training report on bharti infotel

8/7/2019 training report on bharti infotel

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TABLE OF CONTENTS

S. NO. CONTENTS Page No.1. Abstract 22. Acknowledgement 33. Company Profile 44. Introduction to GSM 135. Wireless Concept 176. GSM Basics 197. GSM Transmission process 298. GSM Network 369. GSM Switching System 3710. Base Station System 4411. Operation & Support System 5112. BSS Interfaces 5213. Channel Concepts 5614. GSM Identities 6015. Traffic Cases 6416. Call Processing 72

17. My Departments 7817.1. Operation & Maintenance Centre 7917.1.1. Structure of OMC 8017.1.2. Functions of OMC 8017.1.3. Introduction to OSS 8417.1.4. OSS applications 8617.2. Network Switching subsystem 9117.2.1. Switch hardware 9217.2.2. Mobile Intelligent Network 10717.2.3. CCS#7 Signalling 11218. My Projects During Training 12118.1. Monitoring of Alarms 12218.2. Fault Management 12318.3. TRU additions 12718.4. Definition of New Cell site 12818.5. Roaming Testing 13318.6. Definition of A-Links & A-ter Links 14718.7. Definition of CCS#7 & semi-permanent links 15118.8. B- number Analysis 15518.9. Backups 16419. Bibliography & References 168

ABSTRACT

This report includes all my learning work done during my six months Industrial Training in Operation & Maintenance Centre and Network Switching Subsystem departments at IDEA.

This whole report is divided mainly into three different sections. First sectiongives the knowledge about basic concepts of Global System for Mobile Communicat

ion. The second section gives the overview of both departments in which I completed my training. The third section describes my profile during my six months industrial training.

The first section of report covers all the basic concepts related to GSM like wireless concepts, switching system, channel concepts, various identities relatedto GSM, base station system and operation and support system.

The second section of report covers the overview idea of my departments, variousresponsibilities of both departments, hardware related to switch, CCS#7 signall


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ing and basic concepts of mobile intelligent networks.

The third section covers my profile during six months period. My training profile includes alarm monitoring of nodes and cell sites, fault management, TRU addition, new cell site definition, roaming testing, definition of A-links, A-ter links, definition of C7 signalling routes and semi- permanent connections, backup of different nodes, B- number analysis etc.

ACKNOWLEDGMENT

It is a pleasure of mine to find my self penning down these lines to express mysincere thanks to our Principle Mr. H.B. Sharda, Mr. Sanjeev Dewra (H.O.D; ECE Deptt.) and my training & placement officer Mr.

, S.B.S.C.E.T; Ferozepur, who gave me this opportunity of industrialtraining for 6 months to enhance my professional practice & to get preliminary

industrial exposure in the concerned discipline.

It gives me immense pleasure & honor to express my heartful thanks to Ms. Satinder Ahuja (Senior Manager- Human Resource) & Mr. Bhupinder Singh (D.G.M, Punjab)for giving me opportunity for the training in their reputed industry.

I express my deep sense of gratitude to Mr. Nitesh Suri (Head Operation & Maintence Centre), Mr. Harpreet Singh (Senior Engineer, Haryana), Mr. Gagan Chhabra, Mr. Chirag Chugh, Mr. Sameer Gagneja, Mr. Sumeet Arora, Mr. Vipin Garg, Mr. Vinay

Sharma for giving me knowledge about cellular communication and concepts related to NSS & BSS operations.

The six months at IDEA gave me intense understanding of mobile communication andthe cellular technology concepts. Apart from it gave me a practical Outlook of

the functioning involving both technical and personal communication skills thusenriching my knowledge in various aspects.

My deepest gratitude is to my teachers & all the members of S.B.S.C.E.T; Ferozepur, for always boosting my morale & providing me encouraging environment.

In the last but not the least, I want to thank my Parents without grace of whomnothing was possible.


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(VARUN GUPTA)

Spearheading the Indian telecom revolution for two decades

Bharti Enterprises, Indias leading telecom conglomerate has been at the forefrontof technology and has revolutionized with its world class services. Establishedin 1976, Bharti Enterprises has been a pioneering force in the telecom sector w

ith many firsts and innovations to its credit. Working on the principle of providing end to end communication solution across the telecom value chain from manufacture of hardware to development of telecom software and from fixed line to cellular and wireless services, e-commerce, broadband, domestic long distance, undersea cable, infrastructure development and business solutions. Bharti Enterprises under cable chairmanship of Sunil Bharti Mittal is the only company to have brought to India the excellence and expertise of leading Telecom players of the world. Bharti Telecom, the manufacture division of Bharti is the largest sets under the brand name Beetel. Bharti Tele-ventures, the services division of Bharti has major interests in Basic, long Distance and Cellular, Broadband and Infrastructure Operations in the country.

Highlights

BHARTI announces agreement with VODAFONE marking the entry of the World

s Largest Telecom Operator into India


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BHARTI wins SILVER TROPHY at the CII NATIONAL SIX SIGMA AWARDS

BHARTI TELE-VENTURES adds another first by winning the prestigious MIS ASIA IT EXCELLENCE AWARD 2005

Bharti Enterprises and AXA Asia Pacific Holdings Limited announce Partnership for a life insurance joint venture in India

IDEA Launches future factory - Centers of Innovation to Incubate Pioneering Mobile Applications

As we spread wings to expand our capabilities and explore new horizons, the fundamental focus remains unchanged: seek out the best technology in the world and put it at the service of our ultimate user: our customer.Sunil Bharti Mittal

Bharti Enterprises has successfully focused its strategy on telecom while straddling diverse fields of business. Bharti Tele-Ventures Limited, a part of BhartiEnterprises, is India

s leading provider of telecommunications services. The businesses at Bharti Tele-Ventures have been structured into two main strategic business groups - the Mobility Leaders business group and the Infotel Leaders business group. The Mobility business group provides GSM mobile services across India

in twenty three telecom circles, while the Infotel business group provides broadband & telephone services, long distance services and enterprise services. Allthese services are provided under the IDEA brand. Bharti Tele-Ventures is todayacknowledged as one of India

s finest companies and its flagship brand

IDEA

, with an aggregate of 14.42 million customers as of end of August 05, consisting of

approximately 13.41 million mobile customers across the length and breadth of India.Business StrategyBharti Tele-Ventures

strategic objective is To capitalize on the growth opportunities that the Company believes are available in the Indian telecommunications market and consolidate its position to be the leading integrated telecommunications services provider in key markets in India, with a focus on providing mobile services.The Company has developed the following strategies to achieve its strategic objective: Focus on maximizing revenues and margins; Capture maximum telecommunications revenue potential with minimum geographical coverage; Offer multiple telecommunications services to provide customers with a "one-stop

shop" solution; Position itself to tap data transmission opportunities and offer advanced mobile

data services; Focus on satisfying and retaining customers by ensuring high level of customer satisfaction; Leverage strengths of its strategic and financial partners; and Emphasize on human resource development to achieve operational efficiencies.Businesses:Bharti Tele-Ventures current businesses include - Mobile services Fixed-line National and international long distance services VSAT, Internet services and network solutionsCompetitive StrengthsBharti Tele-Ventures believes that the following elements will contribute to the

Company

s success as an integrated telecommunication services provider in India


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and will provide the Company with a solid foundation to execute its business strategy:

Nationwide Footprint - As of September 30, 2003, approximately 91% of India

s total mobile subscribers resided in the Company

s fifteen mobile circles. These 15circles collectively accounted for approximately 56% of India

s land mass; Focus on telecommunications to enable the Company to better anticipate industrytrends and capitalize on new telecommunications-related business opportunities; The strong brand name recognition and a reputation for offering high quality service to its customers; Quality management team with vision and proven execution skills; and The Company

s strong relationships with international strategic and financial investors such as SingTel, Warburg Pincus, International Finance Corporation, Asian Infrastructure Fund Group and New York Life Insurance.

BRANCHES OF BHARTI

IDEA comes to you from Bharti Tele-ventures Limited - a part of the biggest private integrated telecom conglomerate, Bharti Enterprises.Current News:

IDEA Becomes The First GSM Operator In The Country To Cross The 10 Million-Customer Milestone

IDEA Express Yourself campaign wins two Silvers at the prestigious AAAI awardsrand Campaign of the Year and Best Advertising Film, amidst stiff competition from 37 advertising agencies.

