular communication got its name because of the cell structure of the BTS service

areas. For convenience, a service area is often subdivided into an array of hexagonal cells,

each containing a single BTS. The service areas of individual BTSs are being calculated

more precisely and they are being placed where they can be the most effective rather than

in traditional cells, although the term cellular is still used.


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The arrangement of the cells is basically a form of SDMA, since frequencies can be

reused by other cells that are far enough away not to interfere with the current cell. The

degree of reuse (determined by how far apart cells must be to reuse the same frequency)

is dependent upon the actual implementation of the radio link.

Traditional AMPS-type cell sites are called macro cells as shown in Figure 2.2. These

are spaced anywhere from 3 to 60 km apart (averaging around 6 km), depending on

population density, terrain and other factors. Newer networks are adding micro cells as

shown in Figure 2.2, which are smaller cells (perhaps as close as a few hundred feet apart)

that can be used for spot coverage (e.g., near a tunnel) or to increase the level of SDMA

(adding capacity). Both macro cells and micro cells are part of a service providers network.

Figure 2.2 Macro Cells Vs Micro Cells

Another type of cell is a Pico cell. Pico cells are just small micro cells, although they

need not be part of a service providers network. Individual buildings or even floors within

a building can use Pico cells to have their own cellular service. For example, a company

could provide local cellular service to its employees without paying per-minute airtime

charges to a service provider.

It is also possible to have hybrid arrangements, with micro cells and Pico cells existing

inside of macro cells, as is shown in the Figure 2.3.

Figure 2.3 A Micro Cell with a Macro Cell

2.5 CHALLENGES OF CELLULAR COMMUNICATION

Cellular networks must deal with all of the challenges of traditional telephony, such as

call setup, switching, and so on. However, cellular networks must face additional challenges

due to the mobile, wireless nature of the terminals. They also have to deal with many types

of signal corruption caused by sending the signals through the air.


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This is called discontinuous transmission (DTX) and in the form of power control. Cellular

networks often have complicated power control procedures, which use location

management as well as transmitted power level information to more finely control the

output power of terminals. This is especially important for CDMA networks in order toprevent the signals from nearby terminals from blanking out the signals of further ones.

Cellular networks have additional routing concerns regarding billing, especially when

terminals are in roaming and billing must go through another service provider. Ironically,

billing issues can be even worse when calls are made between two terminals using the

same cell site (especially when one is roaming).

2.5.2 Signal Corruption

There are six major types of signal corruptions that are individually addressed by

most cellular networks. Multipath effects (signal reflections) cause two of them: Riceanfading and Rayleigh fading. Doppler effects are caused by the mobility of terminals. Blocking

is caused by the fact that terminals may be at different distances from the cell sites. Signal

loss can be caused by a variety of things, usually physical barriers. All other types of signal

corruption can be classified as noise.

2.5.2.1. Ricean Fading

Ricean fading, the most general multipath effect, results when a transmitted signal

follows multiple paths to the receiver. After the direct transmission is received, echoes of

the signal reach the receiver. These can cause transmitted symbols to interfere with future

symbols. In Figure 2.4, the direct transmission would reach the car first, followed by anecho off of the building.

Figure 2.4 Ricean Fading

2.5.2.2. Rayleigh Fading

Rayleigh fading is a very similar multipath effect as shown in Figure 2.5, except it

results when obstacles block the direct path from the transmitter to the receiver. Since the

direct transmission is blocked, the reflected signals are not echoes, but the first signals

received. Deconstructive interference (anti-nodes) can cause short-term amplitude dips,

or even complete loss, in the received signal.


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2.5.2.5 Loss of Signal

There are several possible causes for loss of signal, including over-aggressive power

management as shown in Figure 2.8. The most likely cause is simply due to obstacles. Ifthe direct transmission blocked and there are no reflected signals to receive, the signal is

lost and must be renegotiated.

Figure 2.8 Loss of Signal

2.5.2.6 Noise

There are many types of noise, the most common noise is general background noise,

which is additive white Gaussian noise that is dependent only on temperature and cannot

be avoided. The cellular network also introduces its own noise. For example, the blocking

effect can be felt on a smaller scale, when a cell that has greater power (i.e., is closer) than

the cell with which a terminal is communicating is using a nearby frequency, only causing

noise rather then blanking out the other signal. This is called cell-to-cell adjacent channelnoise. Co-channel noise is also present, and is caused by the reuse of frequencies. There

can be additional noise problems when two terminals are using the same cell to communicate

with each other (a type of crosstalk problem). Cellular networks usually deal with all types

of noise by simply protecting the data with error correction techniques. A major effect of

the different types of noise is the limits that they place upon cellular networks (e.g., number

of terminals per cell, frequency reuse factors, etc.).

2.6 HANDOFF STRATEGIES

When a mobile moves into a different cell while a conversation is in progress, the

MSC automatically transfers the call to a new channel belonging to the new base station.This handoff operation not only involves in identifying a new base station, but also requires

that the voice and control signals be allocated to channels associated with the new base

station.

Processing handoffs is an important task in any cellular radio system. Many handoff

strategies prioritize handoff requests over call initiation requests when allocating unused

channels in a cell site. Handoffs must be performed successfully and as in frequently as

possible, and be imperceptible to the users. In order to meet these requirements, system

designers must specify an optimum signal level at which to initiate a handoff. Once a particular

signal level is specified as the minimum usable signal for acceptable voice quality at the


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MSC engages in an intersystem handoff when a mobile signal becomes weak in a given

cell and the MSC cannot find another cell within its system to which it can transfer the call

in progress. There are many issues that must be addressed when implementing an

intersystem handoff. For instance, a local call may become a long-distance call as themobile moves out of its home system and becomes a roamer in a neighboring system.

