1

ACKNOWLEDGEMENT

First of all I would like to pay my gratitude and sincerity to my parents. I am in this position

thanks to their striving effort where they try to make my future a success.

I would like to thank the Head of Division (Electrical Division) in IET, Mr. Weerarathne for

taking all the necessary arrangements related to A & A industrial training program and make our

training successful.

Special thanks to the Project Manager of the A & A Engineering division Mr. E.W Disanayake

for welcoming us to the company and guiding me throughout the project and for his sincere

dedication to grant us a good training at the division.

Special thank to Mr. Roshan Beliketimulla, (Manager - Finance). Thank you very much for

your kind cooperation.

I offer my special thanks to all the Engineers, technical officers and other staff who has

contributed to make our training a success.

In addition, I must thank all trainees for their wonderful corporation & understanding during the

training.

Thank you.

H.S Ganepola

EE/08/7100,

Institute of Engineering Technology,

Katunayake.

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CONTENTS

Acknowledgement

1 - Training Establishment page

1.1 History 4

1.2 A & A Vision and Mission 4

1.3 About A & A 4

1.4 A & A services 4

2 - Mobile Telecommunication 5

2.1 Mobile communication 5

2.2 History Of Mobile Telecommunication 5

2.3 Mobile Standards 7

2.4 Global System For Mobile Communication (GSM) 8

2.4.1 GSM Network Architecture 8

A) Mobile Switching center 9

B) Gateway MSC 11

C) Home Location Register 11

D) Visitor Location Register 12

E) Authentication Center 12

F) Equipment identity register 12

G) Base Station Controller 13

H) Radio Base Station 13

2.4.1 GSM Transmission Process 15

2.4.2 GSM Radio Interface 18

3 – Transmission 19

3.1 Transmission Medium used in Telecom industry 19

3.1.1 Copper Cables 19

A) Twisted Pair cables 19

B) Coaxial cables 26

3.1.2 Electromagnetic waves 31

3.1.3 Optical Fibers 35

3.2 Mobile site Planning 43

3.2.1 Planning New Links 43

3.2.2 Implementing New Links 43

3.3 Indoor Units 43

3.4.1 Access Module Magazine (AMM) 43

3.4.2 Plug-in Units for the AMM 44

3.5 Outdoor Units 45

3.5.1 Radio Unit 45

3.5.2 Antenna 45

3

3.5.3 Maintaining Existing Links 45

3.5.4 TEMS Link Planner 45

4 – GSM identities 47

4.1 Mobile Station ISDN Number (MSISDN) 47

4.2 International Mobile Subscriber Identity (IMSI) 47

4.3 International Mobile Equipment Identity (IMEI) 47

4.4 Temporary Mobile Subscriber Identity (TMSI) 48

4.5 Mobile Station Roaming Number (MSRN) 48

4.6 Location Area Identity (LAI) 48

5 - Operations Section 49

5.1 Antennas Used In Base Stations 49

5.2 RBS (Radio Base Station) 50

5.2.1 CDU- Combine And Distribution Unit 52

5.2.2 CXU- Configuration Switch Unit 53

5.2.3 DCCU- Dc Connection Unit 53

5.2.4 DTRU- Double Transceiver Unit 53

5.2.5 DXU- Distribution Switch Unit 54

5.2.6 TMA- Tower Mounted Amplifier 54

6 – Data Communication using GSM 56

6.1. General Packet Radio Service (GPRS) 56

6.1.1 Key User Features of GPRS 56

6.1.2 Key Network Features of GPRS 56

6.2 3G Technology 60

6.2.1 Features 60

6.2.2 Applications 61

6.3 High-Speed Downlink Packet Access (HSDPA) 61

6.3.1 Technology 61

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1- Training Establishment

A & A Telecommunication Engineering Services (Pvt) Ltd has emerged about three years ago as a

Telecom System implementation Contractor for providing high performance system integrating

solutions that meet its Clients specific requirements.

1.1 A & A Mission

To provide combined expertise with most efficient & reliable services to Telecom sector with the high

ethical standards in timely and professional manner to all A & A stake holders in Sri Lanka and other

grid-deficit countries.

1.2 A & A Vision

To focus be the customer’s first choice and provide services with no setback with commitment to

deliver significant cost & deadline reduction.

1.3 About A & A A & A is a complete solution portfolio with industrial leading experience and capabilities are creating

an integrated management platform for all telecommunication equipment installation & maintenance

requirements.

A & A Technical staff committed to serve with professional system integration needs and to maintain

high quality, efficient and reliable quality services to the customer’s satisfaction and fulfill all their

requirements.

The Company has always taken every effort to fulfill the Customer expectation from the time of award

of the contracts up to final completion stage as a policy of highest priority. A & A was able to achieve

activity targets with the committed efforts of the highly experienced Project Managers, Engineers,

Technical staff and the skilled employees of all grades in the Company.

1.4 A & A Services

A & A is committed to 100% Client satisfaction and to implement quality integrated solutions, while

deliberately seeking new and innovative ways to develop the talent and quality of services provided.

Whether you invest in one component or an entire system, A & A solve your telecommunication

installation requirements with efficient & reliable services with followings.

• Preliminary Network Planning Assistance.

• LOS & Field Surveys.

• Map Analyses

• Path Profiling

• BTS & MW Equipment Deployment & Commissioning.

• Core Network Equipment Deployment. (MSC, BSC, NGN)

• Optical Network Termination Equipment Deployment & Commissioning.

• Broadband Communication Equipment Deployment.

• Installation of Broadcasting Transmitters & Antenna systems.

• Equipment Maintenance Management.

A & A Telecommunication Engineering Services (Pvt) Ltd

No:903/D , Udawatta Road, Malabe, Sri Lanka

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Mobile Telecommunication

2.1 MOBILE COMMUNICATION

Mobile communications is one of the fastest growing and most demanding of all telecommunications

technologies. Currently, it represents an increasingly high percentage of all new telephone

subscriptions worldwide. In many cases, cellular solutions successfully compete with traditional

wire line networks and cordless telephones. In the future, cellular systems employing digital

technology will become the universal method of telecommunication

2.2 HISTORY OF MOBILE TELECOMMUNICATION

The origins of mobile communications followed quickly behind the invention of radio in the late

1800s. The first applications of mobile radio were related to the navigation and safety of ships at

sea. As radio concepts developed, so did its use as a communications tool. The major milestones

in the development of wireless communications are summarized in the following table:

Year Activity

1906 Reginald Fesseden successfully transmits human voice over radio. Up

until that time, radio communications consisted of transmissions of

Morse Code. 1915 J. A. Fleming invents the vacuum tube making it possible to build

mobile radios.

1921 The Detroit police department used a 2 MHz frequency in the

department's first vehicular mobile radio. The system was only one way

and police had to find a wire line phone to respond to radio messages.

1930s Amplitude Modulation (AM) two-way mobile systems were in place in

the U.S. that took advantage of newly developed mobile transmitters and

utilized a "push-to-talk" or half-duplex transmission. By the end of the

decade channel allocation grew from 11 to 40.

1935 Invention of Frequency Modulation (FM) improved audio quality. FM

eliminated the need for large AM transmitters and resulted in radio

equipment which required less power to operate. This made the use of

transmitters in vehicles more practical.

1940s The Federal Communications Commission (FCC) recognized a

communication service it classified as Domestic Public Land Mobile

(DPLM) radio service. The first DPLM system was established in St.

Louis in 1946 and it utilized the 150 MHz band. The following year, a

"highway" system was developed along the New York -Boston corridor

using the 35-40 MHz band.

1947 D.H. Ring, working at Bell Laboratories, envisions the cellular concept.

1948 Shockley, Bardeen and Brittain, at Bell Laboratories, invent the

transistor which enables electronic equipment, including the radio to

be miniaturized.

1949 Radio Common Carriers (RCCs) were recognized.

1949,

1958

Bell Systems made broadband proposals.

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1964 AT&T introduces Improved Mobile Telephone System (IMTS).

1968 The FCC began to address issue of new US spectrum requirements.

1969 Nordic countries of Denmark, Finland, Iceland, Norway and Sweden

agree to form a group to study and recommend areas of cooperation in

telecommunication. This led to the standardization of

telecommunications for all members of the Nordic Mobile Telephone

(NMT) group, the first comprehensive international standardization

group. 1973 The NMT group specifies a feature allowing mobile telephones to be

located within and across networks. This feature would become the

basis for roaming.

1979 The FCC authorized the installation and testing of the first

developmental cellular system in the US (Illinois Bell Telephone

Company).

1981 Ericsson launches the world's first cellular system in Saudi Arabia based

on the analog NMT 450 standard.

1991 The first digital cellular standard (GSM) is launched.

1998 The number of mobile subscribers world-wide has grown to over 200

million.

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2.3 Mobile Standards

Year Standard Mobile

Telephone

System

Technology PRIM k y

1981 NMT450 Nordic Mobile

Telephony

Analogue Europe

,middle East

1983 AMPS Advanced

Mobile Phone

System

Analogue North & South

America

1985 TACS Total Access

Communication

system

Analogue Europe and

China

1986 NMT 900 Nordic Mobile

Telephony

Analogue Europe

,middle East

1991 GSM 900 Global System

for Mobile

communication

900

Digital World-wide

1991

TDMA(IS136)

Time Division

Multiple

Access

(Digital-

AMPS)

Digital North and

South

America

1992 GSM 1800 Global System for Mobile communication 1800

Digital Europe

1993 CDMA One

(IS95)

Code Division

Multiple

Access one

Digital North

America &

Korea

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2.4 GLOBAL SYSTEM FOR MOBILE COMMUNICATION

(GSM)

2.4.1 GSM NETWORK ARCHITECTURE

The GSM network is mainly divided into two parts. As,

• Switching System (SS)

• Base Station System (BSS)

In addition to that all the nodes are monitored and maintained by computerized system

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A) MOBILE SWITCHING CENTRE (MSC)

The MSC is the main component of the NSS. It provides switching functions for calls

between mobile subscribers, and between mobile and fixed network subscribers. It is

interconnected to and provides interfaces with the PSTN, ISDN and public packet or

circuit switched data networks. The MSC functionally incorporates,

• SSP (service switching point)

• RCF (radio control function)

The SSP handles:

• Conventional switching functions:

o Main control

o Switching network

o Time base

o PCM link connection

o Section selection for auxiliaries such as the IWF (Inter Working Function)

o Tones and announcements

o Local or national signaling point (SP)

• Mobile radio application specific functions:

o setting up and releasing link sections from/to the PSTN/ISDN: the exchange

provides call handling related functions under RCP control

o call transfer on change of radio channel: the exchange can transfer a call set

up on one network section to another under RCP control, transaction between

RCP and SSP, under RCP control for an originating call from a mobile

station, signaling points in local and national networks

o Selection of circuits to the BSS: specialized circuit groups are created. The

SSP selects an incoming or outgoing circuit whose identity is transmitted to

the RCP

o Echo canceling: voice frequency channels established with a mobile station

are equipped with an echo canceller to eliminate echo on a call between a

PSTN subscriber and a PLMN subscriber. Echo cancellers are connected on

an individual call basis. They are integrated into the exchange on the PSTN

side.