IDEA adds another first becomes the first private sector mobile service to launch operations in J&K

IDEA Live launched - the most comprehensive mobile portal featuring movies, music, mobile games & sports on the mobileVision:By 2010 IDEA will be the most admired brand in India: Loved by more customers Targeted by top talent Benchmarked by more businessMISSION:

Error free service deliveryInnovative products and servicesCost efficiency

NORTHERN REGION


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After touching the hearts of more than 1 million customers and winning the Techies Award for Best Cellular Services for four consecutive years, Bharti Cellularhas reached Punjab- the land of colors, festivals, industrious people and emerging opportunities, Haryana- the place of handicrafts & textile industry, and Himachal Pradesh the ultimate destination for nature lovers.Punjab is said to be a sweet home-coming for Bharti, launched on Feb 8,2002. With over 25000 bookings on day 1 and having 50,000 customers in just 75 days it is

already on an expressway to success.Ahead of competitors in Himachal, and with grand start in Haryana, Bharti is here to take care of communication needs and live up to the true spirit of Northern

Region of Excellence.

1: Introducing three new circles in the region: Jammu & Kashmir, Rajasthan, U.P(West).

2. Market leaders in Punjab.

3. One of the best cellular companies of Country.

4. Achieving a record 15 lakh customers in 4 years duration. Fastest growingnetwork.

5. Best HR team of the region.

Northern Region of ExcellenceNorthern Region Overview

TECHNICAL DEPARTMENT HIERARCHIES


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DEFINITION

Global system for mobile communication (GSM) is a globally accepted standard fordigital cellular communication. GSM is the name of a standardization group esta

blished in 1982 to create a common European mobile telephone standard that wouldformulate specifications for a pan-European mobile cellular radio system operat

ing at 900 MHz.

INTRODUCTION

The Evolution of Mobile Telephone Systems Cellular is one of the fastest growingand most demanding telecommunications applications. Today, it represents a cont

inuously increasing percentage of all new telephone subscriptions around the world. Currently there are more than 45 million cellular subscribers worldwide, and

nearly 50 percent of those subscribers are located in the United States. It isforecasted that cellular systems using a digital technology will become the universal method of telecommunications. The countries with highest number of subscribers are the United Kingdom and Italy.

Figure 1. Cellular Subscriber Growth Worldwide

The concept of cellular service is the use of low-power transmitters where frequencies can be reused within a geographic area. The idea of cell-based mobile radio service was formulated in the United States at Bell Labs in the early 1970s.However, the Nordic countries were the first to introduce cellular services forcommercial use with the introduction of the Nordic Mobile Telephone (NMT) in 1981. Cellular systems began in the United States with the release of the advancedmobile phone service (AMPS) system in 1983. The AMPS standard was adopted by Asia, Latin America, and Oceanic countries, creating the largest potential market in the world for cellular.In the early 1980s, most mobile telephone systems were analog rather than digital, like today

s newer systems. One challenge facing analog systems was the inability to handle the growing capacity needs in a cost-efficient manner. As a result, digital technology was welcomed. The advantages of digital systems over analo


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g systems include ease of signaling, lower levels of interference, integration of transmission and switching, and increased ability to meet capacity demands.Throughout the evolution of cellular telecommunications, various systems have been developed without the benefit of standardized specifications. This presentedmany problems directly related to compatibility, especially with the development

of digital radio technology. The GSM standard is intended to address these problems.HISTORY OF GSM

This history of GSM is outlined in the following table:

DATE ACTIVITY1982-1985 Confrence Europenne des Postes et Telecommunications (CEPT) began specifying a European digital telecommunications standard in the 900 MHz frequencyband. This standard later became known as Global System for Mobile communication

(GSM).1986 Field tests were held in Paris to select which digital transmission technology to use. The choice was Time Division Multiple Access

(TDMA) or Frequency Division Multiple Access (FDMA).1987 A combination of TDMA and FDMA was selected as the transmission technologyfor GSM. Operators from 12 countries signed a Memorandum of Understanding committing themselves to introducing GSM by 1991.1988 CEPT began producing GSM specifications for a phased implementation.1990 Phase 1 specifications were frozen to allow manufacturers to develop network equipment.1991 The GSM 1800 standard was released.1992 Phase 1 specifications were completed. First commercial Phase 1 GSM networks were launched. The first international roaming agreement was established between Telecom Finland and Vodafone in UK.1993 The number of GSM subscribers reached one million. The first commercial DCS 1800 system was launched in the U.K.1994 The MoU now had over 100 signatories covering 60 countries. More GSM networks were launched. The total number of GSM subscribers exceeded 3 million.1995 The specification for the Personal Communications Services (PCS) was developed in the U.S.A. This version of GSM operates at 1900 MHz. GSM growth trends continued steadily through 1995, with the number of GSM subscribers increasing at the rate of 10,000 per day and rising. In April 1995, there were 188 members of the MoU from 69 countries.1996 The first GSM 1900 systems became available.These comply with the PCS 1900 standard.1998 At the beginning of 1998 the MoU has a total of 253 members in over 100 countries and there are over 70 million GSM subscribers world-wide. GSM subscribers

account for 31% of the worlds mobile market.

GSM WOLDWIDE

GSM worldwide (indicated by darker areas)


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WIRELESS CONCEPT

INTRODUCTION TO ANALOG AND DIGITAL

Analog InformationAnalog information is continuous and does not stop at discrete values. An example of analog information is time. It is continuous and does not stop at specificpoints. An analog watch may have a second-hand which does not jump from one second to the next, but continues around the watch face without stopping.Analog SignalsAn analog signal is a continuous waveform which changes in accordance with the properties of the information being represented.

Digital InformationDigital information is a set of discrete values. Time can also be represented digitally. However, digital time would be represented by a watch which jumps fromone minute to the next without stopping at the seconds. In effect, such a digital watch is taking a sample of time at predefined intervals.Digital SignalsFor mobile systems, digital signals may be considered to be sets of discrete waveforms.

ADVANTAGES OF USING DIGITALHuman speech is a form of analog information. It is continuous and changes in both frequency (higher and lower pitches) and amplitude (whispering and shouting).

At first, analog signals may appear to be a better medium for carrying analog information such as speech. Analog information is continuous and if it were to be

represented by discrete samples of the information (digital signal), then someinformation would be missing (like the seconds on the digital watch). An analogsignal would not miss any values as it too is continuous. All signals, analog and digital, become distorted over distances.In analog, the only solution to this is to amplify the signal. However, in doing

so, the distortion is also amplified. In digital, the signal can be completelyregenerated as new, without the distortion.


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The problem with using digital signals to transfer analog information is that some information will be missing due to the technique of taking samples. However,the more often the samples are taken, the closer the resulting digital values will be to a true representation of the analog information. Overall, if samples are taken often enough, digital signals provide a better quality for transmissionof analog information than analog signals.

GSM BASICSFREQUENCY BANDS

RADIO CHANNELA mobile station communicates with a base station via a radio channel. A radio channel is a bi-directional radio transmission path. Each radio channel has two distinct frequencies; one for downlink and one for uplink.

Downlink is defined as the transmission path from the base station to the mobilestation, while uplink is defined as the transmission path from the mobile station to the base station..

Uplink and downlink on a radio channel

The base station transmits on one frequency while the mobile station transmits on another frequency. This creates a full duplex communication path. That is, simultaneous communication in both directions.FREQUENCY SPECTRUMDifferent frequency bands are used for GSM 900, GSM 1800 and GSM 1900. An operator applies for the available frequencies or, as in the United States; the operator buys frequency bands at an auction

For GSM 900 the available frequency bands are:Uplink 890 - 915 MHzDownlink 935 - 960 MHz

For GSM 1800 the frequency bands are:Uplink 1710 - 1785 MHzDownlink 1805 - 1880 MHz

For GSM 1900 the frequency bands are:Uplink 1850 - 1910 MHzDownlink 1930 - 1990 MHz

TERMINOLOGYDUPLEX DISTANCE

The distance between one uplink frequency and its corresponding downlink frequency is called the duplex distance. The duplex distance varies for different frequency bands, refer to Table below:

Duplex distance

CHANNEL SEPARATIONThe distance between adjacent frequencies on either the uplink or downlink is called channel separation. Channel separation is 200 kHz, regardless of the standards mentioned above. This separation is necessary to reduce interference between

channels.In addition to the duplex distance, every mobile system includes a channel separation. This is the distance on the frequency band between channels being transmitted in the same direction. This is required in order to avoid the overlapping o


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dth compared to ordinary MSK, but it has less resistance against noise.

TRANSMISSION PROBLEMS

Many problems may occur during the transmission of a radio signal. Some of the most common problems are described below.

1. PATH LOSS

Path loss occurs when the received signal becomes weaker and weaker due to increasing distance between MS and BTS, even if there are no obstacles between the transmitting (Tx) and receiving (Rx) antenna. The path loss problem seldom leads to a dropped call because before the problem becomes extreme, a new transmissionpath is established via another BTS.