Also, compatibility between the two MSCs must be determined before implementing an

intersystem handoff.

Different systems have different policies and methods for managing handoff requests.

Some systems handle handoff requests in the same way they handle originating calls. In

such systems, the probability that a handoff request will not be served by a new base

station is equal to the blocking probability of incoming calls. However, from the users

point of view, having a call abruptly terminated while in the middle of a conversation is

more annoying than being blocked occasionally on a new call attempt. To improve thequality of service as perceived by the users, various methods have been devised to prioritize

handoff requests over call initiation requests when allocating voice channels.

2.7 QUESTIONS

1. Explain the common base station subsystem

2. Briefly discuss about the cell organization

3. Explain the challenges for cellular communication

4. Explain the handover process with neat diagram


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CHAPTER 3

TELECOMMUNICATIONS SYSTEMS

3.1 INTRODUCTION

Cellular is one of the fastest growing and most demanding telecommunications

applications. Today, it represents a continuously increasing percentage of all new telephone

subscriptions around the world. It is forecasted that cellular systems using a digital technology

will become the universal method of telecommunications. By the year 2005, forecasters

predict that there will be more than 100 million cellular subscribers worldwide.

Figure 3.1 Cellular Subscriber Growth Worldwide

In the early 1980s, most mobile telephone systems were analog rather than digital,

like todays newer systems. One challenge facing analog systems was the inability to handle

the growing capacity needs in a cost-efficient manner. As a result, digital technology was

welcomed. The advantages of digital systems over analog systems include ease of signaling,

lower levels of interference, integration of transmission and switching, and increased abilityto meet capacity demands. Table 3.1 shows the worldwide development of mobile

telephone systems.


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enhanced full-rate coding, which is finding its way into the IS-54/136 and IS-95 specifications

as well.

3.2.4 IS-95

IS-95 is a DSSS CDMA standard that was developed by QUALCOMM and is

being put into service primarily by Bell South. It uses Pseudo Noise (PN), sequences to

encode channels into pairs of 1.23 MHz bands and is fundamentally different from any of

the TDMA standards. Unlike the TDMA standards, call quality in an IS-95 system improves

when other channels are idle, even if they are being used but have no voice activity in one

direction on a channel. CDMA supporters frequently use theoretical calculations to show

a tremendous increase in capacity, but in actuality there can never be more than 63 traffic

channels per band, and the realistic limit is 45 (the true numbers will be known when a real

system is operational). Thus, 900 IS-95 channels may be carried in the 50 MHz allocatedto a service provider.

IS-95 literature is deceptive about reuse factors. Theoretically, IS-95 has a reuse

factor of 1, meaning that every cell can use every frequency. However, some of the available

PN sequences are not available to all of the cells. For example, if a terminal is directly

between two cells, they will both communicate with it using the same PN sequence. This is

referred to as part of loading, and reduces the number of available channels, likely to at

most 25 per 1.23 MHz band, or 500 per cell.

Since channels are isolated through a CDMA mechanism, the framing structure underIS-95 is much simpler. The traffic channels are divided into 20 ms frames with no super

frames. Two synchronization channels are each also divided into 20 ms frames and use 3-

frame super frames. Layer 2 services are provided that split the physical channels into sub

channels. The type of modulation used is QPSK.

The IS-95 down-link encodes 64 chips per bit with a PN sequence to create 64

channels, each with a maximum bit rate of 19.2 kbps. The up-link uses a more complicated

encoding method to get 64 channels with a maximum bit rate of 28.8 kbps. Of this, at most

9.6 kbps is used for speech and varying amounts are used for error protection and

embedded control (the uplink uses more error protection bits than the downlink). Lowerbit rates may also be used to decrease the overall noise in the system.

3.3 GSM

Throughout the evolution of cellular telecommunications, various systems have been

developed without the benefit of standardized specifications. This presented many problems

directly related to compatibility, especially with the development of digital radio technology.

The GSM standard is intended to address these problems.


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Visitor Location Register (VLR): VLR is a database that contains temporaryinformation about subscribers that is needed by the MSC in order to service visitingsubscribers. The VLR is always integrated with the MSC. When a mobile station

roams into a new MSC area, the VLR connected to that MSC will request dataabout the mobile station from the HLR. Later, if the mobile station makes a call,the VLR will have the information needed for call setup without having to interrogatethe HLR each time.

Authentication Center (AUC):AUC provides authentication and encryptionparameters that verify the users identity and ensure the confidentiality of each call.AUC protects network operators from different types of fraud found in todayscellular world.

Equipment Identity Register (EIR): EIR is a database that contains informationabout the identity of mobile equipment that prevents calls from stolen, unauthorized,

or defective mobile stations.AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR

node.

3.3.3.2 Base Station System

All radio-related functions are performed in the BSS, which consists of BSCs and

BTSs.

BSC: BSC provides all the control functions and physical links between the MSCand BTS. It is a high-capacity switch that provides functions such as handover,cell configuration data, and control of radio frequency (RF) power levels in base

transceiver stations. A number of BSCs are served by an MSC. BTS: BTS handles the radio interface to the mobile station. BTS is the radio

equipment (transceivers and antennas) needed to service each cell in the network.A group of BTSs are controlled by a BSC.