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Operation and maintenance functions:

o interface with the OMC-S over asynchronous links o

generation of data required for observation o generation

of call data records (CDR)

The RCP handles:

• Call security

The RCP provides the mobile radio service with protection mechanisms against

unauthorized call attempts or intrusion into a conversation by third parties. These

mechanisms are used to authenticate the mobile calling and/or called party and to cipher all

information (speech, data and signaling) from the radio interface using a ciphering key

(Kc) which is changed for each call. The RCF adapts the security information interchanged

between BSS and VLR. The VLR manages various security procedures relating to keys

and triplets, authentication, ciphering, subscriber identity confidentiality (TMSI).

• Location update

The location update function tells the network where the mobile subscriber is at all times. It

provides the subscriber with continuity of service throughout the authorized coverage area,

enabling the subscriber, regardless of location to be called using a permanent directory

number, to access the network from anywhere. The VLR updates mobile subscriber

location data and transmits this data to the HLR if the subscriber is from another VLR.

• Call handling

The RCF manages all operations relating to originating or terminating call set-up, dialog

with the SSP to manage the physical resources involved in a call (terrestrial circuit for

dialog with the SSP, radio channel for dialog with the BSC), operations required for

handover, including generating the handover number (HO number) when the mobile

subscriber changes cells.

• Charging

For each originating, terminating, or rerouted call relating to a mobile subscriber, the MSC

transmits call data records (CDR) to the OMC-S. Calls rerouted after call forwarding also

result in CDR transmission.

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Numbering and routing

By dialing an E.164 number, depending on the type of subscription, a mobile subscriber

can reach a national PSTN subscriber, a subscriber of one or more national PLMNs, a

subscriber of any foreign PSTN, a subscriber of any foreign PLMN. The RCP analyzes the

number based on the type of number received, the RCP transmits the number to the SSP

for translation and routing (example: the received number is national or international), asks

the HLR for an MSRN (mobile station roaming number) and transmits it to the SSP

(example: the number received is an HPLMN number), transmits the number of the SSP

with the origin geographic area (example: the number received is an emergency services

number.

B) GATEWAY MSC (GMSC)

Gateway functionality enables an MSC to interrogate a HLR in order to route a mobile

terminating call. It is not used in calls from MS's to any terminal other than another MS.

For example, if a person connected to the PSTN wants to make a call to a GSM mobile

subscriber, then the PSTN exchange will access the GSM network by first connecting the

call to a GMSC. The GMSC requests call routing information from the HLR that provides

information about which MSC/VLR to route the call to. The same is true of a call from an

MS to another MS.

C) HOME LOCATION REGISTER (HLR)

The HLR is a database storing information relating to mobile network subscribers. The SS

can incorporate more than one database according to capability, availability, and operating

criteria selected. A static record in the HLR describes each subscription, giving details of

options and supplementary services accessible to the subscriber. This static information is

combined with dynamic information concerning the subscriber's latest known location or

the operational status of the MS. The HLR stores data characterizing the subscriber

(MSISDN, access rights, etc.).

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D) VISITOR LOCATION REGISTER (VLR)

The role of a VLR in a GSM network is to act as a temporary storage location for

subscription information for MSs which are within a particular MSC service area. Thus,

there is one VLR for each MSC service area. This means that the MSC does not have to

contact the HLR (which may be located in another country) every time the subscriber uses a

service or changes its status. Following occurs when MS's move into a new service area:

1. The VLR checks its database to determine whether or not it has a record for the MS

(based on the subscriber's IMSI)

2. When the VLR finds no record for the MS, it sends a request to the subscriber's HLR

for a copy of the MS's subscription

3. The HLR passes the information to the VLR and updates its location information for

the subscriber. The HLR instructs the old VLR to delete the information it has on the

MS

4. The VLR stores its subscription information for the MS, including the latest location

and status (idle)

E) AUTHENTICATION CENTER (AUC)

The AuC is a database storing confidential information, such as the Ki (the subscriber's

individual authentication key) used by the network to certify the subscriber's identity. The Ki

is stored in coded form which can be deciphered only by the AuC. The AuC generates the

triplets used for the authentication and ciphering procedures. It generates a signed response

(SRES), based on random number (RAND), individual authentication key (Ki) and

algorithm A3, a ciphering key Kc, based on RAND, Ki and algorithm A8. The triplet is

made up of RAND, SRES and Kc. The AuC supplies the VLR with data required for

subscriber authentication and ciphering interchanges between the MS and BSS.

F) EQUIPMENT IDENTITY REGISTER (EIR)

The EIR is an optional database accessible to the MSC. It contains lists of permitted MSs

and barred MSs (stolen or disturbance-generating). Each MS is assigned IMEI

(international mobile equipment identity) providing information not relating to the

subscriber identity, such as the factory serial number or software version. The exchange

interrogates the EIR to check the mobile station's IMEI status. The EIR responds with the

following information:

o White-listed: equipment permitted to use the network,

o Grey-listed: equipment being tracked,

o Black-listed: equipment barred from using the network.

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G) BASE STATION CONTROLLER (BSC)

The BSC controls a major part of the radio network. Its most important task is to ensure the

highest possible utilization of the radio resources. The main functional areas of the BSC are:

> Radio Network Management

> RBS Management

> TRC Handling

> Transmission Network Management

> Internal BSC Operation and Maintenance

> Handling of MS connections

H) RADIO BASE STATION (RBS)

RBS functionality can be divided into the following areas:

> Radio resources

> Signal processing

> Signaling link management

> Synchronization

> Local maintenance handling

> Functional supervision and testing

RADIO RESOURCES

An RBS's main function is to provide connection with the MSs over the air interface. This

includes the following tasks:

> Configuration and system start: site configuration involves loading of software

from the BSC and setting parameters prior to system startup.

> Radio transmission: to transmit several frequencies using the same antenna, a

combiner or a set of combiners are needed. Transmission power is controlled from the

BSC.

> Radio reception: in addition to reception of traffic on the physical channels, a

primary RBS function the detection of channel requests from MSs (e.g. when a call is

being made).

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SIGNAL PROCESSING

An RBS is responsible for the processing of signals before transmission and after

reception. This includes:

> Ciphering using the ciphering key

> Channel coding and interleaving

> Adaptive equalization

> Realization of diversity

> Demodulation

SIGNALING LINK MANAGEMENT

An RBS manages the signaling link between the BSC and MS, applying the appropriate

protocols to the information being sent.

SYNCHRONIZATION

Timing information is extracted from the PCM-links from the BSC and is sent to a timing

module within the RBS. Those enable the RBS to synchronize with the correct frequency

reference and TDMA frame number.

LOCAL MAINTENANCE HANDLING

An RBS enables operation and maintenance functions to be carried out locally at the RBS

site, without BSC connection. In this way, field technicians can maintain RBS equipment

and software on site.

FUNCTIONAL SUPERVISION AND TESTING

Supervision and testing of RBS functions is supported, using either built-in tests during

normal operation or tests executed by command.

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2.4.2 GSM TRANSMISSION PROCESS

ANALOG TO DIGITAL (A/D) CONVERSION

One of the primary functions of an MS is to convert the analog speech information into

digital form for transmission using a digital signal. The analog to digital (A/D) conversion

process outputs a collection of bits: binary ones and zeros which represent the speech

input.

The A/D conversion is performed by using a process called Pulse Code Modulation

(PCM). PCM involves three main steps:

> Sampling

> Quantization

> Coding

SAMPLING

Sampling involves measuring the analog signal at specific time intervals. The accuracy of

describing the analog signal in digital terms depends on how often the analog signal is

sampled. This is expressed as the sampling frequency. The sampling theory states that:

To reproduce an analog signal without distortion, the signal must be sampled

with at least twice the frequency of the highest frequency component in the analog

signal.

Normal speech mainly contains frequency components lower than 3400 Hz. Higher

components have low energy and may be omitted without affecting the speech quality

much. Applying the sampling theory to analog speech signals, the sampling frequency,

should be at least 2 x 3.4 kHz = 6.8 kHz. Telecommunication systems use a sampling

frequency of 8 kHz, which is acceptable based on the sampling theory.

QUANTIZATION

The next step is to give each sample a value. For this reason, the amplitude of the signal at

the time of sampling is measured and approximated to one of a finite set of values. The

figure below shows the principle of quantization applied to an analog signal. It can be seen

that a slight error is introduced in this process when the signal is quantized or

approximated. The degree of accuracy depends on the number of quantization levels used.

Within common telephony, 256 levels are used while in GSM 8,192 levels are used.

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CODING

Coding involves converting the quantized values into binary. Every value is represented by a

binary code of 13 bits (213 = 8192). For example, a quantized value of 2,157 would have a bit

pattern of 0100001101101

Bit 12 11 10 9 8 7 6 5 4 3 2 1 0 Total

Set to 0 1 0 0 0 0 1 1 0 1 1 0 1

Value 0 2048 0 0 0 0 64 32 0 8 4 0 1 2157

Channel Coding

The result from the process of A/D conversion is 8,000 samples per second of 13 bits each.