2. SHADOWING

Shadowing occurs when there are physical obstacles including hills and buildingsbetween the BTS and the MS. The obstacles create a shadowing effect which can decrease the received signal strength. When the MS moves, the signal strength fluctuates depending on the obstacles between the MS and BTS. Shadowing

3. MULTIPATH FADING

Multipath fading occurs when there is more than one transmission path to the MSor BTS, and therefore more than one signal arriving at the receiver. This may be

due to buildings or mountains, either close to or far from the receiving device.Rayleigh fading and time dispersion are forms of Multipath fading.

3.1 Rayleigh fading

This occurs when a signal takes more than one path between the MS and BTS antennas. In this case, the signal is not received on a line of sight path directly from the Tx antenna. Rather, it is reflected off buildings, for example, and is received from several different indirect paths. Rayleigh fading occurs when the obstacles are close to the receiving antenna.

Rayleigh fading

3.2 Time Dispersion

Time dispersion is another problem relating to multiple paths to the Rx antennaof either an MS or BTS. However, in contrast to Rayleigh fading, the reflected signal comes from an object far away from the Rx antenna. Time dispersion causesInter-Symbol Interference (ISI) where consecutive symbols (bits) interfere witheach other making it difficult for the receiver to determine which symbol is the

correct one.

Time dispersion

If the reflected signal arrives one bit time after the direct signal, then the receiver detects a 1 from the reflected wave at the same time it detects a 0 from

the direct wave. The symbol 1 interferes with the symbol 0 and the MS does not


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know which one is correct.4. TIME ALIGNMENT

Each MS on a call is allocated a time slot on a TDMA frame. This is an amount oftime during which the MS transmits information to the BTS. The information mustalso arrive at the BTS within that time slot. The time alignment problem occurswhen part of the information transmitted by an MS does not arrive within the al

located time slot. Instead, that part may arrive during the next time slot, andmay interfere with information from another MS using that other time slot.

The time alignment problem

SOLUTIONS TO TRANSMISSION PROBLEMS

1. CHANNEL CODING

In digital transmission, the quality of the transmitted signal is often expressed in terms of how many of the received bits are incorrect. This is called Bit Er

ror Rate (BER). BER defines the percentage of the total number of received bitswhich are incorrectly detected.

Channel coding is used to detect and correct errors in a received bit stream. It

adds bits to a message. These bits enable a channel decoder to determine whether the message has faulty bits, and to potentially correct the faulty bits.

2. INTERLEAVING

Channel coding is most effective in detecting and correcting single errors and shorterror sequences. It is not suitable for handling longer sequences of bit errors.For this reason, a process called interleaving is used to separate consecutive bits of a message so that these are transmitted in a non-consecutive way.

For example, a message block may consist of four bits (1234). If four message blocks must be transmitted, and one is lost in transmission, without interleavingthere is a 25% BER overall, but a 100% BER for that lost message block. It is not possible torecover from this.

Interleaving

If interleaving is used, as shown in Figure 3-18, the bits of each block may besent in a non-consecutive manner. If one block is lost in transmission, again there is a 25% BER overall. However, this time the 25% is spread over the entire set of message blocks, giving a 25% BER for each. This is more manageable and there is a greater possibility that the errors can be corrected by a channel decoder.

Received interleaved message blocks

3. ANTENNA DIVERSITY

Antenna diversity increases the received signal strength by taking advantage ofthe natural properties of radio waves. There are two primary diversity methods:space diversity and polarization diversity.


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3.1 Space Diversity

Increased received signal strength at the BTS may be achieved by mounting two receiver antennae instead of one. If the two Rx antennae are physically separated,

the probability that both of them are affected by a deep fading dip at the sametime is low. By choosing the best of each signal, the impact of fading can be r

educed. Space diversity offers slightly better antenna gain than polarization diversity, but requires more space.

3.2 Polarization Diversity

With polarization diversity the two space diversity antennae are replaced by onedual polarized antenna. This antenna has normal size but contains two different

ly polarized antenna arrays. The most common types are vertical/horizontal arrays and arrays in 45 degree slant orientation. The two arrays are connected to therespective Rx branches in the BTS. The two arrays can also be used as combined Tx/Rx antennas.

4. ADAPTIVE EQUALIZATION

Adaptive Equalization is a solution specifically designed to counter act the problem of time- dispersion. It works as follows:

1. A set of predefined known bit patterns exist, known as training sequences. These are known to the BTS and the MS (programmed at manufacture). The BTS instructs the MS to include one of these in its transmissions to the BTS.2. The MS include straining sequence to its transmission to BTS.3. The BTS receives the transmission from the MS and examines the trainingsequence within it. The BTS compares the received training sequence with the training sequence which it had instructed the MS to use. If there are differences between the two, it can be assumed that the problems in the radio path affected these bits must have had a similar affect on the non-training sequence bits.4. The BTS begins a process in which it uses its knowledge of what happened

the training sequence to correct the other bit.

Adaptive equalization

5. FREQUENCY HOPPING

As mentioned previously, Rayleigh fading is frequency dependent. This means thatthe fading dips occur at different places for different frequencies. To benefitfrom this fact, it is possible for the BTS and MS to hop from frequency to freq

uency during a call. The frequency hopping of the BTS and MS is synchronized.In GSM there are 64 patterns of frequency hopping, one of which is a simple cyclic or sequential pattern. The remaining 63 are known as pseudo-random patterns which an operator can choose from.

Frequency hopping

6. TIMING ADVANCE

Timing advance is a solution specifically designed to counteract the problem oftime alignment. It works by instructing the misaligned MS to transmit its burstearlier than it normally would.

Timing advance


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GSM TRANSMISSION PROCESSThe following figure summarizes the GSM transmission process. The details of transmission from an MS are described later in this section.

GSM transmission process

1. 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 zeroswhich represent the speech input.

The A/D conversion is performed by using a process called Pulse Code Modulation(PCM). PCM involves three main steps:

SamplingQuantizationCoding

SAMPLINGSampling involves measuring the analog signal at specific time intervals.

Analog signal sampling

The accuracy of describing the analog signal in digital terms depends on how often the analog signal is sampled, among other things. 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 analogsignal

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 asampling frequency of 8 kHz, which is acceptable based on the sampling theory.QUANTIZATIONThe next step is to give each sample a value. For this reason, the amplitude ofthe 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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QuantizationCODINGCoding involves converting the quantized values into binary. Every value is represented by a binary code of 13 bits (213= 8192). For example, a quantized valueof 2,157 would have a bit pattern of 0100001101101:

Coding of quantized value 2157

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

2. SEGMENTATION

The key to reducing the bit rate is to send information about the speech instead

of the speech itself. This can be explained with the following analogy:In GSM, the speech coding process analyses speech samples and outputs parameters of what the speech consists of: the tone, length of tone, pitch, etc. This isthen transmitted through the network to another MS which generates the speech based on these parameters.

The process of segmentation and speech coding is explained in more detail as follows:

The human speech process starts in the vocal chords or speech organs, where a tone is generated. The mouth, tongue, teeth, etc. act as a filter, changing the nature of this tone. The aim of speech coding in GSM is to send only informationabout the original tone itself and about the filter.Segmentation: Given that the speech organs are relatively slow in adapting to changes, the filter parameters representing the speech organs are approximately constant during 20 ms. For this reason, when coding speech in GSM, a block of 20 ms is coded into one set of bits. In effect, it is similar to sampling speech ata rate of 50 times per second instead of the 8,000 used by A/D conversion.

3. SPEECH CODING

Instead of using 13 bits per sample as in A/D conversion, GSM speech coding uses260 bits. This calculates as 50 x 260 = 13 kbits/s. This provides a speech qual

ity which is acceptable for mobile telephony and comparable with wire-line PSTNphones. Many types of speech coders offer better speech quality, at the expenseof a higher bit rate (waveform coders). Others use lower bit rates, at the expense of lower speech quality (vocoders). The hybrid coder which GSM uses providesgood speech quality with a relatively low bit rate, at the expense of speech coder complexity.

4. CHANNEL CODINGChannel coding in GSM uses the 260 bits from speech coding as an input and outputs 456 encoded bits. The 260 bits are split according to their relative importance:

Block 1: 50 very important bitsBlock 2: 132 important bits andBlock 3: 78 not so important bits

The first block of 50 bits is sent through a block coder, which adds three parity bits to result in 53 bits. It is these three bits which are used to detect errors in a received message.


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These 53 bits, the 132 bits in the second block and 4 tail bits (total = 189) are sent to a 1:2 convolutional coder which outputs 378 bits. The bits added by the convolutional coder enable the correction of errors when the message is received.