3.3.3.3 Operation and Support System

Operations and Maintenance Center (OMC) is connected to all equipment in the

switching system and to the BSC. The implementation of OMC is called the Operation

and Support System (OSS). The OSS is the functional entity from which the network

operator monitors and controls the system. The purpose of OSS is to offer cost-effective

support for centralized, regional and local operational and maintenance activities that arerequired for a GSM network. An important function of OSS is to provide a network

overview and support the maintenance activities of different operation and maintenance

organizations.

3.3.3.4 Additional Functional Elements

Other functional elements are as follows:

Message Center (MXE): MXE is a node that provides integrated voice, fax,and data messaging. Specifically, the MXE handles short message service, cell

broadcast, voice mail, fax mail, email, and notification.


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Figure 5.1 Generation of DAB Signal

5.3.1.2 Reception of a DAB signal

Figure 5.2demonstrates a conceptual DAB receiver. The DAB ensemble is selected

in the analogue tuner, the digitized output of which is fed to the OFDM demodulator and

channel decoder to eliminate transmission errors. The information contained in the FIC is

passed to the user interface for service selection and is used to set the receiver appropriately.

The MSC data is further processed in an audio decoder to produce the left and right audio

signals or in a data decoder (Packet Demux) as appropriate.

Figure 5.2 Conceptual DAB Receiver


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All together, this chain represents a classical combination of MPEG-4 elements

transported with an MPEG-2 Transport Stream. BIFS as one of those MPEG elements/

norms represents quite a powerful tool for data provision and interactivity.

5.4.2 Benefits of DMB

A wide range of TV and interactive services to be broadcast simultaneously on thesame multiplex (video services, DAB and DAB+ radio services, file downloading(podcasting), electronic programme guide, slide show, broadcast website, BIFS)

Existing DAB transmitter network to be adapted to carry these new services

Robust reception of mobile TV at highway speeds (>300km/h)

Multimedia content to be delivered without the risk of network congestion

Both DMB and DAB services to be accessed on the same receiver

DMB is an open European Standard

DMB demands less spectrum commitment than other mobile TV standards, which

typically use 6-8 MHz blocks. In contrast, DMB can offer both TV and radio services

within a multiplex of just 1.5 MHz. Whilst this spectrum would deliver a range of

approximately 7 DMB services, extra services can be made available simply by adding

further multiplexes.

DMB has the further benefit of being broadcast in Band III or L-Band, where higher

powers give rise to broader and more comprehensive coverage. Other mobile TV

standards must use UHF Bands IV or V. As a result, transmitter powers are low andcoverage areas from a single transmitter are typically small. However, since DMB is in

Band III and L-Band higher powers give rise to broader and more comprehensive coverage.

5.5 QUESTIONS

1. What is broadcast disk?

2. What is podcast?

3. Define scheduling

4. Briefly discuss about DAB

5. Explain about DMB


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The standard also defines the concept of a Portal. A portal is a device that interconnects

between a 802.11 and another 802 LAN. This concept is an abstract description of part

of the functionality of a translation bridge.

6.2.2 IEEE 802.11 Layers Description

As any 802.x protocol, the 802.11 protocol covers the MAC and Physical Layer.

The Standard currently defines a single MAC which interacts with three PHYs ( as shown

in Figure 6.2) (all of them running at 1 and 2 Mbit/s) as follows:

Frequency Hopping Spread Spectrum in the 2.4 GHz Band

Direct Sequence Spread Spectrum in the 2.4 GHz Band

Figure 6.2 802.11 Layered Architecture

Beyond the standard functionality usually performed by MAC Layers, the 802.11MAC performs other functions that are typically related to upper layer protocols, such as

Fragmentation, Packet Retransmissions, and Acknowledges.

6.3.2 The MAC Layer

The MAC Layer defines two different access methods, they are: Distributed

Coordination Function (DCF) and Point Coordination Function (PCF).

6.3.2.1 The Basic Access Method: CSMA/CA

DCF is the basic access mechanism. Basically DCF is a Carrier Sense Multiple Accesswith Collision Avoidance (CSMA/CA). CSMA protocols are well-known in the industry,

the most popular being the Ethernet, which is a CSMA/CD protocol (CD standing for

Collision Detection).

A CSMA protocol works as follows: A station desiring to transmit senses the medium.

If the medium is busy (i.e. some other station is transmitting) then the station defers its

transmission to a later time. If the medium is sensed free then the station is allowed to

transmit.


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These kinds of protocols are very effective when the medium is not heavily loaded

since it allows stations to transmit with minimum delay. But there is always a chance of

stations simultaneously sensing the medium as being free and transmitting at the same time,

causing a collision.

These collision situations must be identified, so the MAC layer can retransmit the

packet by itself and not by upper layers, which would cause significant delay. In the Ethernet

case this collision is recognized by the transmitting stations which will get into a retransmission

phase based on an exponential random backoff algorithm.

While these Collision Detection mechanisms are good on a wired LAN, they cannot

be used on a Wireless LAN environment for two main reasons:

Implementing a Collision Detection mechanism would require the implementation

of a Full Duplex radio capable of transmitting and receiving at once, an approachthat would increase the price significantly.

In a Wireless environment we cannot assume that all stations hear each other(which is the basic assumption of the Collision Detection scheme), and the factthat a station wants to transmit and senses the medium as free doesnt necessarilymean that the medium is free around the receiver area.

In order to overcome these problems, the 802.11 uses a Collision Avoidance (CA)

mechanism together with a Positive Acknowledge scheme, as follows:

A station wanting to transmit senses the medium. If the medium is busy then itdefers. If the medium is free for a specified time called Distributed Inter FrameSpace (DIFS), then the station is allowed to transmit.