This is a bit rate of 104 kbits/s. When it is considered that 8 subscribers use one radio

channel, the overall bit rate would be 8 x 104 kbits/s = 832 kbits/s. Recalling the general

rule of 1 bit per Hertz, this bit rate would not fit into the 200 kHz available for all 8

subscribers. The bit rate must be reduced somehow - this is achieved using segmentation

and speech coding.

SPEECH CODING

The GSM group studied several voice coding algorithms on the basis of subjective

speech quality and complexity (which is related to cost, processing delay, and power

consumption once implemented) before arriving at the choice of a Regular Pulse Excited -

Linear Predictive Coder (RPELPC) with a Long Term Predictor loop. Basically,

information from previous samples, which does not change very quickly, is used to predict

the current sample. The coefficients of the linear combination of the previous samples,

plus an encoded form of the residual, the difference between the predicted and actual

sample, represent the signal. Speech is divided into 20 millisecond samples, each of which is

encoded as 260 bits, giving a total bit rate of 13 kbps.

CHANNEL CODING

Due to natural or manmade electromagnetic interference, the encoded speech or data

transmitted over the radio interface must be protected as much as is practical. The GSM

system uses,

> Encoding

> Block interleaving

To achieve this protection the method used for speech blocks will be described below.

Recall that the speech codec produces a 260-bit block for every 20 ms speech sample.

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From subjective testing, it was found that some bits of this block were more important for

perceived speech quality than others. The bits are thus divided into three classes:

Class Ia 50 bits - most sensitive to bit errors Class Ib

132 bits - moderately sensitive to bit errors Class II78

bits - least sensitive to bit errors

INTERLEAVING

To further protect against the burst errors common to the radio interface, each sample is

diagonally interleaved. The 456 bits output by the convolution encoder are divided into 8

blocks of 57 bits, and these blocks are transmitted in eight consecutive timeslot bursts.

Since each timeslot burst can carry two 57-bit blocks, each burst carries traffic from two

different speech samples.

MODULATION

Recall that each timeslot burst is transmitted at a gross bit rate of 270.833 kbps. This

digital signal is modulated onto the analog carrier frequency, which has a bandwidth of

200 kHz, using Gaussian filtered Minimum Shift Keying (GMSK). GMSK was selected

over other modulation schemes as a compromise between spectral efficiency, complexity of

the transmitter, and limited spurious emissions. The complexity of the transmitter is related

to power consumption, which should be minimized for the mobile station. The spurious

radio emissions, outside of the allotted bandwidth, must be strictly controlled so as to limit

adjacent channel interference, and allow for the coexistence of GSM and the older analog

systems (at least for the time being).

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2.4.3 GSM RADIO INTERFACE

The GSM radio interface is the bandwidth given to communicate through the air interface. In

GSM 900 system this is 890-915 MHz for uplink and 935-960 MHz for down link. In GSM

1800 system this is 1710-1785 MHz for uplink and 1805-1880 MHz for down link to provide full

duplex communication between Mobile Station and GSM network, for each communication a

separate carrier from uplink and down link is assigned. These carriers are time intervals from a

particular frequency those divided using Frequency Division Multiple Access (FDMA) and Time

Division Multiple Access (TDMA).

TIME DIVISION MULTIPLE ACCESS (TDMA)

Most digital cellular systems use the technique of Time Division Multiple Access (TDMA) to

transmit and receive speech signals. With TDMA, one carrier is used to carry a number of calls,

each call using that carrier at designated periods in time. These periods of time are referred to as

time slots. Each MS on a call is assigned one time slot on the uplink frequency and one on

the downlink frequency. Information sent during one time slot is called a burst. In GSM, a

TDMA frame consists of 8 time slots. This means that a GSM radio carrier can carry 8 calls.

TDMA FRAME

> Each carrier mentioned above is time divided. Each time division is

called a time slots [TS] or burst window.

> Each TDMA frame contains 8 TS having duration of 4.615ms.

> Each TS have duration of 577 microseconds.

> TS can carry a speech, data, or signaling packet of a given channel.

> A physical channel is composed by TS n [n=0 to 7] of consecutive

TDMA frames.

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3- Transmission

3.1Transmission Mediums used in Telecom industry

3.1.1Copper Cables

A)Twisted Pair cables

Twisted pair cabling is a type of wiring in which two conductors (the forward and return

conductors of a single circuit) are twisted together for the purposes of canceling out

electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation

from Unshielded Twisted Pair (UTP) cables, and crosstalk between neighboring pairs.

25 pair colour code chart

In balanced pair operation, the two wires carry equal and opposite signals and the destination

detects the difference between the two. This is known as differential mode transmission. Noise

sources introduce signals into the wires by coupling of electric or magnetic fields and tend to

couple to both wires equally. The noise thus produces a common-mode signal which is cancelled

at the receiver when the difference signal is taken. This method starts to fail when the noise

source is close to the signal wires; the closer wire will couple with the noise more strongly and

20

the common-mode rejection of the receiver will fail to eliminate it. This problem is especially

apparent in telecommunication cables where pairs in the same cable lie next to each other for

many miles. One pair can induce crosstalk in another and it is additive along the length of the

cable. Twisting the pairs counters this effect as on each half twist the wire nearest to the noise-

source is exchanged. Providing the interfering source remains uniform, or nearly so, over the

distance of a single twist, the induced noise will remain common-mode. Differential signaling

also reduces electromagnetic radiation from the cable, along with the associated attenuation

allowing for greater distance between exchanges.

The twist rate (also called pitch of the twist, usually defined in twists per meter) makes up part of

the specification for a given type of cable. Where nearby pairs have equal twist rates, the same

conductors of the different pairs may repeatedly lie next to each other, partially undoing the

benefits of differential mode. For this reason it is commonly specified that, at least for cables

containing small numbers of pairs, the twist rates must differ.

In contrast to FTP (foiled twisted pair) and STP (shielded twisted pair) cabling, UTP (unshielded

twisted pair) cable is not surrounded by any shielding. It is the primary wire type for telephone

usage and is very common for computer networking, especially as patch cables or temporary

network connections due to the high flexibility of the cables.

Unshielded twisted pair (UTP)

Twisted pair cables were first used in telephone systems by Alexander Graham Bell in 1881. By

1900, the entire American telephone line network was either twisted pair or open wire with

similar arrangements to guard against interference. Today, most of the millions of kilometres of

twisted pairs in the world are outdoor landlines, owned by telephone companies, used for voice

service, and only handled or even seen by telephone workers.

UTP cables are found in many Ethernet networks and telephone systems. For indoor telephone

applications, UTP is often grouped into sets of 25 pairs according to a standard 25-pair color

code originally developed by AT&T. A typical subset of these colors (white/blue, blue/white,

white/orange, orange/white) shows up in most UTP cables.

For urban outdoor telephone cables containing hundreds or thousands of pairs, the cable is

divided into smaller but identical bundles. Each bundle consists of twisted pairs that have

different twist rates. The bundles are in turn twisted together to make up the cable. Pairs having

the same twist rate within the cable can still experience some degree of crosstalk. Wire pairs are

selected carefully to minimize crosstalk within a large cable.

21

22

Cable shielding

Twisted pair cables are often shielded in attempt to prevent electromagnetic interference.

Because the shielding is made of metal, it may also serve as a ground. However, usually a

shielded or a screened twisted pair cable has a special grounding wire added called a drain wire.

This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is

applied to the collection of pairs, this is referred to as screening. The shielding must be grounded

for the shielding to work.

Screened unshielded twisted pair (S/UTP)

Also known as Fully shielded[ (or Foiled) Twisted Pair (FTP), is a screened UTP cable

(ScTP).

Shielded twisted pair (STP or STP-A)

STP cabling includes metal shielding over each individual pair of copper wires. This type

of shielding protects cable from external EMI (electromagnetic interferences). e.g. the

150 ohm shielded twisted pair cables defined by the IBM Cabling System specifications

and used with token ring networks.

Screened shielded twisted pair (S/STP or S/FTP)

S/STP cabling, also known as Screened Fully shielded Twisted Pair (S/FTP), is both

individually shielded (like STP cabling) and also has an outer metal shielding covering

the entire group of shielded copper pairs (like S/UTP). This type of cabling offers the best

protection from interference from external sources, and also eliminates alien crosstalk

Note that different vendors and authors use different terminology (i.e. STP has been used to

denote both STP-A, S/STP, and S/UTP).

23

24

25

Advantages

It is a thin, flexible cable that is easy to string between walls.

More lines can be run through the same wiring ducts.

UTP costs less per meter/foot than any other type of LAN cable.

Disadvantages

Twisted pair’s susceptibility to electromagnetic interference greatly depends on the pair

twisting schemes (usually patented by the manufacturers) staying intact during the

installation. As a result, twisted pair cables usually have stringent requirements for

maximum pulling tension as well as minimum bend radius. This relative fragility of

twisted pair cables makes the installation practices an important part of ensuring the

cable’s performance.

In video applications that send information across multiple parallel signal wires, twisted

pair cabling can introduce signaling delays known as skew which results in subtle color

defects and ghosting due to the image components not aligning correctly when

recombined in the display device. The skew occurs because twisted pairs within the same

cable often use a different number of twists per meter so as to prevent common-mode

crosstalk between pairs with identical numbers of twists. The skew can be compensated

26

by varying the length of pairs in the termination box, so as to introduce delay lines that

take up the slack between shorter and longer pairs, though the precise lengths required are

difficult to calculate and vary depending on the overall cable length.

B) Coaxial cables

Coaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a flexible,

tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from

the inner conductor and the outer shield sharing the same geometric axis. Coaxial cable was

invented by English engineer and mathematician Oliver Heaviside, who first patented the design

in 1880.