The remaining bits of block 3 are not protected.

Channel coding

5. INTERLEAVING

First level of interleavingThe channel coder provides 456 bits for every 20 ms of speech. These are interleaved, forming eight blocks of 57 bits each, as shown in the figure below.

Interleaving of 20 ms of encoded speech

As can be seen in Figure, in any one burst, there is space for two of these blocks. (The remaining bits are explained later in this book.) Thus, if one burst transmission is lost, there is a 25% BER for the entire 20 ms of speech (2/8 = 25%).

Normal burstSecond level of interleavingIf only one level of interleaving is used, a loss of this burst results in a total loss of 25%. This is too much for the channel decoder to correct. A second level of interleaving can be introduced to further reduce the possible BER to 12.5%.Instead of sending two blocks of 57 bits from the same 20 ms of speech within one burst, a block from one 20 ms and a block from another 20 ms are sent together. This causes a delay in the system, because the MS must wait for the next 20 ms

of speech. However, the system can now afford to loose a whole burst because the loss only affects 12.5% of the bits from each speech frame. This rate can be corrected by a channel decoder.

Speech frame6. CIPHERING/ENCRYPTION

The purpose of ciphering is to encode the burst so that it cannot be interpretedby any device other than the intended receiver. The ciphering algorithm in GSM

is called the A5 algorithm. It does not add bits to the burst, meaning that theinput and output to the ciphering process is the same as the input: 456 bits per

20 ms.7. BURST FORMATING

As previously explained, every transmission from an MS/BTS must include some extra information such as the training sequence. The process of burst formatting is

to add these bits (along with some others such as tail bits) to the basic speech/data being sent. This increases the overall bit rate, but is necessary to counteract problems encountered on the radio path. In GSM, the input to burst format


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ting is the 456 bits received from ciphering. Burst formatting adds a total of 136 bits per block of 20 ms, bringing the overall total to 592.However, each time slot on a TDMA frame is 0.577 ms long. This provides enough time for 156.25 bits to be transmitted (each bit takes 3.7 ms), but a burst onlycontains 148 bits. The rest of the space, 8.25 bit times, is empty and is called

the Guard Period (GP). This time is used to enable the MS/BTS ramp up and ramp down. To ramp up means to get power from the battery/power supply for transmission.Ramping down is performed after each transmission to ensure that the MS is not using battery power during time slots allocated to other MSs.

The output of burst formatting is a burst of 156.25 bits or 625 bits per 20 ms.However, in order to regulate the modulator, some dummy bits are used on eitherside of the burst. This brings the total to 676 bits per 20 ms of speech. When it is considered that there are 8 subscriber per TDMA frame, the overall bit rate

for GSM can be calculated to be 270.4 kbits/s.5.8 MODULATION & TRANSMISSIONThe 676 bits per 20 ms of speech must then be sent over the air using a carrierfrequency. As previously explained, GSM uses the GMSK modulation technique. Thebits are modulated onto a carrier frequency (e.g. 912.2 MHz) and transmitted.

THE GSM NETWORKThe GSM network is divided into three major systems:

Switching system (SS),Base station system (BSS),Operation and support system (OSS).

The basic GSM network elements are shown in figure: GSM SWITCHING SYSTEMINTRODUCTION

The Switching System in Ericssons GSM systems contains the following components:

Switching System

1. MOBILE SERVICES SWITCHING CENTER/VISITOR LOCATION REGISTER (MSC/VLR)

MSC FUNCTIONS

The primary node in a GSM network is the MSC. It is the node which controls call


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s both to MSs and from MSs. The primary functions of an MSC include the following:

Switching and Call Routing: an MSC controls call set-up, supervision and releaseand may interact with other nodes to successfully establish a call. This includ

es routing of calls from MSs to other networks such as a PSTN. Charging: an MSC contains functions for charging mobile calls and information about the particular charge rates to apply to a call at any given time or for a given destination. During a call it records this information and stores it after the call, e.g. for output to a billing centre. Service provisioning: supplementary services are provided and managed by an MSC.

In addition, the SMS service is handled by MSCs. Communication with HLRs: the primary occasion on which an MSC and HLR communicate is during the set-up of a call to an MS, when the HLR requests some routing information from the MSC. Communication with the VLR: associated with each MSC is a VLR, with which it communicates for subscription information, especially during call set-up and release. Communication with other MSCs: it may be necessary for two MSCs to communicate w

ith each other during call set-up or handovers between cells belonging to different MSCs. Control of connected BSCs: as the BSS acts as the interface between the MSs andthe SS, the MSC has the function of controlling the primary BSS node: the BSC. Each MSC may control many BSCs, depending on the volume of traffic in a particular MSC service area. An MSC may communicate with its BSCs during; for example, call set-up and handovers between two BSCs.

VLR FUNCTIONS

The role of a VLR in a GSM network is to act as a temporary storage location forsubscription 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 doesnot have to contact the HLR (which may be located in another country) every tim

e the subscriber uses a service or changes its status.

The following occurs when MSs move into a new service area: The VLR checks its database to determine whether or not it has a record for theMS (based on the subscribers IMSI). When the VLR finds no record for the MS, it sends a request to the subscribers HLR for a copy of the MSs subscription. 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. The VLR stores its subscription information for the MS, including the latest location and status (idle).

VLR-HLR interaction

For the duration which the MS is within in its MSC service area, a VLR containsa complete copy of the necessary subscription details, including the following information for each MS: Identity numbers for the subscriber Supplementary service information (e.g. whether the subscriber has call forwarding on busy activated or not) Activity of MS (e.g. idle) Current LA of MS


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2. 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 MSs to any terminal other than another MS. For example, if a person connected to the PSTN wants to make a all 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 which provides information about which MSC/VLR to route the call to. The same is true of a call from an MS to another MS.

3. HOME LOCATION REGISTER (HLR)

The HLR is a centralized network database that stores and manages all mobile subscriptions belonging to a specific operator. It acts as a permanent store for apersons subscription information until that subscription is cancelled. The information stored includes: Subscriber identity (i.e. IMSI, MSISDN)

Subscriber supplementary services Subscriber location information (i.e. MSC service area) Subscriber authentication information

The primary functions of the HLR include:

Subscription Database Management: as a database, the HLR must be able to processdata quickly in response to data retrieval and update requests from other netwo

rk nodes. For this reason it acts as a database management system. Each subscriber record contains a substantial amount of parameters. Communication with MSCs: when setting up calls to an MS, it is necessary for the

HLR to contact the MSC serving the MS for routing information. Communication with GMSCs: during call set-up to an MS, the GMSC requests MS location information from the HLR, which then provides this in the form of routing information. Also, if the subscriber is detached the HLR will inform the GMSC that there is no need to perform further routing of the call. By analysing the IMSI, a GMSC knows which HLR to contact worldwide for that MSs subscription. Communication with AUCs: before any activity involving change or use of subscription information takes place, the HLR must retrieve new authentication parameters from an AUC. Communication with VLRs/ILRs: when an MS moves into a new MSC service area the VLR for that area requests information about the MS from the HLR of the subscriber. The HLR provides a copy of the subscription details, updates its MS locationinformation and instructs the old VLR to delete the information it has about that MS. As the ILR acts as a VLR for AMPS subscribers, the HLR communicates with it in a similar way.

4. AUTHENTICATION CENTER (AUC) AND EQUIPMENT IDENTITY REGISTER (EIR)

PLMNs need a higher level of protection than traditional telecommunication networks. Therefore, to protect GSM systems, the following security functions have been defined:

Subscriber Authentication: by performing authentication, the network ensures that no unauthorized users can access the network, including those which are attempting to impersonate others. Radio Information Ciphering: the information sent between the network and an MSis ciphered. An MS can only decipher information intended for itself. Mobile Equipment Identification: because the subscriber and equipment are separate in GSM, it is necessary to have a separate authentication process for the MSequipment. This ensures, e.g. that a mobile terminal which has been stolen is no


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t able to access the network. Subscriber Identity Confidentiality: during communication with an MS over a radio link, it is desirable that the real identity (IMSI) of the MS is not always transmitted. Instead a temporary identity (TMSI) can be used. This helps to avoidsubscription fraud.

AUC FUNCTIONS

The primary function of an AUC is to provide information which is then used by an MSC/VLR to perform subscriber authentication and to establish ciphering procedures on the radio link between the network and MSs.