The receiving station checks the CRC of the received packet and sends anacknowledgment packet (ACK). Receipt of the acknowledgment indicates to thetransmitter that no collision occurred. If the sender does not receive theacknowledgment then it retransmits the fragment until it receives acknowledgmentor is thrown away after a given number of retransmissions.

6.2.3.2 Virtual Carrier Sense

In order to reduce the probability of two stations colliding because they cannot heareach other, the standard defines a Virtual Carrier Sense mechanism:

A station wanting to transmit a packet first transmits a short control packet called

RTS (Request To Send), which includes the source, destination, and the duration of the

following transaction (i.e. the packet and the respective ACK), the destination station

responds (if the medium is free) with a response control Packet called CTS (Clear to

Send), which includes the same duration information.


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6.3 HOW DOES A STATION JOIN AN EXISTING CELL (BSS)

When a station wants to access an existing BSS (either after power-up, sleep mode,

or just entering the BSS area), the station needs to get synchronization information fromthe Access Point (or from the other stations when in ad-hoc mode, which will be discussed

later). The station can get this information by one of two means:

Passive Scanning: In this case the station just waits to receive a Beacon Framefrom the AP, (the beacon frame is a frame sent out periodically by the AP containingsynchronization information) or

Active Scanning: In this case the station tries to locate an Access Point bytransmitting Probe Request Frames, and waits for Probe Response from the AP

Both methods are valid. A method is chosen according to the power consumption/

performance trade-off.

6.4 THE AUTHENTICATION PROCESS

Once the station has located an Access Point, and decides to join its BSS, it goes

through the Authentication Process. This is the interchange of information between the AP

and the station, where each side proves the knowledge of a given password.

6.5 THE ASSOCIATION PROCESS

Once the station is authenticated, it then starts the Association Process, which is the

exchange of information about the stations and BSS capabilities, and which allows theDSS (the set of APs) to know about the current position of the station). A station is capable

of transmitting and receiving data frames only after the association process is completed.

6.6 ROAMING

Roaming is the process of moving from one cell (or BSS) to another without losing

connection. This function is similar to the cellular phones handover, with two main differences:

On a packet-based LAN system, the transition from cell to cell may be performedbetween packet transmissions, as opposed to telephony where the transition may

occur during a phone conversation, this makes the LAN roaming a little easier On a voice system, a temporary disconnection may not affect the conversation,

while in a packet based environment it significantly reduces performance becauseretransmission is then performed by the upper layer protocols

The 802.11 standard does not define how roaming should be performed, but defines

the basic tools. These include active/passive scanning, and a re-association process, where

a station which is roaming from one Access Point to another becomes associated with the

new one1.


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6.6.1 Synchronization

Stations need to keep synchronization, which is necessary for keeping hopping

synchronized and other functions like Power Saving. On an infrastructure BSS, this isachieved by all the stations updating their clocks according to the APs clock, using the

following mechanism: The AP periodically transmits frames called Beacon Frames. These

frames contain the value of the APs clock at the moment of transmission (note that this is

the moment when transmission actually occurs, and not when it is put in the queue for

transmission. Since the Beacon Frame is transmitted using CSMA rules, transmission may

be delayed significantly). The receiving stations check the value of their clocks at the moment

the signal is received, and correct it to keep in synchronization with the APs clock. This

prevents clock drifting which could cause loss of synch after a few hours of operation.

6.7 SECURITY

Security is one of the first concerns that people have when deploying a Wireless

LAN. The 802.11 committee has addressed the issue by providing what is called WEP

(Wired Equivalent Privacy). Users are primarily concerned that an intruder should not be

able to:

Access the Network resources by using similar Wireless LAN equipment

Capture Wireless LAN traffic (eavesdropping)

6.7.1 Preventing Access to Network Resources

This is done by the use of an Authentication mechanism where a station needs to

prove knowledge of the current key. This is very similar to Wired LAN privacy, in the

sense that an intruder needs to enter the premises (by using a physical key) in order to

connect his workstation to the wired LAN.

6.7.1.1 Eavesdropping

Eavesdropping is prevented by using the WEP algorithm which is a Pseudo Random

Number Generator initialized by a shared secret key. This PRNG outputs a key sequence

of pseudo-random bits equal in length to the largest possible packet which is combinedwith the outgoing/incoming packet producing the packet transmitted in the air.

The WEP is a simple algorithm based on RSAs RC4 which has the following properties:

Reasonably strong: Brute-force attack to this algorithm is difficult because everyframe is sent with an Initialization Vector which restarts the PRNG for each frame.

Self Synchronizing: The algorithm re-synchronizes for each message. This isnecessary in order to work in a connection-less environment, where packets mayget lost (as any LAN).


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7.2.1 Bluetooth Connections

The major difference between Bluetooth wireless connectivity and the cellular radio

architecture is that Bluetooth enables ad hoc networking. Rather than depending on abroadband system, which relies on terminals and base stations for maintaining connections

to the network via radio links, Bluetooth implements peer-to-peer connectivity, no base

stations or terminals are involved. A typical Bluetooth network scenario is shown in Figure

7.1.

Using peer-to-peer connectivity, Bluetooth technology simplifies personal area wireless

connections, enabling all digital devices to communicate spontaneously. Early applications

are expected to include cable replacement for laptops, PDAs, mobile phones, and digital

cameras. Because Bluetooth supports voice transmissions, headsets also are in line to

become wireless. The Bluetooth technology offers the following advantages: Voice/data access points

Cable is replaced by a Bluetooth chip that transmits information at a special radiofrequency to a receiver Bluetooth chip

Ad hoc networking enables personal devices to automatically exchange informationand synchronize with each other.