Coaxial cable is used as a transmission line for radio frequency signals, in applications such as

connecting radio transmitters and receivers with their antennas, computer network (Internet)

connections, and distributing cable television signals. One advantage of coax over other types of

transmission line is that in an ideal coaxial cable the electromagnetic field carrying the signal

exists only in the space between the inner and outer conductors. This allows coaxial cable runs to

be installed next to metal objects such as gutters without the power losses that occur in other

transmission lines, and provides protection of the signal from external electromagnetic

interference.

Coaxial cable differs from other shielded cable used for carrying lower frequency signals such as

audio signals, in that the dimensions of the cable are controlled to produce a repeatable and

predictable conductor spacing needed to function efficiently as a radio frequency transmission

line.

A: outer plastic sheath

B: woven copper shield

C: inner dielectric insulator

D: copper-plated core

RG-59 flexible coaxial cable.

27

How it works

Like any electrical power cord, coaxial cable conducts AC electric current between locations.

Like these other cables, it has two conductors, the central wire and the tubular shield. At any

moment the current is traveling outward from the source in one of the conductors, and returning

in the other. However, since it is alternating current, the current reverses direction many times a

second. Coaxial cable differs from other cable because it is designed to carry radio frequency

current. This has a frequency much higher than the 50 or 60 Hz used in mains (electric power)

cables, reversing direction millions to billions of times per second. Like other types of radio

transmission line, this requires special construction to prevent power losses:

If an ordinary wire is used to carry high frequency currents, the wire acts as an antenna, and the

high frequency currents radiate off the wire as radio waves, causing power losses. To prevent

this, in coaxial cable one of the conductors is formed into a tube and encloses the other

conductor. This confines the radio waves from the central conductor to the space inside the tube.

To prevent the outer conductor, or shield, from radiating, it is connected to electrical ground,

keeping it at a constant potential.

The dimensions and spacing of the conductors must be uniform. Any abrupt change in the

spacing of the two conductors along the cable tends to reflect radio frequency power back toward

the source, causing a condition called standing waves. This acts as a bottleneck, reducing the

amount of power reaching the destination end of the cable. To hold the shield at a uniform

distance from the central conductor, the space between the two is filled with a semirigid plastic

dielectric. Manufacturers specify a minimum bend radius[2]

to prevent kinks that would cause

reflections. The connectors used with coax are designed to hold the correct spacing through the

body of the connector.

Each type of coaxial cable has a characteristic impedance depending on its dimensions and

materials used, which is the ratio of the voltage to the current in the cable. In order to prevent

reflections at the destination end of the cable from causing standing waves, any equipment the

cable is attached to must present an impedance equal to the characteristic impedance (called

'matching'). Thus the equipment "appears" electrically similar to a continuation of the cable,

preventing reflections. Common values of characteristic impedance for coaxial cable are 50 and

75 ohms.

28

Coaxial cable cutaway

Description

Coaxial cable design choices affect physical size, frequency performance, attenuation, power

handling capabilities, flexibility, strength and cost. The inner conductor might be solid or

stranded; stranded is more flexible. To get better high-frequency performance, the inner

conductor may be silver plated. Sometimes copper-plated iron wire is used as an inner conductor.

The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or may be air

with spacers supporting the inner wire. The properties of dielectric control some electrical

properties of the cable. A common choice is a solid polyethylene (PE) insulator, used in lower-

loss cables. Solid Teflon (PTFE) is also used as an insulator. Some coaxial lines use air (or some

other gas) and have spacers to keep the inner conductor from touching the shield.

Many conventional coaxial cables use braided copper wire forming the shield. This allows the

cable to be flexible, but it also means there are gaps in the shield layer, and the inner dimension

of the shield varies slightly because the braid cannot be flat. Sometimes the braid is silver plated.

For better shield performance, some cables have a double-layer shield. The shield might be just

two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some

cables may invest in more than two shield layers, such as "quad-shield" which uses four

alternating layers of foil and braid. Other shield designs sacrifice flexibility for better

performance; some shields are a solid metal tube. Those cables cannot take sharp bends, as the

shield will kink, causing losses in the cable.

29

For high power radio-frequency transmission up to about 1 GHz coaxial cable with a solid

copper outer conductor is available in sizes of 0.25 inch upwards. The outer conductor is rippled

like a bellows to permit flexibility and the inner conductor is held in position by a plastic spiral to

approximate an air dielectric.

Coaxial cables require an internal structure of an insulating (dielectric) material to maintain the

spacing between the center conductor and shield. The dielectric losses increase in this order:

Ideal dielectric (no loss), vacuum, air, Polytetrafluoroethylene (PTFE), polyethylene foam, and

solid polyethylene. A low relative permittivity allows for higher frequency usage. An

inhomogeneous dielectric needs to be compensated by a non-circular conductor to avoid current

hot-spots.

Most cables have a solid dielectric; others have a foam dielectric which contains as much air as

possible to reduce the losses. Foam coax will have about 15% less attenuation but can absorb

moisture—especially at its many surfaces—in humid environments, increasing the loss. Stars or

spokes are even better but more expensive. Still more expensive were the air spaced coaxials

used for some inter-city communications in the middle 20th Century. The center conductor was

suspended by polyethylene discs every few centimeters. In a miniature coaxial cable such as an

RG-62 type, the inner conductor is supported by a spiral strand of polyethylene, so that an air

space exists between most of the conductor and the inside of the jacket. The lower dielectric

constant of air allows for a greater inner diameter at the same impedance and a greater outer

diameter at the same cutoff frequency, lowering ohmic losses. Inner conductors are sometimes

silver plated to smooth the surface and reduce losses due to skin effect. A rough surface prolongs

the path for the current and concentrates the current at peaks and thus increases ohmic losses.

The insulating jacket can be made from many materials. A common choice is PVC, but some

applications may require fire-resistant materials. Outdoor applications may require the jacket to

resist ultraviolet light and oxidation. For internal chassis connections the insulating jacket may be

omitted.

The ends of coaxial cables are usually made with RF connectors.

Signal propagation

Open wire transmission lines have the property that the electromagnetic wave propagating down

the line extends into the space surrounding the parallel wires. These lines have low loss, but also

have undesirable characteristics. They cannot be bent, twisted or otherwise shaped without

changing their characteristic impedance, causing reflection of the signal back toward the source.

They also cannot be run along or attached to anything conductive, as the extended fields will

induce currents in the nearby conductors causing unwanted radiation and detuning of the line.

30

Coaxial lines solve this problem by confining the electromagnetic wave to the area inside the

cable, between the center conductor and the shield. The transmission of energy in the line occurs

totally through the dielectric inside the cable between the conductors. Coaxial lines can therefore

be bent and moderately twisted without negative effects, and they can be strapped to conductive

supports without inducing unwanted currents in them. In radio-frequency applications up to a

few gigahertz, the wave propagates primarily in the transverse electric magnetic (TEM) mode,

which means that the electric and magnetic fields are both perpendicular to the direction of

propagation. However, above a certain cutoff frequency, transverse electric (TE) and/or

transverse magnetic (TM) modes can also propagate, as they do in a waveguide. It is usually

undesirable to transmit signals above the cutoff frequency, since it may cause multiple modes

with different phase velocities to propagate, interfering with each other. The outer diameter is

roughly inversely proportional to the cutoff frequency. A propagating surface-wave mode that

does not involve or require the outer shield but only a single central conductor also exists in coax

but this mode is effectively suppressed in coax of conventional geometry and common

impedance. Electric field lines for this TM mode have a longitudinal component and require line

lengths of a half-wavelength or longer.

Connectors

Coaxial connectors are designed to maintain a coaxial form across the connection and have the

same well-defined impedance as the attached cable. Connectors are often plated with high-

conductivity metals such as silver or gold. Due to the skin effect, the RF signal is only carried by

the plating and does not penetrate to the connector body. Although silver oxidizes quickly, the

silver oxide that is produced is still conductive. While this may pose a cosmetic issue, it does not

degrade performance.

A coaxial connector (male N-type)

31

3.1.2 Electromagnetic waves

Electromagnetic waves were first postulated by James Clerk Maxwell and subsequently

confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic

equations, revealing the wave-like nature of electric and magnetic fields, and their symmetry.

Because the speed of EM waves predicted by the wave equation coincided with the measured

speed of light, Maxwell concluded that light itself is an EM wave.

According to Maxwell's equations, a spatially-varying electric field generates a time-varying

magnetic field and vice versa. Therefore, as an oscillating electric field generates an oscillating

magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These

oscillating fields together form an electromagnetic wave.

A quantum theory of the interaction between electromagnetic radiation and matter such as

electrons is described by the theory of quantum electrodynamics.

The physics of electromagnetic radiation is electrodynamics. Electromagnetism is the physical

phenomenon associated with the theory of electrodynamics. Electric and magnetic fields obey

the properties of superposition so that a field due to any particular particle or time-varying

electric or magnetic field will contribute to the fields present in the same space due to other

causes: as they are vector fields, all magnetic and electric field vectors add together according to

vector addition. For instance, a travelling EM wave incident on an atomic structure induces

oscillation in the atoms of that structure, thereby causing them to emit their own EM waves,

emissions which alter the impinging wave through interference. These properties cause various

phenomena including refraction and diffraction.

Since light is an oscillation it is not affected by travelling through static electric or magnetic

fields in a linear medium such as a vacuum. However in nonlinear media, such as some crystals,

interactions can occur between light and static electric and magnetic fields — these interactions

include the Faraday Effect and the Kerr effect.

In refraction, a wave crossing from one medium to another of different density alters its speed

and direction upon entering the new medium. The ratio of the refractive indices of the media

determines the degree of refraction, and is summarized by Snell's law. Light disperses into a

visible spectrum as light is shone through a prism because of the wavelength dependent

refractive index of the prism material (Dispersion).