The information provided is called a triplet and consists of:

1. A non predictable RANDom number (RAND)

2. A Signed RESponse (SRES)

3. A ciphering Key (Kc)

Provision of Triplets

At subscription time, each subscriber is assigned a subscriber authentication Key (Ki). Ki is stored in the AUC along with the subscribers IMSI. Both are used in

the process of providing a triplet. The same Ki and IMSI are also stored in theSIM. In an AUC the following steps are carried out to produce one triplet:

1. A non-predictable random number, RAND, is generated.

2. RAND and Ki are used to calculate SRES and Kc, using two different algorithms, A3 and A8 respectively.

3. RAND, SRES and Kc are delivered together to the HLR as a triplet.

Authentication Procedure

1. The MSC/VLR transmits the RAND to the MS.2. The MS uses RAND in the A3 and A8 algorithms to compute the SRES and Ki.3. The signature SRES is sent back to MSC/VLR which performs authentication, bychecking whether the SRES from the MS and the SRES from the AUC match. If so, the subscriber is permitted to use the network. If not, the subscriber is barred from network access.

Authentication can be performed during:

Each registration Each call setup attempt Location updating Before supplementary service activation and deactivation There can be exceptions

for subscribers belonging to other PLMNs.

5. MESSAGE CENTER (MC)


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MC FUNCTIONS

An MC may be added to a GSM network to provide one or more of the following messaging services:

Voice mail Fax mail Short Message Service (SMS) text messages SMS Cell Broadcast (SMSCB) text messages

7. EQUIPMENT IDENTITY REGISTER

In GSM there is a distinction between subscription and mobile equipment. As mentioned above, the AUC checks the subscription at access. The EIR checks the mobile equipment to prevent a stolen or non-type-approved MS from being used.

Equipment Identification Procedure

The equipment identification procedure uses the identity of the equipment itself(IMEI) to ensure that the MS terminal equipment is valid.

1. The MSC/VLR requests the IMEI from the MS.2. MS sends IMEI to MSC.3. MSC/VLR sends IMEI to EIR.4. On reception of IMEI, the EIR examines three lists:

A WHITE LIST containing all number series of all equipment identities that havebeen allocated in the different participating GSM countries. A BLACK LIST containing all equipment identities that have been barred. A GRAY LIST (on operator level) containing faulty or non approved mobile equipment.

5. The result is sent to MSC/VLR, which then decides whether or not to allow network access for the terminal equipment.

Equipment identification

The decision to identify equipment remains with individual operators. GSM specifications recommend identification for each attempted call set-up.

8. INTERWORKING LOCATION REGISTER (ILR)

Ericssons ILR offers roaming capabilities between mobile telephony systems complying with different standards. The ILR is specific to the CMS 40 product portfolio and enables AMPS network subscribers to roam to a GSM 1900 network. The ILR consists of an AMPS HLR, a GSM 1900 VLR and interfacing functions.

BASE STATION SYSTEM

INTRODUCTION

The Base Station system is responsible for all the radio related functions in the system, such as:

Radio communication with the mobile units


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Handover of calls in progress between cells

Management of all radio network resources and cell configuration data.

Ericssons BSS consists of three components:

Base station Controller (BSC): BSC is the central node within a BSS and co-ordinates the actions of TRCs and RBSs.

Transcoder controller (TRC): The TRC provides the BSS with rate adaptation capabilities. This is necessary because the rate used over the air interface and that

used by MSC/VLR are different 33.8 kbits/s and 64 kbits/s respectively. A device which performs rate adaptation is called a Transcoder.

Radio Base Station (RBS): RBS acts as interface between MSs and the network, byproviding radio coverage functions from their antennae.

BSC FUNCTIONS

The Base Station Controller, BSC, controls and supervises the radio resources inBTS. Together with BTS, the BSC forms the Base Station System (BSS), responsibl

e for the management and cell configuration data in the radio network. The mainfunctions of the BSC are:

administration of resources in BSS, supervision of BTS, connection handling of mobile stations, locating and handover, administration of paging, transmission network management, operation and maintenance of BSS

TRANSCODERS

TRANSCODING AND RATE ADAPTION

The Transcoder Rate Adaptation (TRA) function performs encoding and decoding ofspeech and rate adaptation of data. It multiplexes a number of TCHs onto one 64kbps channel improving transmission efficiency between the BSC and the BTS.

TRA functions include:

Transcoding of speech information: Speech at 64 kbps to/from the MSC is transcodedto 13 kbps towards the RBS enabling four compressed channels to be multiplexed onto one 64 kbps channel. additional control information (3 kbps) is added to the transcoded rate of 13 kbpstowards the RBS giving a final output of 16 kbps. rate adaptation of data information (maximum data rate supported at present in GSMis 9.6kbit/s). DTX functions on the uplink, which allows the mobile radio transmitter to be powered down most of the time during speech pauses.

Radio Base Station (RBS)

The RBS handles the radio interface to the mobile station. One RBS can serve 1,2 or 3 cells. A group of RBSs is controlled by 1 BSC. Ericsson has 3 base station families and they are RBS 200 & RBS 2000 series (2202 & 2206).RBS 2202 is expl


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ained below:

RBS 2202:

The RBS 2202 is a member of the RBS 2000 family and is used in indoor applications with up to six transceivers. It can be configured for omni cells or multi-cells up to three-sector cells.

The RBS 2202 can be installed in any indoor environment. RBS 2202 uses the samereplaceable units as all RBSs in the RBS 2000 Macro family. RBS 2202 supports all the standard features of the RBS 2000 family,Such as

frequency hopping receiver diversity

duplex filters

dynamic power regulation

discontinuous transmission/reception

encryption/ciphering

RBS 2202 is designed to apply to the most common voltage systems. In order to reduce the cabinet size, all required transmission equipment and backup battery must be housed outside the RBS 2202 cabinet. The RBS 2202 cabinet contains the radio equipment, power supply and the Climate equipment (fans). RBS 2202 is designed to fulfill applicable parts of the GSM standards.

The RBS 2202 cabinet consists of the radio cabinet mounted on a base frame. On top of the cabinet it is possible to mount a cowl.The radio cabinet contains a number of units. These are all easily accessible from the front of the cabinet. The base frame is used as a mounting base. It is mounted on the floor in order to hold the radio cabinet in place.

Product Architecture of RBS

Replaceable Units (RUs)

The RBS 2202 consists of the following RUs:-

Distribution Switch Unit (DXU)Transceiver Unit(TRU)Energy Control Unit (ECU)

Combining and Distribution Unit (CDU) Power Supply Unit (PSU)

RBS 2202

Distribution Switch Unit (DXU)

The Distribution Switch Unit (DXU) is the central control unit of the RBS. There


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is one DXU per RBS. In multi cabinet configurations the DXU is located in the Master Cabinet only. DXU node handles the Managed Object Central Functions (CF).

The DXU consists of four/five main blocks:

PCM-partCentral Processing Unit (CPU)Central Timing Unit (CTU)High Level Data Link Controller (HDLC) concentrator

TG Synchronization (TG Sync)

Functions:

The functions of the DXU are common to one RBS. These include:

Distribution Switch Timing Unit (A timing reference for the RBS is generated by extracting the synchronization information from the PCM link or from an internal source). Collects up to 16 external alarms (product dependent)

Local bus interface RS 485 (Acts as master on the bus and communicates with distributed main RUs). PCM interface G.703 (Supervision of transmission faults) Manages the A-bis link resources Concentrates the control links (LAPD signaling to the BSC) Stores software for the entire RBS in a non-volatile memory Maintains the Installation Database (IDB) which is integrated with the DXU (The IDB contains information regarding the installed hardware - each RU identity, itsphysical position and related configuration parameters).

These functions enable the DXU to establish connection with the BSC (the PCM Link) and cross connect individual time slots to certain transceivers.

Transceiver Unit (TRU)

The Transceiver Unit (TRU) is a transmitter/receiver and signal processing unitwhich transmits and receives the radio frequency signals that are passed to andfrom the mobile station. There are different versions of TRU depending on the frequency band. One TRU can serve eight full rate duplex channels or 16 half ratechannels. The TRU has one transmit antenna terminal and two receive terminals.The TRU supports diversity reception. Diversity is used to improve the receiverperformance. It is achieved by having two independent receiver paths. The signals are combined in the signal processing in the Digital Block.

The TRU consists of three main blocks.

Digital Block Transmitter Block (TX-block) Receiver Block (RX-Block)

Digital BlockThe digital block serves as the TRX controller. It communicates with other RBS components via the Local Bus, CDU Bus, and Timing Bus and X Bus. The digital block performs uplink and downlink digital signal processing such as channel coding,

interleaving, ciphering, and burst-formatting and equalization.

Transmitter Block (TX-block)The transmitter block carries out the Transmitter (TX) functions including GMSKmodulation, RF generation and power amplification. When base-band hopping is employed each TX transmits on the same frequency and the physical channel data will


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be sent from different TXs with each burst. With synthesizer hopping the physical channel data will be sent from the same TX all the time but will use a new frequency with every burst.