Figure 7.1 Connecting with Bluetooth

7.2.2 Reliable and Secure Transmission

Bluetooth technology also provides fast, secure voice and data transmissions. The

range for connectivity is up to 10 meters, and line of sight is not required. The Bluetooth

radio unit


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Functions even in noisy radio environments, ensuring audible voice transmissionsin severe conditions

Protects data by using error-correction methods

Provides a high transmission rate

Encrypts and authenticates for privacy

As with any wireless interface, Bluetooth must address issues involving reliable delivery

of information. To help deliver accurate information, Bluetooth provides two error-correction

mechanisms: Forward Error Correction (FEC) and Automatic Repeat Request (ARQ).

Typically, FEC is applied to voice traffic for which the timeliness of the delivery takes

precedence over the accuracy. ARQ mechanisms are used for data applications.

Bluetooth operates in the unlicensed ISM frequency band, it competes with signals

from other devices, such as garage door openers and microwave ovens. In order forBluetooth devices to operate reliably, each Bluetooth network is synchronized to a specific

frequency pattern. The Bluetooth unit moves through 1,600 different frequencies per second,

and the pattern is unique to each network.

Bluetooth also implements various security measures, including authentication and

encryption. Authentication is used to verify the identity of the device sending information,

and encryption is used to ensure the integrity of the data.

7.2.3 Low Power Architecture

Bluetooth is intended for mobile devices, it implements low power architecture in

which units move into lower power modes when not actively participating on the network.

Bluetooth units also consume less power during operation. For example, the Bluetooth

radio consumes less than 3 percent of the power that a mobile phone consumes.

7.2.4 Interoperability, standards, and specifications

Another key concept in the Bluetooth environment is the idea of interoperability among

Bluetooth units regardless of manufacturer. Because Bluetooth is an open specification for

short range wireless communication, all Bluetooth products must conform to a standard.

This ensures that wireless connections will be globally available, and Bluetooth units madeanywhere in the world will be able to connect with and communicate information and

services to other Bluetooth devices.

To this end, the Bluetooth SIG has developed detailed specifications for the hardware

and software elements of Bluetooth units. The specifications consist of Core and Profile

documentations. The Core document discusses elements such as the radio, baseband, link

manager, and interoperability with different communication protocols. The Profile document

delineates the protocols and procedures to be used for specific classes of applications.

The specifications are intended to prevent discrepancies in end products due to different


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interpretations of the Bluetooth standard. The SIG also has implemented a qualification

process. This process defines criteria for Bluetooth product qualification, ensuring the

Bluetooth standards are met in any product that sports the Bluetooth name.

7.3 BLUETOOTH TERMINOLOGY

Bluetooth draws heavily on existing radio communications and networking technologies,

which enables it to be operationally compatible with the existing devices that also use these

technologies. Many of the various terms and concepts used in Bluetooth are borrowed

from other areas and included in the specification of Bluetooths elements, such as baseband,

RF communications, and many of the upper and lower layer protocols. What makes

Bluetooth unique is how it applies its proprietary components and the existing technologies

to define its central core operations and its application profiles. Regardless of their source,

the terms that are integral to Bluetooth are worth examining a little more closely.

7.3.1 Bluetooth Stack

Baseband or radio module is the hardware that enables wireless communication

between devices. The building block of this technology is the Bluetooth stack, which includes

the hardware and software portions of the system. Figure 7.2 shows a graphic representation

of the stack. The stack contains a physical level protocol (baseband) and a link level

protocol (Link Manager Protocol, or LMP) with an adaptation layer (Logical Link Control

and Adaptation Layer Protocol, or L2CAP), enabling upper layer protocols to interact

with the lower layer.The Bluetooth stack has the following components:

The radio frequency (RF) portion provides the digital signal processing componentof the system, and the baseband processes these signals

The link controller handles all the baseband functions and supports the link manager.It sends and receives data, identifies the sending device, performs authentication,and determines the type of frame to use for sending transmissions. The link controlleralso directs how devices listen for transmissions from other devices and can movedevices into power-saving modes


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CHAPTER 8

WIRELESS ATM

8.1 INTRODUCTION

Asynchronous Transfer Mode (ATM) has been advocated as an important technology

for all types of services and networks. Most people believe that ATM will be the standard

for the future Broadband Integrated Services Digital Network (B-ISDN). From the service

point of view, ATM combines both the data and multimedia information into the wired

networks while scales well from backbones to the customer premises networks. To ensure

the success of ATM, lots of the design issues have been standardized by ATM Forum.

A wireless personal communication network has been growing very fast in the last

decade. Now a day, laptop, cellular phone, and pagers are very popular. Many systems

have been developed to provide different services, such as, Personal Communications

Service (PCS), Portable Telephone Systems, and Satellite Communications System. Usually,

these services do not guarantee QoS, so they are not suitable for the fast growing multimedia

applications.

Due to the success of ATM on wired networks, wireless ATM (WATM) is a direct

result of the ATM everywhere movement. WATM can be viewed as a solution for next-generation personal communication networks, or a wireless extension of the B-ISDN

networks, which will support integrated data transmission (data, voice, and video) with

guaranteed QoS.

8.2 WIRELESS ATM REFERENCE MODELS

The overall system consists of a fixed ATM network infrastructure and a radio access

segment. In the fixed ATM network, the switches, which communicate directly with wireless

station or wireless end user devices, are mobility enhanced ATM switches. These switches

setup connections on behalf of the wireless devices. They serve as entrance to the

infrastructure wired ATM networks. The other ATM switching elements in the wired ATM

networks remain unchanged.