EM radiation exhibits both wave properties and particle properties at the same time (see wave-

particle duality). Both wave and particle characteristics have been confirmed in a large number

of experiments. Wave characteristics are more apparent when EM radiation is measured over

32

relatively large timescales and over large distances while particle characteristics are more evident

when measuring small timescales and distances. For example, when electromagnetic radiation is

absorbed by matter, particle-like properties will be more obvious when the average number of

photons in the cube of the relevant wavelength is much smaller than 1. Upon absorption of light,

it is not too difficult to experimentally observe non-uniform deposition of energy. Strictly

speaking, however, this alone is not evidence of "particulate" behavior of light, rather it reflects

the quantum nature of matter

There are experiments in which the wave and particle natures of electromagnetic waves appear in

the same experiment, such as the self-interference of a single photon. True single-photon

experiments (in a quantum optical sense) can be done today in undergraduate-level labs.[2]

When

a single photon is sent through an interferometer, it passes through both paths, interfering with

itself, as waves do, yet is detected by a photomultiplier or other sensitive detector only once.

Shows three electromagnetic modes (blue, green and red) with a distance scale in micrometres

along the x-axis.

33

Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of

electric and magnetic fields. This diagram shows a plane linearly polarized wave propagating

from right to left. The electric field is in a vertical plane, the magnetic field in a horizontal plane.

Electromagnetic spectrum

Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and

near fields) is classified by wavelength into radio, microwave, infrared, the visible region we

perceive as light, ultraviolet, X-rays and gamma rays. Arbitrary electromagnetic waves can

always be expressed by Fourier analysis in terms of sinusoidal monochromatic waves which can

be classified into these regions of the spectrum.

The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter

wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with

single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.

Spectroscopy can detect a much wider region of the EM spectrum than the visible range of

400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to

2500 nm. Detailed information about the physical properties of objects, gases, or even stars can

34

be obtained from this type of device. It is widely used in astrophysics. For example, hydrogen

atoms emit radio waves of wavelength 21.12 cm.

Legend: γ = Gamma rays

HX = Hard X-rays

SX = Soft X-Rays

EUV = Extreme ultraviolet

NUV = Near ultraviolet

Visible light

NIR = Near infrared

MIR = Moderate infrared

FIR = Far infrared

Radio waves: EHF = Extremely high

frequency (Microwaves)

SHF = Super high

frequency (Microwaves)

UHF = Ultrahigh

frequency

VHF = Very high

frequency

HF = High frequency

MF = Medium frequency

LF = Low frequency

VLF = Very low frequency

VF = Voice frequency

ULF = Ultra low

frequency

SLF = Super low

frequency

ELF = Extremely low

frequency

35

3.1.3Optical fibers

An optical fiber is made up of the core (carrying the light pulses), the cladding (reflecting the

light pulses back into the core) and the buffer coating (protecting the core and cladding from

moisture, damage, etc). Together, all of this creates a fiber optic which can carry up to 10 million

messages at any time using light pulses. Fiber optics is the overlap of applied science and

engineering concerned with the design and application of optical fibers. Optical fibers are widely

used in fiber-optic communications, which permits transmission over longer distances and at

higher bandwidths (data rates) than other forms of communications. Fibers are used instead of

metal wires because signals travel along them with less loss and are also immune to

electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so

they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers

are used for a variety of other applications, including sensors and fiber lasers.

Light is kept in the core of the optical fiber by total internal reflection. This causes the fiber to act

as a waveguide. Fibers which support many propagation paths or transverse modes are called

multi-mode fibers (MMF), while those which can only support a single mode are called single-

mode fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for

short-distance communication links and for applications where high power must be transmitted.

Single-mode fibers are used for most communication links longer than 550 meters (1,800 ft).

Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of

the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing

them together with an electric arc. Special connectors are used to make removable connections.

A bundle of optical fibers

36

Applications

Optical fiber communication

Optical fiber can be used as a medium for telecommunication and networking because it is

flexible and can be bundled as cables. It is especially advantageous for long-distance

communications, because light propagates through the fiber with little attenuation compared to

electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the

per-channel light signals propagating in the fiber have been modulated at rates as high as 111

gigabits per second by NTT, although 10 or 40 Gb/s is typical in deployed systems. Each fiber

can carry many independent channels, each using a different wavelength of light (wavelength-

division multiplexing (WDM)). The net data rate (data rate without overhead bytes) per fiber is

the per-channel data rate reduced by the FEC overhead, multiplied by the number of channels

(usually up to eighty in commercial dense WDM systems as of 2008). The current laboratory

fiber optic data rate record, held by Bell Labs in Villarceaux, France, is multiplexing 155

channels, each carrying 100 Gb/s over a 7000 km fiber.[19]

Nippon Telegraph and Telephone

Corporation have also managed 69.1 Tb/s over a single 240km fibre (multiplexing 432 channels,

equating to 171 Gb/s per channel). Bell Labs also broke a 100 Petabit per second kilometer

barrier.

For short distance applications, such as creating a network within an office building, fiber-optic

cabling can be used to save space in cable ducts. This is because a single fiber can often carry

much more data than many electrical cables, such as 4 pair Cat-5 Ethernet cabling.[vague]

Fiber is

also immune to electrical interference; there is no cross-talk between signals in different cables

and no pickup of environmental noise. Non-armored fiber cables do not conduct electricity,

which makes fiber a good solution for protecting communications equipment located in high

voltage environments such as power generation facilities, or metal communication structures

prone to lightning strikes. They can also be used in environments where explosive fumes are

present, without danger of ignition. Wiretapping is more difficult compared to electrical

connections, and there are concentric dual core fibers that are said to be tap-proof.

Fiber optic sensors

Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical

fiber. In other cases, fiber is used to connect a non-fiberoptic sensor to a measurement system.

Depending on the application, fiber may be used because of its small size, or the fact that no

electrical power is needed at the remote location, or because many sensors can be multiplexed

along the length of a fiber by using different wavelengths of light for each sensor, or by sensing

37

the time delay as light passes along the fiber through each sensor. Time delay can be determined

using a device such as an optical time-domain reflectometer.

Optical fibers can be used as sensors to measure strain, temperature, pressure and other quantities

by modifying a fiber so that the quantity to be measured modulates the intensity, phase,

polarization, wavelength or transit time of light in the fiber. Sensors that vary the intensity of

light are the simplest, since only a simple source and detector are required. A particularly useful

feature of such fiber optic sensors is that they can, if required, provide distributed sensing over

distances of up to one meter.

Extrinsic fiber optic sensors use an optical fiber cable, normally a multi-mode one, to transmit

modulated light from either a non-fiber optical sensor, or an electronic sensor connected to an

optical transmitter. A major benefit of extrinsic sensors is their ability to reach places which are

otherwise inaccessible. An example is the measurement of temperature inside aircraft jet engines

by using a fiber to transmit radiation into a radiation pyrometer located outside the engine.

Extrinsic sensors can also be used in the same way to measure the internal temperature of

electrical transformers, where the extreme electromagnetic fields present make other

measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation,

displacement, velocity, acceleration, torque, and twisting.

Principle of operation

An optical fiber is a cylindrical dielectric waveguide (nonconducting waveguide) that transmits

light along its axis, by the process of total internal reflection. The fiber consists of a core

surrounded by a cladding layer, both of which are made of dielectric materials. To confine the

optical signal in the core, the refractive index of the core must be greater than that of the

cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber,

or gradual, in graded-index fiber

Total internal reflection

When light traveling in a dense medium hits a boundary at a steep angle (larger than the "critical

angle" for the boundary), the light will be completely reflected. This effect is used in optical

fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the

boundary. Because the light must strike the boundary with an angle greater than the critical

angle, only light that enters the fiber within a certain range of angles can travel down the fiber

without leaking out. This range of angles is called the acceptance cone of the fiber. The size of

this acceptance cone is a function of the refractive index difference between the fiber's core and

cladding.

38

In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber

so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the

numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and

work with than fiber with a smaller NA. Single-mode fiber has a small NA.

Multi-mode optical fiber

Fiber with large core diameter (greater than 10 micrometers) may be analyzed by geometrical

optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis (see below). In a

step-index multi-mode fiber, rays of light are guided along the fiber core by total internal

reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line

normal to the boundary), greater than the critical angle for this boundary, are completely

reflected. The critical angle (minimum angle for total internal reflection) is determined by the

difference in index of refraction between the core and cladding materials. Rays that meet the

boundary at a low angle are refracted from the core into the cladding, and do not convey light

and hence information along the fiber. The critical angle determines the acceptance angle of the

fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate

down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of

light into the fiber. However, this high numerical aperture increases the amount of dispersion as

rays at different angles have different path lengths and therefore take different times to traverse

the fiber.

In graded-index fiber, the index of refraction in the core decreases continuously between the axis

and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather

than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce

multi-path dispersion because high angle rays pass more through the lower-index periphery of

39

the core, rather than the high-index center. The index profile is chosen to minimize the difference

in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close

to a parabolic relationship between the index and the distance from the axis.

Single-mode fiber

The structure of a typical single-mode fiber.

1. Core: 8 µm diameter

2. Cladding: 125 µm dia.

3. Buffer: 250 µm dia.

4. Jacket: 400 µm dia.

Main article: Single-mode optical fiber

Fiber with a core diameter less than about ten times the wavelength of the propagating light

cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic

structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation.

The electromagnetic analysis may also be required to understand behaviors such as speckle that

occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber

supports one or more confined transverse modes by which light can propagate along the fiber.

Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of

larger-core multi-mode fiber can also be modeled using the wave equation, which shows that

such fiber supports more than one mode of propagation (hence the name). The results of such

modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if

the fiber core is large enough to support more than a few modes.

The waveguide analysis shows that the light energy in the fiber is not completely confined in the

core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound

mode travels in the cladding as an evanescent wave.

The most common type of single-mode fiber has a core diameter of 8–10 micrometers and is

designed for use in the near infrared. The mode structure depends on the wavelength of the light

used, so that this fiber actually supports a small number of additional modes at visible

wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as

50 micrometers and as large as hundreds of micrometers. The normalized frequency V for this

fiber should be less than the first zero of the Bessel function J0 (approximately 2.405).

40

Practical issues

In practical fibers, the cladding is usually coated with a tough resin buffer layer, which may be

further surrounded by a jacket layer, usually plastic. These layers add strength to the fiber but do

not contribute to its optical wave guide properties. Rigid fiber assemblies sometimes put light-

absorbing ("dark") glass between the fibers, to prevent light that leaks out of one fiber from

entering another. This reduces cross-talk between the fibers, or reduces flare in fiber bundle

imaging applications.