Receiver Block (RX-block)The receiver block carries out the Receiver (RX) functions for reception and demodulation. There are two RX per TRU. The receiver is capable of frequency hopping. The radio loop between the TX and RX makes it possible to test the entire TRU

by generating test signals.

Functions

The TRU includes all functions related to one radio carrier supporting eight Basic Physical Channels (BPC) on a TDMA frame. The functions include:

Radio transmitting GMSK modulation RF generation Power amplification

Radio receiving Base band hopping/synthesizer hopping Diversity Air interface signal processing TRX management

Energy Control Unit (ECU)

The Energy Control Unit (ECU) controls and supervises the power equipment (PSU,BFU, battery, AC Connection Unit) and climate equipment (fans, heater, cooler and heat exchanger). The ECU observes alarm signals from power and climate system.

The purpose of the ECU is to protect equipment within the RBS from conditions that could reduce lifetime and reliability. The ECU protects the equipment during

power failure conditions and cold-start up.

The ECU consists of five main blocks:

Central Processing Unit I/O-block Power and cold start Local bus Optical interface

Combining & Distribution Unit (CDU)

The Combining and Distribution Unit (CDU) is the interface between Transceivers(TRUs) and the antenna system. The CDU allows several TRUs to share antennas.To support different configurations a range of CDU types have been developed. This description is of the CDU as a unit.

Block diagram, CDU

CDUs consist of the following main blocks:

Combiner RXDA Duplexer RF Filtering


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CombinerIt combines a number of transmitter outputs to one antenna. Several combiners may be used to combine two groups of transmitters, although there is a practical limit as to how far this may be taken. See (A) in the functional diagram above.

RXDAReceiver (RX) Divider Amplifier splits received signals into several independent

circuits, thus permitting many receivers to operate from one antenna. See (B) in the simplified functional diagram above.

DuplexerAllows a transmitter signal to pass an antenna at the same time as allowing receive signals to pass from the same antenna to the receiver.RF FilteringDue to the possibility of interference from unwanted signals and other undesirable products, it is necessary to incorporate RF filtering inside the CDUs. This is implemented by means of band pass filters (BP) in both Transmit and Receive paths which reject signals outside the operating band in use. The receiver BP is connected in the circuits from antennas before any active amplifying device or si

gnal path splitter circuit. The transmitter BP is inserted into the signal pathafter all transmitter combining circuits.

Functions of the RBS This provides a system interface to the A-bis interface and is used to cross connect individual time slots to transceivers. The DXU also provides the RBS synchronization timing reference for RBS operation.

This contains the receiver and transmitter circuitry needed for handling 8 timeslots of information on the air interface. The TRU contains RF measurement circuits used for testing transmitter and receiver properties.

This is responsible for combining transmitted signals from various transceiversand distributing received signals to all transceivers.

This supervises and controls the power equipment (PSUs), and regulates the environment conditions inside the cabinet.THE OPERATION AND SUPPORT SYSTEMThe operations and maintenance centre (OMC) is connected to all equipment in the

switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offerthe customer cost-effective support for centralized, regional and local operational and maintenance activities that are required for a GSM network. An important

function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

One of the most important tasks in a mobile telephony system is to continuouslykeep track of where mobile stations are located. One primary function of the Mobile Services Switching Centre (MSC) and Visitor Location Register (VLR) is to store information, such as location area, about different mobile stations.It is the responsibility of the mobile station to always inform the network about changes in its location and it must also continuously verify that it is tunedto the strongest frequency.

BSS INTERFACESINTRODUCTIONThere are four primary interfaces within the BSS where traffic and signalling information is received and transmitted. The A interface exchanges information between the MSC/VLR and the TRC, the A-ter Interface between the TRC and BSCs, the

A-bis Interface transmits information between the BSC and BTS, while the Air Interface operates between the BTS and MS.


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There are basically two ways of building the interfaces:2 Mbps PCM interface. The E1 physical channel is divided into 32 time slots, each with a bit rate of 64 kbps. This is the normal configuration in a GSM 900 andGSM 1800 network1.5 Mbps PCM interface. The T1 physical channel is divided into 24 time slots, each with a bit rate of 64 kbps. This is the normal configuration in a GSM 1900 network.

A INTERFACEThe A-Interface provides two distinct types of information, signalling and traffic, between the MSC and the BSS. The speech is transcoded in the TRC and the SS7

signalling is transparently connected through the TRC or on a separate link tothe BSC. The picture below shows the mapping of the traffic information on the PCM-link:

A-TER INTERFACEThe A-ter interface is the link between the TRC and the BSC. In the TRC the speech is transcoded from 64 kbit/s to 16 kbit/s: 13 kbit/s of speech information and 3 kbit/s of in-band signalling information. The pictures below show how the traffic information is mapped to the PCM links:

A-BIS INTERFACEThe A-bis Interface is responsible for transmitting traffic and signalling information between the BSC and the BTS. The transmission protocol used for sending signalling information on the A-bis interface is Link Access Protocol on the D Channel.

AIR INTERFACEThe Air Interface uses the Time Division Multiple Access (TDMA) technique to transmit and receive traffic and signalling information between the BTS and MS. The

TDMA technique is used to divide each carrier into eight time slots. These timeslots are then assigned to specific users, allowing up to eight conversations t

o be handled simultaneously by the same carrier.


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CHANNEL CONCEPTS

AIR INTERFACE CHANNELS

The path used to carry information between a Mobile Station and a Base Transceiver Station is known as the Physical Channel. The Control and Traffic Channels are further subdivided; there are two types of Traffic Channels and three categories of Control Channels with a total of nine different types.

INTRODUCTION TO PHYSICAL AND LOGICAL CHANNELS

Each timeslot on a TDMA frame is called a physical channel. Therefore, there are8 physical channels per carrier frequency in GSM. Physical channels can be usedto transmit speech, data or signaling information.

The TDMA channel conceptA physical channel may carry different messages, depending on the information which is to be sent. These messages are called logical channels. For example, on one of the physical channels used for traffic, the traffic itself is transmittedusing a Traffic Channel (TCH) message, while a handover instruction is transmitted using a Fast Associated Control Channel (FACCH) message.

Bursts

The information contained in one time slot on the TDMA frame is called a burst.There are five types of bursts:

Normal Burst: used to carry information on traffic and control channels.Frequency Correction Burst: used for frequency synchronization of the mo

bile.Synchronization Burst: used for frame synchronization of the mobile.Access Burst: used for random access and handover access.Dummy Burst: used when no other type of burst is to be sent.

LOGICAL CHANNELS

There are 12 logical channels in the system. Two are used for traffic, nine forcontrol signaling and one for message distribution. Many types of logical channels exist; each designed to carry a different message to or from an MS. All information to and from an MS must be formatted correctly, so that the receiving device can understand the meaning of different bits in the message.

Logical channels and bursts

CONTROL CHANNELS

When an MS is switched on, it searches for a BTS to connect to. The MS scans theentire frequency band, or, optionally, uses a list containing the allocated car

rier frequencies for this operator. When the MS finds the strongest carrier, itmust then determine if it is a control channel. It does so by searching for a particular logical channel called Broadcast Control Channel (BCCH). A frequency carrying BCCH contains important information for an MS, including e.g. the current

LA identity, synchronization information and network identity. Without such inf


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ormation, an MS cannot work with a network. This information is broadcast at regular intervals, leading to the term Broadcast Channel (BCH) information.

According to their functions, four different classes of control channels are defined: Broadcast channels. Common control channels. Dedicated control channels. Associated control channels.

Broadcast channels (BCH)

The BCH channels are used, by the base station, to provide the mobile station with the sufficient information it needs to synchronize with the network. Three different types of BCHs can be distinguished: The Broadcast Control Channel (BCCH), which gives to the mobile station the parameters needed in order to identify and access the network The Synchronization Channel (SCH), carries information about the TDMA frame number and the BASE STATION IDENTITY CODE of BTS.

The Frequency-Correction Channel (FCCH), which supplies the mobile station withthe frequency reference of the system in order to synchronize it with the networkWhen the MS has finished analyzing the information on a BCH, it then has all the

information required to work with a network. However, if the MS roams to another cell, it must repeat the process of reading FCCH, SCH and BCCH in the new cell. If the mobile subscriber then wishes to make or receive a call, the Common Control CHannels (CCCH) must be used.Common Control Channels (CCCH)The CCCH channels help to establish the calls from the mobile station or the network. Three different types of CCCH can be defined: The Paging Channel (PCH). It is used to alert the mobile station of an incomingcall The Random Access Channel (RACH), which is used by the mobile station to request

access to the network The Access Grant Channel (AGCH). It is used, by the base station, to inform themobile station about which channel it should use. This channel is the answer ofa base station to a RACH from the mobile stationAt this stage the MS and BSS are ready to begin call set-up procedures. For this

the MS and BSS use Dedicated Control CHannels (DCCHs).