Based on the different types of wireless applications, the radio access segment falls

into a number of areas which may need different solutions.

8.2.1 Fixed Wireless Components

In fixed wireless LANs, or network interconnection via satellite or microwaves links,

the end user devices and switching devices are fixed. They establish connections with each

other via wireless channel, not through cable. In these kinds of applications, the data


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8.3.2.3 Data Link Control (DLC)

Data Link Control is responsible for providing service to ATM layer. Mitigating the

effect of radio channel errors should be done in this layer before cells are sent to the ATM

layer. In order to fulfill this requirement, error detection/retransmission protocols and forwarderror correction methods are recommended.

8.3.2.4 Radio Resource Control (RRC)

RRC is needed for support of control plane functions related to the radio access

layer. It should support radio resource control and management functions for PHY, MAC,

and DLC layers. The design issues of RRL will include control/management syntax for

PHY, MAC and DLC layers; meta-signaling support for mobile ATM; and interface to

ATM control plane.

8.3.3 Mobile ATM

To support mobility, new higher layer control/signaling functions are needed to handling

handover, location management, routing, addressing, and traffic management. The item,

which defines the design the functions of control/signaling, are called Mobile ATM.

8.3.3.1 Handover

In WATM networks, a mobile end user establishes a virtual circuit (VC) to

communicate with another end user (either mobile or ATM end user). When the mobile

end user moves from one AP (access point) to another AP, proper handover is required.

To minimize the interruption to cell transport, an efficient switching of the active VCs from

the old data path to new data path is needed. Also the switching should be fast enough tomake the new VCs available to the mobile users.

When the handover occurs, the current QoS may not be support by the new data

path. In this case, a negotiation is required to set up new QoS. Since a mobile user may be

in the access range of several APs, it will select the one which can provide the best QoS.

During the handover, an old path is released and a new path is then re-established.

There is a possibility that some cells will get lost during this process (when the connection

is broken). In case no cell lost is allowed. Cell buffering is used to guarantee that no cell is

lost and cell sequence is preserved. Cell buffering consists of Uplink Buffering and DownlinkBuffering. If VC is broken when the mobile user is sending cells to APs, Uplink Buffering

is required. The mobile user will buffer all the outgoing cells. When the connection is up, it

sends out all the buffered cells so no cells are lost unless the buffers are overflowed.

Downlink Buffering is performed by APs to preserve the downlink cells for sudden link

interruptions, congestion, or retransmissions. It may also occur when handover is executed.

8.3.3.2 Location Management

When a connection is needed to be established between a mobile ATM end point and

another ATM end point, the mobile ATM end point is needed to be located. There are two


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2. Home Agent: Home Agent is a router on the home network serving as the anchorpoint for communication with the Mobile Node; it tunnels packets from a deviceon the Internet, called a Correspondent Node, to the roaming Mobile Node. (A

tunnel is established between the Home Agent and a reachable point for the MobileNode in the foreign network.)

3. Foreign Agent: Foreign Agent is a router that may function as the point ofattachment for the Mobile Node when it roams to a foreign network, deliveringpackets from the Home Agent to the Mobile Node

Figure 9.1 Mobile IP Components and Relationship

The care-of address is the termination point of the tunnel toward the Mobile Node

when it is on a foreign network. The Home Agent maintains an association between thehome IP address of the Mobile Node and its care-of address, which is the current location

of the Mobile Node on the foreign or visited network.

9.3 HOW MOBILE IP WORKS

This section explains how Mobile IP works. The Mobile IP process has three main

phases, which are discussed in the following sections.

1. Agent Discovery: A Mobile Node discovers its Foreign and Home Agents duringagent discovery

2. Registration: The Mobile Node registers its current location with the ForeignAgent and Home Agent during registration

3. Tunneling: A reciprocal tunnel is set up by the Home Agent to the care-of address(current location of the Mobile Node on the foreign network) to route packets tothe Mobile Node as it roams

9.3.1 Agent Discovery

During the agent discovery phase, the Home Agent and Foreign Agent advertise their

services on the network by using the ICMP Router Discovery Protocol (IRDP). The


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Mobile Node listens to these advertisements to determine if it is connected to its home

network or foreign network.

The IRDP advertisements carry Mobile IP extensions that specify whether an agent isa Home Agent, Foreign Agent, or both; its care-of address; the types of services it will

provide such as reverse tunneling and Generic Routing Encapsulation (GRE) and the allowed

registration lifetime or roaming period for visiting Mobile Nodes. Rather than waiting for

agent advertisements, a Mobile Node can send out an agent solicitation. This solicitation

forces any agents on the link to immediately send an agent advertisement. If a Mobile

Node determines that it is connected to a foreign network, it acquires a care-of address.

There are two types of care-of addresses exist:

1. Care-of address acquired from a Foreign Agent

2. Co-located care-of address

A Foreign Agent care-of address is an IP address of a Foreign Agent that has an

interface on the foreign network being visited by a Mobile Node. A Mobile Node that

acquires this type of care-of address can share the address with other Mobile Nodes. A

co-located care-of address is an IP address temporarily assigned to the interface of the

Mobile Node itself. A co-located care-of address represents the current position of the

Mobile Node on the foreign network and can be used by only one Mobile Node at a time.

When the Mobile Node hears a Foreign Agent advertisement and detects that it has moved

outside of its home network, it begins registration.

9.3.2 Registration

The Mobile Node is configured with the IP address and mobility security association

(which includes the shared key) of its Home Agent. In addition, the Mobile Node is

configured with either its home IP address, or another user identifier, such as a Network

Access Identifier.