Modern cables come in a wide variety of sheathings and armor, designed for applications such as

direct burial in trenches, high voltage isolation, dual use as power lines installation in conduit,

lashing to aerial telephone poles, submarine installation, and insertion in paved streets. The cost

of small fiber-count pole-mounted cables has greatly decreased due to the high Japanese and

South Korean demand for fiber to the home (FTTH) installations.

Fiber cable can be very flexible, but traditional fiber's loss increases greatly if the fiber is bent

with a radius smaller than around 30 mm. This creates a problem when the cable is bent around

corners or wound around a spool, making FTTX installations more complicated. "Bendable

fibers", targeted towards easier installation in home environments, have been standardized as

ITU-T G.657. This type of fiber can be bent with a radius as low as 7.5 mm without adverse

impact. Even more bendable fibers have been developed. Bendable fiber may also be resistant to

fiber hacking, in which the signal in a fiber is surreptitiously monitored by bending the fiber and

detecting the leakage.

Another important feature of cable is cable withstanding against the horizontally applied force. It

is technically called max tensile strength defining how much force can applied to the cable

during the installation of a period.

Telecom Anatolia fiber optic cable versions are reinforced with aramid yarns or glass yarns as

intermediary strength member. In commercial terms, usage of the glass yarns are more cost

effective while no loss in mechanical durability of the cable. Glass yarns also protect the cable

core against rodents and termites.

Termination and splicing

Optical fibers are connected to terminal equipment by optical fiber connectors. These connectors

are usually of a standard type such as FC, SC, ST, LC, or MTRJ.

Optical fibers may be connected to each other by connectors or by splicing, that is, joining two

fibers together to form a continuous optical waveguide. The generally accepted splicing method

41

is arc fusion splicing, which melts the fiber ends together with an electric arc. For quicker

fastening jobs, a "mechanical splice" is used.

Fusion splicing is done with a specialized instrument that typically operates as follows: The two

cable ends are fastened inside a splice enclosure that will protect the splices, and the fiber ends

are stripped of their protective polymer coating (as well as the more sturdy outer jacket, if

present). The ends are cleaved (cut) with a precision cleaver to make them perpendicular, and are

placed into special holders in the splicer. The splice is usually inspected via a magnified viewing

screen to check the cleaves before and after the splice. The splicer uses small motors to align the

end faces together, and emits a small spark between electrodes at the gap to burn off dust and

moisture. Then the splicer generates a larger spark that raises the temperature above the melting

point of the glass, fusing the ends together permanently. The location and energy of the spark is

carefully controlled so that the molten core and cladding do not mix, and this minimizes optical

loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on

one side and measuring the light leaking from the cladding on the other side. A splice loss under

0.1 dB is typical. The complexity of this process makes fiber splicing much more difficult than

splicing copper wire.

Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need

for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held

together by a precision-made sleeve, often using a clear index-matching gel that enhances the

transmission of light across the joint. Such joints typically have higher optical loss and are less

robust than fusion splices, especially if the gel is used. All splicing techniques involve the use of

an enclosure into which the splice is placed for protection afterward.

Fibers are terminated in connectors so that the fiber end is held at the end face precisely and

securely. A fiber-optic connector is basically a rigid cylindrical barrel surrounded by a sleeve

that holds the barrel in its mating socket. The mating mechanism can be "push and click", "turn

and latch" ("bayonet"), or screw-in (threaded). A typical connector is installed by preparing the

fiber end and inserting it into the rear of the connector body. Quick-set adhesive is usually used

so the fiber is held securely, and a strain relief is secured to the rear. Once the adhesive has set,

the fiber's end is polished to a mirror finish. Various polish profiles are used, depending on the

type of fiber and the application. For single-mode fiber, the fiber ends are typically polished with

a slight curvature, such that when the connectors are mated the fibers touch only at their cores.

This is known as a "physical contact" (PC) polish. The curved surface may be polished at an

angle, to make an "angled physical contact" (APC) connection. Such connections have higher

loss than PC connections, but greatly reduced back reflection, because light that reflects from the

angled surface leaks out of the fiber core; the resulting loss in signal strength is known as gap

loss. APC fiber ends have low back reflection even when disconnected.

42

In the mid 1990's fiber optic cable termination was very labor intensive with many different parts

per connector, fiber polishing and the need for an oven to bake the epoxy in each connector made

terminating fiber optic very hard and labor intensive.

Today many different connectors are on the market and offer an easier less labor intensive way

of terminating fiber optic cable.

Some of the most popular connectors have already been polished from the factory and include a

gel inside the connector and those two steps help save money on labor especially on large

projects. A cleave is made at a required length in order to get as close to the polished piece

already inside the connector, with the gel surrounding the point where the two piece meet inside

the connector very little light loss is exposed. Here’s an example of a newer style connector

being terminated.

ST connectors on multi-mode fiber

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3.2 Mobile Site Planing

As RND section responsible for the radio network between mobile stations to radio base

station. From RBS to switch path is maintained by transmission section. Then it is obvious that

they must be connected to the switch to work the network. Transmission people do this job.

Their work can be classified as follows.

> Planning new links

> Implementing new links

> Maintaining existing links

3.2.1 PLANNING NEW LINKS

For this the suitable path should be found and the parameters of the link should be checked for

required levels. This is done using different analyzing methods and instruments. When planning

a microwave link, between two sites, there are several aspects to be considered. Out of these

factors the most important factor is the obstruction free path between the relevant points. It is

termed as "Line Of Sight" (Figure 01). Therefore the first factor of a microwave link is the

LOS. An optical line of sight exists if an imaginary straight line can be drawn connecting the

antennas on either side of the link.

Line Of Sight Path between Two Towers

3.3 IMPLEMENTING NEW LINKS

After planning they try to implement the new link. Normally two teams go to the

implementing process. One team does the job at one side and the other team does their job at

the other side.

3.4 INDOOR UNITS

3.4.1 ACCESS MODULE MAGAZINE (AMM)

The AMM houses the plug-in units and is designed for fitting in a 19" rack or cabinet. There are

two types of AMM.

> AMM 1U-1 is used for 1+0 terminals and can house one MMU and one TRU.

> AMM 2U-4 is mainly used for 1+1 or two 1+0 terminals and can house up to four

units; two MMUs and two TRUs.

44

The plug-in units are inserted into the AMM from the front. The connection between the plug-

in units is made through the backplane of the AMM. All indicators and external connector

interfaces are located on the fronts of the plug-in units.

Cables are routed to the left and right hand side of the front. The AMM has a front panel to

protect the cables and connections. Indicators are visible through the front panel. Tools, used for

removal of the plug-in units, are attached to the inside of the front panel.

3.4.2 PLUG-IN UNITS FOR THE AMM

> Modem unit (MMU)

> Traffic Unit (TRU)

MODEM UNIT (MMU)

The MMU is the indoor interface with the radio unit and contains a

modulator/demodulator. The MMU provides traffics and capacity of 155Mbit/s and is

frequency independent. One MMU per radio unit is required.

TRAFFIC UNIT (TRU)

The main functions of the TRU is the generating and terminating an SDH STM-1 or SONET

OC-3 signal and transmit it to or receive it from the MMU. It also contains a protection

switching function used for protected terminal configuration. One TRU per terminal is

required. Besides the main traffic (155Mbit/s), there are three auxiliary channels; one

channel for wayside traffic and two service channels.

The TRU comes in two versions; the TRU EL. With electrical traffic interface, and the TRU

EL/OPT with both electrical and optical traffic interfaces.

FAN UNIT

To guarantee sufficient cooling for the plug-in units, a fan unit is always fitted on top of the

AMM. One fan unit per AMM is required.

The cooling air enters at front of the AMM, flows between the units and out through openings

at the back of the AMM.

DC DISTRIBUTION UNIT (DDU)

The optional Dc distribution unit is used for distribution of primary Dc power to a

maximum of five MMUs or fan units. Each output is protected by an automatic type fuse (6A)

combined with an on/off switch.

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3.5 OUTDOOR UNITS

> Radio Unit

> Antenna

3.5.1 RADIO UNIT

The radio unit is a microwave radio with RF transceivers, which transmit and receive RF

signals. Traffic signals from the indoor units are processed and converted to transmitter

frequency and sent over the hop.

The radio unit is fitted directly to the antenna as standard. It can also be installed

separately and connected by a flexible waveguide to any antenna with standard waveguide

interface (154 IEC-UBR). It can be disconnected and replaced without affecting the antenna

alignment. There are connections for antenna alignment, radio cable and grounding. Two

LEDs indicate alarm and power on/off.

3.5.2 ANTENNA

Five different antenna types, fitting directly to the radio units, are available. 0.2m, 0.3m,

0.6m, 1.2m, and 1.8m are the available compact antennas. All antennas can also be

installed separately and connected to the radio unit by a flexible waveguide.

It is possible to choose between vertical and horizontal polarization. The antenna is fitted

on an antenna support and does not have to be removed during maintenance after

alignment.

3.5.3 MAINTAINING EXISTING LINKS

When the network grows up traffic demand get also increase. Also the new technologies come

to the market and company uses those new technologies then existing links must update. The

fault recovery of existing links has done by transmission section.

3.5.4 TEMS LINK PLANNER

This is the software used in link planning. This is an Ericsson product. According to our

requirements we can plan the link in this software and we can find out the availability and

performance of that according to the predefined performance criteria's fed in to the

software.

TEMS Link Planner uses a digital map database of Sri Lanka which is in Geobox format

(Geobox format is an Ericsson internally developed format). This map has very high

resolution is rich with all the geographical information of Sri Lanka. It contains

information such as,

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• Over view/ key map

• Elevation

• Land usage

• Main roads and other roads

• Rivers and Lakes

On TEMS link planner we can define different map version to help us and to protect our work,

there are map versions such as training version which use for training purposes, stage 1 ,2 and

3 versions which represents links Mobitel's different projects. It is very easy to design a TX link

using TEMS Link Planner. In order to design a link first we have to select a proper map version

and we must import necessary data to that map such as height data, Land usage, and existing

sites. Then we can implement the link by selecting the two end position of the link. In order to

measure the actual performance of the path we created we have to define several parameters and

configure the path.