Dedicated Control Channels (DCCH)

The DCCH channels are used for message exchange between several mobiles or a mobile and the network. Two different types of DCCH can be defined: The Standalone Dedicated Control Channel (SDCCH), which is used in order to exchange signaling information in the downlink and uplink directions. The Slow Associated Control Channel (SACCH). It is used for channel maintenanceand channel control. The Fast Associated Control Channels (FACCH) replace all or part of a traffic channel when urgent signaling information must be transmitted. The FACCH channelscarry the same information as the SDCCH channels.

Traffic Channels

Once call set-up procedures have been completed on the control physical channel,the MS tunes to a traffic physical channel. It uses the Traffic CHannel (TCH) l

ogical channel. There are two types of TCH:

1. Full rate (TCH): transmits full rate speech (13kbits/s). A full rate TCH


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occupies one physical channel.2. Half rate (TCH/2): transmits half rate speech (5.6kbits/s). Two half rateTCHs can share one physical channel, thus doubling the capacity of a cell.

GSM IDENTITIESTo switch a call to a mobile subscriber, the right identities need to be involved. It is therefore important to address them correctly. The numbers used to identify the identities in a GSM network are described in this chapter. Numbering plans are used to identify different networks.MOBILE STATION ISDN NUMBER (MSISDN)The MSISDN is a number which uniquely identifies a mobile telephone subscription

in the public switched telephone network numbering plan. These are the digits dialed when calling a mobile subscriber.

GSM 900In GSM 900, the MSISDN consists of the following:MSISDN = CC + NDC + SNCC = Country CodeNDC = National Destination CodeSN = Subscriber Number A NDC is allocated to each PLMN. In some countries, more than one NDC may be required for each PLMN.

INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI)

The IMSI is a unique identity allocated to each subscriber to allow correct identification over the radio path and through the network and is used for all signalling in the PLMN. All network related subscriber information is connected to the IMSI. The IMSI is stored in the SIM, as well as in the HLR and in the servingVLR.

MCC = Mobile Country CodeMNC = Mobile Network CodeMSIN = Mobile Station Identification NumberAccording to the GSM specifications, IMSI has a maximum length of 15 digits. MNC

expansion In order to make it possible to define more than 100 operators underone MCC, the MNC is extended from two to three decimal digits in Ericssons GSM system. The MNC parameter is also used in the Cell Global Identity and in the Location Area Identity. In order to maintain backward compatibility and to allow the

Ericsson BSS to be connected to equipment from other vendors, the Ericsson implementation has the possibility to switch from two to three MNC digits on both the air interface and the A interface. A changeable exchange property is used to decide if the third digit is used.TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)The TMSI is a temporary number used instead of IMSI to identify a MS. The TMSI is used for the subscribers confidentiality on the air interface. The TMSI has only local significance (that is, within the MSC/VLR area) and is changed at certai


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n events or time intervals. The TMSI structure can be chosen by each operator but should not consist of more than four octets (8 digits).MOBILE STATION ROAMING NUMBER (MSRN)A MSRN is used during the call setup phase for mobile terminating calls. Each mobile terminating call enters the GMSC in the PLMN. The call is then re-routed by

the GMSC, to the MSC where the called mobile subscriber is located. For this purpose, a unique number (MSRN) is allocated by the MSC and provided to the GMSC.The MSRN is seized for the call setup phase only and released immediately afterwards. The call setup takes place in the following way:1. GMSC receives a signalling message "Initial Address Message" for the incoming

call (MSISDN).2. GMSC sends a signalling message "Send Routing Information" to the HLR where the subscriber data is stored (MSISDN).3. HLR uses MSISDN to find the subscriber data in the database. The Supplementary Service (Call forward unconditional not active) is verified. The VLR address that corresponds to the subscriber location and the IMSI are retrieved. HLR sends

a signalling message "Provide Roaming Number" using the VLR address as the destination (IMSI).4. VLR having received the message, requests MSC to seize an idle MSRN and to as

sociate it with the IMSI received. VLR sends back the result to the HLR (MSRN).5. HLR sends back the result to the GMSC (MSRN).6. GMSC uses MSRN to re-route the call to the MSC. MSC receives a signalling message "Initial Address Message" for the incoming call (MSRN). MSC performs digitanalysis on the received MSRN. The result is "Mobile terminating". The MSC finds

the association between the MSRN and the IMSI. The MSRN is released and the IMSI is used for the final establishment of the call. The Use of MSRN:

The interrogation call routing function (request for a MSRN) is a part of the Mobile Application Part (MAP). All data exchanged between the GMSC-HLR-MSC/VLR for

the purpose of interrogation is sent over the signalling network. The Mobile Station Roaming Number (MSRN) consists of three parts:MSRN = CC + NDC + SNCC = Country CodeNDC = National Destination CodeSN = Subscriber Number CELL GLOBAL IDENTITY (CGI)The CGI is used for cell identification within a location area. This is done byadding a Cell Identity (CI) to the components of a LAI. CI has a maximum lengthof 16 bits.CGI consists of:CGI = MCC + MNC + LAC + CI BASE STATION IDENTITY CODE (BSIC)BSIC allows a mobile station to distinguish between different neighboring base stations. BSIC consists of:BSIC = NCC + BCCNCC = Network Colour Code (3 bits), identifies the PLMN. Note that it does not uniquely identify the operator. NCC is primarily used to distinguish between operators on each side of a border.BCC = Base Station Colour Code (3 bits), identifies the Base Station to help distinguish between BTS using the same BCCH frequencies. TRAFFIC CASES

MS IN IDLE MODE:

When any subscriber (MS) roam with in the same PLMN or in other than it continuo


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usly changes its location area, than to access the new network it has to make location update. In other words when MS roam and select a network in IDLE MODE itis called LOCATION UPDATE.

LOCATION UPDATE

A roaming mobile subscriber, moves freely within the GSM network. To keep the system updated with the current subscriber location information, the MS must inform the system whenever it changes location area. A location area consists of oneor more cells in which a MS can move around without needing to update the system

on its location. A location area is controlled by one or more Base Station Controllers (BSCs) but by only one Mobile services Switching Center (MSC). A MSC can

control more than one location area.The BSC sends paging messages to the Radio Base Station (RBS) defined within a certain location area. If the MS moves between cells belonging to different location areas, then network must be informed via a procedure called location updating.

There are four different types of location updating:

Normal IMSI detach IMSI attach Periodic registration

NORMAL LOCATION UPDATING

The Base Transceiver Station (BTS) of every cell continuously transmits the location area identity on the control channel (BCCH). When the MS detects that the broadcast location area identity is different from the one stored in the SIM-card, it performs a location update.If the mobile subscriber is unknown to the Mobile services Switching Center/Visitor Location Register (MSC/VLR), i.e. the broadcast location area belongs to a new MSC/VLR serving area, then the new MSC/VLR must be updated with subscriber information. This subscriber information comes from the Home Location Register (HLR).

This location updating procedure is described in the following steps and shown in Figure below:

1. The MS requests a location update to be carried out in the new MSC/VLR. The IMSI is used to identify the MS. An International Mobile Equipment Identity (IMEI) check is also performed if carried out in the network.

2. In the new MSC/VLR, an analysis of the IMSI number is carried out. The resultof this analysis is a modification of the IMSI to a Mobile Global Title (MGT) w

hich is used to address the HLR.

3. The new MSC/VLR requests the subscriber information for the MS from the HLR.

4. The HLR stores the address of the new MSC/VLR.

5. The HLR sends the subscriber data to the new MSC/VLR.

6. The HLR also orders the old serving MSC/VLR to cancel all information for the


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subscriber because the mobile subscriber is now served by another MSC/VLR.

7. When the new MSC/VLR receives the information from the HLR, it sends a location updating confirmation message to the MS.

Normal Location up date (to a new MSC/VLR)

IMSI DETACH

In the system information broadcast on the control channel (BCCH), the MS receives information on whether the IMSI attach/detach function is used or not. If itis used, the MS must inform the network when it is entering an inactive state (detach).

The procedure is as follows:1. At power off or when the SIM card is taken out, the MS asks for a signaling channel.