The Mobile Node uses this information along with the information that it learns from

the Foreign Agent advertisements to form a Mobile IP registration request. It adds the

registration request to its pending list and sends the registration request to its Home Agent

either through the Foreign Agent or directly if it is using a co-located care-of address and

is not required to register through the Foreign Agent. If the registration request is sent

through the Foreign Agent, the Foreign Agent checks the validity of the registration request,

which includes checking that the requested lifetime does not exceed its limitations, the

requested tunnel encapsulation is available, and that reverse tunnel is supported. If the

registration request is valid, the Foreign Agent adds the visiting Mobile Node to its pending

list before relaying the request to the Home Agent. If the registration request is not valid,

the Foreign Agent sends a registration reply with appropriate error code to the Mobile

Node.


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The Home Agent checks the validity of the registration request, which includes

authentication of the Mobile Node. If the registration request is valid, the Home Agent

creates a mobility binding (an association of the Mobile Node with its care-of address), a

tunnel to the care-of address, and a routing entry for forwarding packets to the homeaddress through the tunnel.

The Home Agent then sends a registration reply to the Mobile Node through the

Foreign Agent (if the registration request was received via the Foreign Agent) or directly to

the Mobile Node. If the registration request is not valid, the Home Agent rejects the request

by sending a registration reply with an appropriate error code.

The Foreign Agent checks the validity of the registration reply, including ensuring that

an associated registration request exists in its pending list. If the registration reply is valid,

the Foreign Agent adds the Mobile Node to its visitor list, establishes a tunnel to the HomeAgent, and creates a routing entry for forwarding packets to the home address. It then

relays the registration reply to the Mobile Node.

Finally, the Mobile Node checks the validity of the registration reply, which includes

ensuring an associated request is in its pending list as well as proper authentication of the

Home Agent. If the registration reply is not valid, the Mobile Node discards the reply. If a

valid registration reply specifies that the registration is accepted, the Mobile Node is confirmed

that the mobility agents are aware of its roaming. In the co-located care-of address case,

it adds a tunnel to the Home Agent. Subsequently, it sends all packets to the Foreign

Agent.

The Mobile Node reregisters before its registration lifetime expires. The Home Agent

and Foreign Agent update their mobility binding and visitor entry, respectively, during re-

registration. In the case where the registration is denied, the Mobile Node makes the

necessary adjustments and attempts to register again. For example, if the registration is

denied because of time mismatch and the Home Agent sends back its time stamp for

synchronization, the Mobile Node adjusts the time stamp in future registration requests.

Thus, a successful Mobile IP registration sets up the routing mechanism for transporting

packets to and from the Mobile Node as it roams.

9.3.3. Tunneling

The Mobile Node sends packets using its home IP address, effectively maintaining

the appearance that it is always on its home network. Even while the Mobile Node is

roaming on foreign networks, its movements are transparent to correspondent nodes.

Data packets addressed to the Mobile Node are routed to its home network, where

the Home Agent now intercepts and tunnels them to the care-of address toward the Mobile

Node. Tunneling has two primary functions: encapsulation of the data packet to reach the


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tunnel endpoint, and de-capsulation when the packet is delivered at that endpoint. The

default tunnel mode is IP Encapsulation within IP Encapsulation. Optionally, GRE and

minimal encapsulation within IP may be used.

Typically, the Mobile Node sends packets to the Foreign Agent, which routes them

to their final destination, the Correspondent Node, as shown in Figure 9.2.

Figure 9.2 Packet Forwarding

However, this data path is topologically incorrect because it does not reflect the true

IP network source for the data-rather, it reflects the home network of the Mobile Node.

Because the packets show the home network as their source inside a foreign network, anaccess control list on routers in the network called ingress filtering drops the packets

instead of forwarding them. A feature called reverse tunneling solves this problem by having

the Foreign Agent tunnel packets back to the Home Agent when it receives them from the

Mobile Node as shown in Figure 9.3.

Tunnel MTU discovery is a mechanism for a tunnel encapsulate such as the Home

Agent to participate in path MTU discovery to avoid any packet fragmentation in the

routing path between a Correspondent Node and Mobile Node. For packets destined to

the Mobile Node, the Home Agent maintains the MTU of the tunnel to the care-of address

and informs the Correspondent Node of the reduced packet size. This improves routingefficiency by avoiding fragmentation and reassembly at the tunnel endpoints to ensure that

packets reach the Mobile Node.


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Figure 9.3 Reverse Tunnel

9.4 SECURITY

Mobile IP uses a strong authentication scheme for security purposes. All registration

messages between a Mobile Node and Home Agent are required to contain the Mobile-

Home Authentication Extension (MHAE). The integrity of the registration messages is

protected by a pre-shared 128-bit key between a Mobile Node and Home Agent. The

keyed Message Digest Algorithm 5 (MD5) in prefix+suffix mode is used to compute the

authenticator value in the appended MHAE, which is mandatory. Mobile IP also supports

the Hash-based Message Authentication Code (HMAC-MD5). The receiver compares

the authenticator value it computes over the message with the value in the extension to

verify the authenticity. Optionally, the Mobile-Foreign Authentication Extension and Foreign-

Home Authentication Extension are appended to protect message exchanges between a

Mobile Node and Foreign Agent and between a Foreign Agent and Home Agent,

respectively. Replay protection uses the identification field in the registration messages as a

timestamp and sequence number. The Home Agent returns its time stamp to synchronize

the Mobile Node for registration.