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4 – GSM identities

4.1 Mobile Station ISDN Number (MSISDN)

This is the directory number of the subscriber. This is the number that should be dialed in order

to initiate a conversation. This is allocated to an IMSI number in the HLR. It consists of three

parts,

MSISDN = CC + NDC + SN

CC : Country Code

NDC : National Destination Code

SN : Subscriber Number

4.2 International Mobile Subscriber Identity (IMSI)

IMSI is a unique identity allocated to each subscriber. It is written in the SIM card and never

known by the subscriber. It is used for the correct identification over the radio path and through

the GSM network. The IMSI is also stored in the AuC and used for authentication. According to

the GSM specifications, IMSI has a maximum length of 15 digits. This number consists of three

parts,

IMSI = MCC + MNC + MSIN

MCC : Mobile Country Code

MNC : Mobile Network Code

MSIN : Mobile Station Identification Number

(The 1st two digits H1, H2 identify the HLR)

4.3 International Mobile Equipment Identity (IMEI)

This number is used for equipment identification and uniquely identifies Mobile Station

hardware as a separate assembly of equipment. This number is used for emergency calls with no

SIM card. IMEI can be used to identify a stolen or not approved type of MS by utilizing an EIR

though this feature is not implemented in the Dialog Network. The IMEI has the following

format,

IMEI = TAC + FAC + SNR + SVN

TAC : Type Approval Code – code given to the manufacturer for approved mobile equipment.

FAC : Final Assembly Code - identifies the factory code.

MCC (3) MNC (2) MSIN(10)

National Identity

IMSI

CC NDC SN

National Mobile Number

International Mobile Number (MSISDN)

TAC SVN SNR FAC

6 Digits 6 Digits 6 Digits 6 Digits

IMEI

IMEISV

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SNR : Serial Number - an individual serial number of six digits which uniquely identifies all

equipment within each TAC and FAC.

SVN : Software Version Number - identify different software versions.

4.4 Temporary Mobile Subscriber Identity (TMSI)

This identity is used to protect the subscriber’s privacy on the air interface. The TMSI is

significant only within the MSC/VLR area and hence its structure can be determined by each

operator.

4.5 Mobile Station Roaming Number (MSRN)

When a call is made to a mobile subscriber, the HLR requests the current MSC/VLR to provide

an MSRN as a temporary routing number for the subscriber that gets the call. Upon reception of

the MSRN, the HLR sends it to the GMSC that is now able to use this number to route the call to

the MSC/VLR exchange where the subscriber that got the call is registered. The MSRN is also

used to protect the subscriber’s privacy on the air interface.

4.6 Location Area Identity (LAI)

A LAI makes possible to identify each location area in the world where a subscriber can be. A

single Location Area can be composed of several cells managed by one or several BSCs but

depending only on a single MSC/VLR. LA is used during paging and location update procedures.

The LAI comprises the following,

LAI = MCC + MNC + LAC

MCC : Mobile Country Code – identify the home country

MNC : Mobile Network Code – identify the operator

LAC : Location Area Code – identify the location area in the PLMN

CGI : Cell Global Identification – unique identity given to each cell

CI : Cell Identifier

LAI

MCC MNC LAC CI

CGI

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5 - Operations Section

Operation section is the department which is responsible of installment, maintenance and

repairing of radio base stations. Radio base station (RBS) is the interface between mobile

subscriber and the network. It provides radio coverage to the subscriber through the radio

antenna. Operations department look after radio base station equipments as well as cooling

(AC) units and power supplies used in the base stations. Mobitel uses types of RBS

equipments they are namely,

• RBS 2206

• RBS 2207

Both of them are products of Erricsson. There is no significant difference between these two

equipments except for the fact that RBS 2206 and all its related units operates in GSM 1800

frequency and RBS 2207 and all its related equipments operates in GSM 900 frequency band.

There are some TDMA (Time division multiple access) radio base stations also used in

Mobitel but there are only few sites operates in TDMA. TDMA is the technology which is

used before GSM. Mobitel still has some customers who are using TDMA, but they are

encouraged to migrate to GSM.

5.1 ANTENNAS USED IN BASE STATIONS

IN its base stations Mobitel uses sector antennas to provide radio coverage to the

subscribers. These sector antennas normally transmit with a transmission power around -35 to

- 40 db. This power level is adjusted by RBS. Sector antennas that we use in Mobitel

support polarization diversity in order to increase its receiver sensitivity. In polarization

diversity the receiver antenna has two antenna arrays one with -45º and other with +45º angles.

Some times we use dual frequency antennas. Dual frequency antennas are capable of

operating in two separate frequency bands. These are used in sites where there is both GSM

900 and GSM 1800 radio base stations are present, so without having two separate antennas

we can have single dual frequency antenna.

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5.2 RBS (RADIO BASE STATION)

RBS is a high capacity indoor base station. This is capable of handling up to 6 carriers.

RBS is a fully assembled cabinet. All plug-in units in the cabinet are accessible from the

front of the cabinet.

MAIN FEATURES

• Capability of using 1, 2 or 3 sectors in one cabinet

• Support co-siting (Antenna sharing)

• Discontinuous transmission and reception

• Duplex filters

• Encryption and ciphering

• Support for EDGE technology

• Frequency hopping

• External alarms

• Receiver diversity support

• Support different transmission interfaces( T1 1544 kbit/s with 100 Ω, E1 2048

kbit/s with 75 Ω and E1 2048 kbit/s with 120 Ω)

• Wide range power input (120 - 250 V AC, 50/60 Hz)

STANDARD HARDWARE UNITS

• CDU (Combine and distribution unit)

• CXU (Configuration switch unit)

• ACCU (AC connection unit)

• DCCU (DC connection unit)

• DTRU (Double transceiver unit)

• DXU (Distribution switch unit)

• FAU (Fan unit)

• IDM (Internal distribution module)

• PSU (Power supply unit)

• DC filter

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OPTIONAL HARDWARE UNITS

• ASU (Antenna sharing unit)

• BBS (Battery back-up system)

• ddTMA (Dual duplex tower mounted amplifier- normally TMA is an mandatory

equipment for GSM 1800)

• TMA-CM (Tower mounted amplifier control module)

• Bias injector

• DXX (Digital cross connector)

diagram of RBS 2206 equipment

Connection fields 11 and power connection

CXU, ASU and OXU

dummies

> DTRU

PSU, DXU

and OXU dummies

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5.2.1 CDU- COMBINE AND DISTRIBUTION UNIT

The CDU is the interface between the transceivers and the antenna system. All signals

are filtered before transmission and after reception using Bandpass filter. CDU allows

several antennas to share antennas. There a maximum of three CDUs in one RBS 2206 or

2207.

The CDU combines transmitted signals from several transceivers to one Tx signal, and

distributes the received signal to several transceivers. It provides simultaneous

transmission and reception on one antenna. CDU support base band hopping. This

amplifies two Rx signals from two Rx antennas for further distribution in the CXU.

There two CDUs available,

• CDU-F , This is intended for high capacity solutions

• CDU-G, Can be configured either foe high capacity or high coverage

Diagram of CDU-G

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5.2.2 CXU- CONFIGURATION SWITCH UNIT

The CXU cross connects the CDU and the DTRU in the receiver path. CXU supports

both GMSK and 8- PSK. One CXU can support up to three CDUs. CXU is configured by

means of software. Following is a figure of CXU.

5.2.3 DCCU- DC CONNECTION UNIT

The DCCU handles distribution and connection/disconnection of the incoming DC

power supply voltages to the PSUs. This unit also contains a filter unit in order to filter

the supply voltages.

5.2.4 DTRU- DOUBLE TRANSCEIVER UNIT

The DTRU is a replaceable unit for two transceiver units. A transceiver is a

transmitter/receiver and signal-processing unit, which transmit and receives one carrier.

The DTRU has two Tx antenna terminals and four Rx antenna terminals. The DTRU has a

built-in hybrid combiner; this hybrid combiner can be used to combine the two Tx

antenna terminals in to one common terminal. Two of the TX antenna terminals are used

for 2-brabch polarization diversity reception, the DTRU is hardware prepared for 4-

branch diversity reception through the remaining two antennas. Following figure shows a

DTRU,

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5.2.5 DXU- DISTRIBUTION SWITCH UNIT

DXU is the central control unit for the RBS. It is simply the CPU of the RBS. It acts as a

interface between transmission network and the transceivers. It also extracts timing

information from transmission interfaces and generates a timing reference for the RBS. The

DXU handles incoming traffic, controls and supervises information and sends it to its

destination within the RBS. It the power and climate equipment for the RBS and it collects

and transmit alarms to the BSC.

5.2.6 TMA- TOWER MOUNTED AMPLIFIER

The tower mounted amplifier compensates for signal loses in the receiver antenna cables in

amplifies the receiving signal, reduces the system noise and improve the uplink sensitivity.

The ddTMA (Dual duplex tower mounted amplifier) consists of a duplex filter. Duplex is

the function that allows communication in two directions (sending and receiving) on one

channel. Two TMAs are required for one sector antenna. One for the Rx 1 and other one for

the Rx 2 (Polarization diversity).

The DC power supply to the TMA is provided through a bias injector over the Tx/Rx feeder

cables using the TMA-CM (Tower mounted amplifier control module). This control module

is also used to identify TMA faults. One TMA -CM can handle up to 6 bias injectors. And

one bias injector is needed for each TMA.

Following is a diagram showing interconnection between CDU, CXU and DTRU. Here two

DTRUs are used for a single sector. So that means this sector transmits and receives signals

related to four carrier frequencies.