2. The MS uses this signaling channel to send the IMSI detach message to the MSC/VLR.

3. In the VLR, an IMSI detach flag is set for the subscriber. This is used to reject incoming call to the MS.

Implicit Detach

If the MS sends an IMSI detach message to the system and the radio link qualityis poor, the system might not be able to decode the information. Because no acknowledgment is sent to the MS, no further attempt is made. In this case, the system still regards the MS as attached. If periodic registration is in use, the system will soon determine that the MS is detached. The VLR then performs an implicit detach, marking the MS as detached.

MS Purging

MS purging is used to inform the HLR that the VLR is about to remove a subscriber record from the VLR. The HLR then sets the MS purged flag and treats the subscriber as unreachable. This saves unnecessary network signaling and database lookup. For example, a UK MS travels to Australia and performs a location update inan MSC/VLR in Australia. Later, the subscriber travels back to the UK, which takes some time. During this period, the subscriber is not active. If MS purging is

not used, when a caller makes a call to the MS, the HLR identifies the MS as registered in the Australian MSC/VLR and routes the call to it. The MSC/VLR then informs the HLR that the subscriber is unreachable. If MS purging is used, the UK

subscribers record will have been purged from the Australian MSC/VLR. When a call is made to the subscriber, the HLR identifies the MS as unreachable and does not contact the Australian MSC/VLR.

IMSI ATTACH

IMSI attach is a complement to the IMSI detach procedure. It is used by the mobile subscriber to inform the network that it has re-entered an active state and is still in the same location area. If the MS changes location area while being s


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witched off, a normalLocation update takes place.

The IMSI attach procedure is as follows:1. The MS requests a signaling channel.2. The MSC/VLR receives the IMSI attach message from the MS.3. The MSC/VLR sets the IMSI attach in the VLR. The mobile is now ready for normal call handling.4. The VLR returns an acknowledgment to the MS.

PERIODIC LOCATION UPDATING

Periodic location updating is used to avoid unnecessary paging of the MS in cases where the MSC does not receive the IMSI detach message.The periodic location updating procedure is as follows:

1. The MS receives information on whether periodic registration is used. If periodic registration is used, the MS is told how often to inform the system that

it is reachable.2. The procedure is controlled by timers both in the MS and in the MSC . In theMSC there is a time-scanning function for the MSs.3. When the timer in the MS expires, the phone is forced to perform periodic location updating The timers in the MS and the MSC then restart. If the cellular phone does not register within the determined time interval plus a safety interval, then the MSC scanning function detects this and the MS is marked detached.

MS IN ACTIVE MODE

An MS is in active mode when there is a call (speech, fax or data), or a call set up procedure taking place.Whenever MS roams in active mode means in busy condition and changes location area than it is necessary that call would remain continue, under this condition when MS makes LU in different LA it is called HANDOVER.

HANDOVER

The process of changing cells during a call is called handover in GSM terminology. To choose the best target cell, measurements are performed by the MS and theRBS. Because the MS contributes to the handover decision, this type of handoveris often called Mobile Assisted Hand-Over (MAHO).

Locating

An MS continuously measures signal strength and quality on its own cell and signal strength on the BCCH carriers of the neighboring cells. The measurements are carried out on the downlink while MS is in active mode. The measurement results are sent to the RBS on SACCH at regular intervals. The serving RBS measures signal strength and quality on the uplink.

Fig: Measurements sent to BSC

The measurements from the RBS and MS are sent to the BSC in the form of measurements reports. Based on these reports, the BSC decides if a handover is necessary.and to which cell. This is called locating. As soon as a neighboring cell is con


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sidered to be better than the serving cell, a handover is attempted. Another reason for attempting a handover, apart from signal strength and quality, is when the Timing Advance (TA) used by MS exceeds a threshold value set by the operator.

This usually happens when the MS is moving over the cell border to another cell. When the MS has changed cells, the new RBS informs the MS about the new neighboring BCCH carriers so measurements can be taken again. If the MS has also switched to a new LA, a location updating type normal takes place after the call hasfinished.

There are several types of handover, including:

Intra-MSC handover both cells belong to the same MSC. There are two types of intra-MSC handovers defined: Intra-BSC handover both cells belong to the same BSC. In this case, the BSC manages most of the handover. Inter-BSC handover the two cells belong to different BSCs, but to the same MSC.

In this case, the MSC is involved in the signaling.

Inter-MSC handover the two cells belong to different MSCs. In this case, at least two MSCs are involved. This case has some sub-cases which are covered later. Inte

r-MSC handover is defined as national handover only. According to GSM, it workswithin one network only. That is, MSCs belonging to one operator.

INTER-MSC HANDOVER

Handover between cells controlled by different MSC/VLRs can only be performed within one PLMN and not between two PLMNs. Cells controlled by different MSC/VLRsalso means that they are controlled by different BSCs.

The sequence follows as :

1. The serving (old) BSC sends a Handover required message to the serving MSC (MSCA), with the identity of the target cell.2. MSCA identifies that this cell belongs to another MSC, (MSCB), and requests help.3. MSCB allocates a handover number to reroute the call. A Handover Request is then sent to the new BSC.4. The new BSC orders the target RBS to activate a TCH.5. MSCB receives the information, and passes it on to MSC A together with the handover number.6. A link is set up to MSCB, possibly via PSTN.7. MSCA sends a handover command to the MS, via the old BSC.8. The MS tunes to the new frequency and transmits handover access bursts in the

correct time slot.9. When the new RBS detects the handover bursts it sends information about TA.10. The MS sends Handover Complete message to the old MSC via the new BSC and the new MSC/VLR.11. A new path in the group switches in MSCA is established, and the call is switched through.12. The old TCH is deactivated by the old BSC (not shown in the picture). The old MSC, MSCA, retains main control of the call until the call is cleared. This isbecause it contains the information about the subscriber and call details such as charging. After call release, the MS must perform location updating because an

LA never belongs to more than one MSC/VLR service area. The HLR is updated by the VLRB, and will in turn tell VLRA to delete all information about the mobile subscriber.

Handover: cells controlled by different MSCs


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INTER-MSC HANDOVER (Basic Handover-MAP V2)

GSM phase 2+ uses different MAP versions: (MAP 1, MAP 2, and MAP 3). This section describes the Basic Handover procedures for MAP 2 only.

1. When the MS moves between cells, the serving BSC detects that a handover to anew cell is necessary. The BSC sends a HANDOVER REQUIRED message to the servingMSC (MSC-A), identifying the cell to which the MS should be handed over to.

2. Once the HANDOVER REQUIRED message is received from the serving MSC, MSC-A selects and analyzes the cell in the Cell Identifier List.3. MSC-B now allocates a free radio resource in the target cell by contacting the target BSC (BSC-B).If the allocation is successful, the MSC-B sends a PREPAREHANDOVER reply containing a HANDOVER REQUEST ACKNOWLEDGE message and the handover number, if requested, to MSC-A.4. After MSC-B has allocated a radio resource, MSC-A sets up a speech connection

with the received handover number to MSC-B through the PLMN or PSTN/ISDN network.

5. MSC-A orders the MS to tune to the new radio channel by sending the HANDOVERCOMMAND message to the serving BSC.6. MSC-A then waits for a response, such as a HANDOVER DETECT message, the answer signal, or the SEND END SIGNAL containing a HANDOVER COMPLETE message. The Group Switch (GS) only operates if a circuit connection had been established and verified via a response before the GS trigger timer expires.

7. If the MSC-A receives a HANDOVER COMPLETE message, the handover is consideredcomplete and the A interface resources toward BSC-A are released. Now MSCB has

the main radio resource control for the call and is regarded as the serving MSC,but not the anchor MSC.

8. MSC-A has the overall call control until the call is cleared. When the calledor calling party initiates clearing of the call, DTAP messages are used to info

rm the other party of the call clearing.

Now MSC-A releases the speech connection and ends the TCAP dialogue with the ENDSIGNAL message. This signal is used by the MSC-B to clear its radio side. The R

ELEASE signal is used to clear the speech path between the MSCs.

CALL PROCESSING

CALL PROCESSING: MS ORIGINATED CALL

Overview

The following highlights the actions required as well as the actions being performed by the BSS as part the MS originated call. The events required for an MS originated call are shown in a diagram in this section.


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CALL SET UP MS channel request

After the dialed digits are entered, the MS transmits a Channel Request on the RACH. After receiving this request, the BTS decodes the message. The BSS software

immediately assigns the MS to a SDCCH with an Immediate Assignment message senton the AGCH channel.

SERVICE REQUEST

MS responseThe MS responds to the immediate assignment message and switches to the assigned

SDCCH. Once on the SDCCH, the MS transmits the Set Asynchronous Balanced Mode (SABM). The network responds to SABM with UA to establish the Layer 2 radio link.

Within the SABM the MS transmits a Service Request indicating to the BSS what type of service, for example, a call or location update is required. This service

request is pr

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