9.5 SOLUTION TO NETWORK MOBILITY

Network mobility is enabled by Mobile IP, which provides a scalable, transparent,

and secure solution. It is scalable because only the participating components need to be

Mobile IP aware -the Mobile Node and the endpoints of the tunnel. No other routers in

the network or any hosts with which the Mobile Node is communicating need to be changed

or even aware of the movement of the Mobile Node. It is transparent to any applications

while providing mobility. Also, the network layer provides link-layer independence; interlink

layer roaming, and link-layer transparency. Finally, it is secure because the set up of packet

redirection is authenticated.


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9.6 OVERVIEW OF AD-HOC NETWORKING

Ad Hoc networks are multi-hop wireless networks, where nodes may be mobile.

These types of networks are used in situations where temporary network connectivity isneeded. Ad hoc networks are formed on a dynamic basis, i.e. a number of users may wish

to exchange information and services between each other on an ad hoc basis, in order to

do this they will need to form an Ad Hoc network. An example of this may be found in a

disaster relief situation. Here an Ad Hoc network could enable emergency services to co-

ordinate emergency services more effectively. Smart spaces are defined as environments

that allow people to perform tasks efficiently by offering unprecedented levels of access to

information and assistance from computers. Ad Hoc networks will play a significant part in

these environments, allowing people to exchange information and services; for example,

people at a meeting could create an Ad Hoc network using their PDAs or Laptops and

exchange information relevant to the meeting. Indeed there are endless examples of wheretheir use could be found.

9.6.1 Routing in Ad Hoc Networks

An Ad Hoc network consists of wireless hosts that move around, i.e. they have no

permanent physical location. In order to facilitate communication within the network, a

routing protocol is used to discover routes between nodes before the exchange of IP data

packets. The routing protocols in Ad Hoc wireless networks are generally categorized as:

1. Proactive

2. Reactive

3. Hybrid

9.5.1.1 Proactive

These protocols require each node to maintain one or more tables to store up to date

routing information and to propagate updates throughout the network. These protocols try

and maintain valid routes to all communication mobile nodes all the time, which means

before a route is actually needed. Periodic route updates are exchanged in order to

synchronize the tables. Some examples of table driven ad hoc routing protocols includeDynamic Destination Sequenced Distance-Vector Routing Protocol (DSDV), Optimized

Link State Routing Protocol (OLSR) and Fisheye State Routing Protocol (FSR). These

protocols differ in the number of routing related tables and how changes are broadcasted

in the network structure. The problem with these protocols is the overhead; the protocols

propagate and maintain routing information, regardless of whether or not it is needed.

9.5.2.1 Reactive

These protocols create routes only when desired by a source node, therefore a route

discovery process is required within the network. Once a route has been established, it is


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maintained by a route maintenance procedure until either the destination becomes

inaccessible or until the route isnt needed any longer. Some examples of source initiated

ad hoc routing protocols include the Dynamic Source Routing Protocol (DSR), Ad Hoc

On Demand Distance Vector Routing Protocol (AODV) and Temporally Ordered RoutingAlgorithm (TORA). No periodic updates are required for these protocols but routing

information is only available when needed.

9.5.3.1 Hybrid

These protocols try to incorporate various aspects of proactive and reactive routing

protocols. They are generally used to provide hierarchical routing; routing in general can

be either flat or hierarchical. In a flat approach, the nodes communicate directly with each

other. The problem with this is that it does not scale well, it also does not allow for route

aggregation of updates. In a hierarchical approach, the nodes are grouped into clusters,within each cluster there is a cluster head, this acts as a gateway to other clusters, it serves

as a sort of default route. The advantage of a hierarchical structure is that within a cluster,

an on demand routing protocol could be used which is more efficient in small-scale networks.

For inter cluster communication then a table driven protocol could be used which, would

allow the network to scale better. An example of such a hybrid routing protocol is the Zone

Routing Protocol (ZRP).

9.7 QUESTIONS

1. Explain the components of Mobile IP

2. Explain the working principle of Mobile IP

3. What is the difference between care of address and co-located care of address

4. Explain reverse tunneling

5. Explain various ad-hoc routing techniques


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UNIT IV


CHAPTER 10


10.1 OVERVIEW

TCP is a connection oriented transport protocol which provides a reliable byte stream

to the application layer. Application data submitted to TCP is divided into Protocol Data

Units (PDUs) (called segments) before transmission. Reliability is achieved since TCP

uses an ARQ mechanism based on positive acknowledgments. Each byte is numbered

and the number of the first byte in a segment is used as a sequence number in the TCP

header. A receiver transmits a cumulative acknowledgment in response to an incoming

segment which implies that many segments can be acknowledged at the same time.

TCP manages a retransmission timer which is started when a segment is transmitted.

If the timer expires before the segment is acknowledged, then TCP retransmits the segment.

The Retransmission Time Out (RTO) value is calculated dynamically based on measurements

of the Round Trip Time (RTT) (i.e. the time it takes from the transmission of a segment until

the acknowledgment is received). In October 1986 the Internet had its first congestion

collapse. The end hosts transmitted more data than the routers were able to handle, and

did not lower the transmission rate even though many packets were lost. Hence the

congested state persisted in the routers. Since then TCP has been extended with mechanismsfor congestion control. Today all TCP implementations are required to use algorithms for

congestion control, namely, slow start, congestion avoidance, fast retransmit, and fast

recovery.

10.1.1. Slow Start and Congestion Avoidance

The purpose of slow start and congestion avoidance is to control the transmission

rate in order to prevent congestion from occurring. TCP is described as a self-clocking

protocol, since the transmission rate is determined by the rate of incoming acknowledg

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