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6 – Data Communication using GSM

6.1 GENERAL PACKET RADIO SERVICE (GPRS)

GPRS is a packet switched technology used in GSM to data transfer. It provides a basic

solution for Internet Protocol (IP) communication between Mobile Stations (MS) and

Internet Service Hosts (IH) or a corporate LAN. Here the radio network resources are used

only when data is being actually transmitted. Thus billing for GPRS is based on amount of

data transmitted, not on the duration as in packet switched technologies

6.1.1KEY USER FEATURES OF GPRS

> Speed

> Immediacy

> New applications and better applications

SPEED

Theoretical maximum speeds of up to 171.2 kbps are achievable with GPRS using all eight

timeslots at the same time. This is about three times as fast as the data transmission speeds

possible over today's fixed telecommunications networks and ten times as fast as current

Circuit Switched Data services on GSM networks. By allowing information to be

transmitted more quickly, immediately and efficiently across the mobile network, GPRS

may well be a relatively less costly mobile data service compared to SMS and Circuit

Switched Data.

IMMEDIACY

GPRS facilitates instant connections whereby information can be sent or received

immediately as the need arises, subject to radio coverage. No dial-up modem connection is

necessary. This is why GPRS users are sometimes referred to be as being "always

connected". Immediacy is one of the advantages of GPRS when compared to Circuit

Switched Data. High immediacy is a very important feature for time critical applications

such as remote credit card authorization where it would be unacceptable to keep the

customer waiting for even thirty extra seconds.

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6.1.2 KEY NETWORK FEATURES OF GPRS

> Packet Switching

> Spectrum Efficiency

PACKET SWITCHING

GPRS involves overlaying a packet based air interface on the existing circuit

switched GSM network. This gives the user an option to use a packet-based data

service. To supplement circuit switched network architecture with packet switching

is quite a major upgrade. With GPRS, the information is split into separate but

related "packets" before being transmitted and reassembled at the receiving end.

Packet switching is similar to a jigsaw puzzle- the image that the puzzle

represents is divided into pieces at the manufacturing factory and put into a plastic

bag. During transportation of the now boxed jigsaw from the factory to the end user,

the pieces get jumbled up. When the recipient empties the bag with all the pieces,

they are reassembled to form the original image. All the pieces are all related and fit

together, but the way they are transported and assembled varies. The Internet itself is

another example of a packet data network, the most famous of many such network

types.

SPECTRUM EFFICIENCY

Packet switching means that GPRS radio resources are used only when users are actually

sending or receiving data. Rather than dedicating a radio channel to a mobile data

user for a fixed period of time, the available radio resource can be concurrently

shared between several users. This efficient use of scarce radio resources means

that large numbers of GPRS users can potentially share the same bandwidth and be

served from a single cell. The actual number of users supported depends on the

application being used and how much data is being transferred. Because of the

spectrum efficiency of GPRS, there is less need to build in idle capacity that is only

used in peak hours. GPRS therefore lets network operators maximize the use of

their network resources in a dynamic and flexible way, along with user access to

resources and revenues.

GPRS uses cording systems to code actual data to be sent over the network. There

are 4 coding systems present,

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Coding System Maximum data rate

supported (kbps)

CS1 9.6

CS2 13.6

CS3 15

CS4 21.4

We can see that the maximum data rate that can be achieved on a channel is 21.4

kbps. So GPRS can achieve a data rate of 171.2 kbps (8* 21.4 kbps). But as the

number of subscribers supported by the carrier increases, the data rate per user

decreases

.

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External IP Net wo r (Internet

IP-Backbone Network External IP

Network (Corporate

External X.25 Network Traffic and signaling

1 Signaling

"E Terminal Equipment

MT Mobile Terminal

MS Mobile Station

BSS Base Station System

BTS Base Transceiver Station

BSC Base Station Controller

GMSC Gateway Mobile Services Switching

Center

MSC Mobile Switching Centre

VLR Visitor Location Register

HLR Home Location Register

AUC Authentication Centre

EIR Equipment Identity Register

SGSN Serving GPRS Support Node

GGSN Gateway GPRS Support Node

Urn Air Interface

A, Abis Interfaces (GSM)

G* Interfaces GPRS)

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6.2 3G Technology

International MobileTelecommunications-2000 (IMT-2000), better known as 3G or 3rd

Generation, is a family of standards for mobile telecommunications fulfilling specifications

by the International Telecommunication Union, which includes UMTS, and CDMA2000 as

well as the non-mobile wireless standards DECT[and WiMAX

While the GSM EDGE

standard also fulfils the IMT-2000 specification, EDGE phones are typically not branded 3G.

Services include wide-area wireless voice telephone, video calls, and wireless data, all in a

mobile environment. Compared to 2G and 2.5G services, 3G allows simultaneous use of

speech and data services and higher data rates (at least 200 kbit/s peak bit rate to fulfill to

IMT-2000 specification). Today's 3G systems can in practice offer up to 14.0 Mbit/s on the

downlink and 5.8 Mbit/s on the uplink.

6.2.1Features

Data rates

ITU has not provided a clear definition of the data rate users can expect from 3G equipment

or providers. Thus users sold 3G service may not be able to point to a standard and say that

the rates it specifies are not being met. While stating in commentary that "it is expected that

IMT-2000 will provide higher transmission rates: a minimum data rate of 2 Mbit/s for

stationary or walking users, and 384 kbit/s in a moving vehicle," the ITU does not actually

clearly specify minimum or average rates or what modes of the interfaces qualify as 3G, so

various rates are sold as 3G intended to meet customers expectations of broadband data.

Security

3G networks offer greater security than their 2G predecessors. By allowing the UE (User

Equipment) to authenticate the network it is attaching to, the user can be sure the network is

the intended one and not an impersonator. 3G networks use the KASUMI block crypto

instead of the older A5/1 stream cipher. However, a number of serious weaknesses in the

KASUMI cipher have been identified [16]

.

In addition to the 3G network infrastructure security, end-to-end security is offered when

application frameworks such as IMS are accessed, although this is not strictly a 3G property.

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6.2.2 Applications

The bandwidth and location information available to 3G devices gives rise to applications not

previously available to mobile phone users. Some of the applications are:

Mobile TV – a provider redirects a TV channel directly to the subscriber's phone

where it can be watched.

Video on demand – a provider sends a movie to the subscriber's phone.

Video conferencing – subscribers can see as well as talk to each other.

Tele-medicine – a medical provider monitors or provides advice to the potentially

isolated subscriber.

Location-based services – a provider sends localized weather or traffic conditions to

the phone, or the phone allows the subscriber to find nearby businesses or friends

6.3 High-Speed Downlink Packet Access (HSDPA)

High-Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third generation)

mobile telephony communications protocol in the High-Speed Packet Access (HSPA) family,

also coined 3.5G, 3G+ or turbo 3G, which allows networks based on Universal Mobile

Telecommunications System (UMTS) to have higher data transfer speeds and capacity.

Current HSDPA deployments support down-link speeds of 1.8, 3.6, 7.2 and 14.0 Mbit/s.

Further speed increases are available with HSPA+, which provides speeds of up to 42 Mbit/s

downlink and 84 Mbit/s with Release 9 of the 3GPP standards.

6.3.1Technology

HS-DSCH channel

For HSDPA, a new transport layer channel, High-Speed Downlink Shared Channel (HS-

DSCH), has been added to W-CDMA specification. It is implemented by introducing three

new physical layer channels: HS-SCCH, HS-DPCCH and HS-PDSCH. The High Speed-

Shared Control Channel (HS-SCCH) informs the user that data will be sent on the HS-DSCH

2 slots ahead. The Uplink High Speed-Dedicated Physical Control Channel (HS-DPCCH)

carries acknowledgment information and current channel quality indicator (CQI) of the user.

62

This value is then used by the base station to calculate how much data to send to the user

devices on the next transmission. The High Speed-Physical Downlink Shared Channel (HS-

PDSCH) is the channel mapped to the above HS-DSCH transport channel that carries actual

user data.

Hybrid automatic repeat-request (HARQ)

Data is transmitted together with error correction bits. Minor errors can thus be corrected

without retransmission.

If retransmission is needed, the user device saves the packet and later combines it with

retransmitted packet to recover the error-free packet as efficiently as possible. Even if the

retransmitted packets are corrupted, their combination can yield an error-free packet.

Retransmitted packet may be either identical (Chase combining) or different from the first

transmission (incremental redundancy).

The round-trip time for retransmissions is improved since the retransmissions are done from

base station instead of radio network controller.

Fast packet scheduling

The HS-DSCH downlink channel is shared between users using channel-dependent

scheduling to make the best use of available radio conditions. Each user device continually

transmits an indication of the downlink signal quality, as often as 500 times per second.

Using this information from all devices, the base station decides which users will be sent data

on the next 2 ms frame and how much data should be sent for each user. More data can be

sent to users which report high downlink signal quality.

The amount of the channelisation code tree, and thus network bandwidth, allocated to

HSDPA users is determined by the network. The allocation is "semi-static" in that it can be

modified while the network is operating, but not on a frame-by-frame basis. This allocation

represents a trade-off between bandwidth allocated for HSDPA users, versus that for voice

and non-HSDPA data users. The allocation is in units of channelisation codes for Spreading

Factor 16, of which 16 exist and up to 15 can be allocated to HSDPA. When the base station

decides which users will receive data on the next frame, it also decides which channelisation

codes will be used for each user. This information is sent to the user devices over one or more

"scheduling channels"; these channels are not part of the HSDPA allocation previously

mentioned, but are allocated separately. Thus, for a given 2 ms frame, data may be sent to a

number of users simultaneously, using different channelisation codes. The maximum number

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of users to receive data on a given 2 ms frame is determined by the number of allocated

channelisation codes. By contrast, in CDMA2000 1xEV-DO, data is sent to only one user at a

time.

Adaptive modulation and coding

The modulation scheme and coding is changed on a per-user basis depending on signal

quality and cell usage. The initial scheme is Quadrature phase-shift keying (QPSK), but in

good radio conditions 16QAM and 64QAM can significantly increase data throughput rates.

With 5 Code allocation, QPSK typically offers up to 1.8 Mbit/s peak data rates, while

16QAM offers up to 3.6. Additional codes (e.g. 10, 15) can also be used to improve these

data rates or extend the network capacity throughput significantly.

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