1-Introduction,Bw Mangmnt,Gsm Architecture

8/3/2019 1-Introduction,Bw Mangmnt,Gsm Architecture

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CHAPTER-1

INTRODUCTION

1.1 Revolution in telecomThe telephone has long been important in modern living, but it use has been

constrained by connecting wires. The advent of mobile radio telephony and particularly

the cellular radio has removed this restriction and led to explosive growth in mobile

throughout the world. The phone is really on move now.

With the phenomenal and unprecedented growth of more than forty fold in just

ten years, a strong demand for mobile cellular services has created an industry which now

accounts for more than one third of all telephone lines. It is expected that mobile phone

will soon exceed the traditional fixed line phones. In fact the trend of fixed and mobile

convergence is already being talked about.

1.2 Concept of mobile communication

Fixed telephones, using wired access network, are meant to be used at a particular

location only. We can have telephones at our office/business and our residence. The fixed

telephones are linked to a place but the modern day life style demands that we should

have telephone facility while on move also. Mobile communication facilitates telephonic

conversation in a fast moving vehicle. This means that phones moves along with a person

thereby moving telephone is linked to a person and not to a place. In these words our

reach becomes broader and world shrinks into a Global village. Wireless communication

is all around us. The day is not far off; the future generations will wonder as to why

wires are required for a telephone to work!!!

1.3 Mobile communication objectives

The important objectives of the mobile communication are

Any time anywhere communication

Mobility & Roaming

High capacity & subs. density


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Efficient use of radio spectrum

Seamless Network Architecture

Low cost

Innovative Services

Standard Interfaces

1.4 History of mobile communication

1946 Appeared in St .Louis USA (By AT & T) at 150 MHz band FM 120 KHz BW

1960 450 MHz Band FM 30 KHz BW

1970 BELL LAB introduced Cellular Principle

1979 Advanced Mobile Phone System in US1985 Total Access Communication System (TACs in UK)

1986 Nordic Mobile Telephony Systems (NMT)

1990 Digital Systems

1.5 Different generations Analog and digital systems

1946- 1960s 1980s 1990s 2000s

Appearance 1G 2G 3G

Analog Digital Digital

Multi Standard Multi Standard UnifiedStandard

. Terrestrial Terrestrial Terrestrial &

Satellite

1 G I st Generation --Analog (cellular revolution)

-only mobile voice services

2 G - 2 nd Generation -- digital (breaking digital barrier)

- Mostly for voice services & data delivery possible

3 G - Voice & data (breaking data barrier) mainly data


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INTERNATIONAL MOBILE TELECOM 2000. (IMT-2000)

THIRD GENERATION (3 G) STANDARD

A future standard in which a single inexpensive mobile terminal can truly provide

communications any time and any where.

INTERNATIONAL MOBILE TELECOM 2000. (IMT-2000)

INTERNATIONAL MOBILE TELECOM 2000. (IMT-2000) is an initiative of ITU that

seeks to integrate the various satellites, terrestrial, fixed and mobile systems currently

being deployed and developed under a single standard to promote global service

capabilities and interoperability.

1.6DEVELOPMENT AND INTRODUCTION OF THE GSM

STANDARD

The chronological development of GSM standard is given below.

YEAR EVENTS/DECISIONS/ACHIVEMENTS

1982 CEPT (CONFERENCE EUROPEAN POSTSANDTELEGRAPHS) Decides to establish Grouped

special mobile (the initial origin of the GSM) to develop a set of common

standards for future pan European cellular mobile network.

1984 Establishment of three working parties (WP1-3) to define and describe the

services offered in a GSM PLMN (GSM Public Land Mobile Network) the

radio interface, transmission, signaling protocols, interfaces and network

architecture.

1986 A so called permanent nucleus is established to continuously coordinate the work,which is intensely supported by industry delegates.

1987 Initial memorandum of understanding (MOU) signed by network operator

organizations (representing 12 countries) with major objectives as:

* coordinating the introduction of the standard and time scales.


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*Planning of service introduction

*Routing, billing, and tariff coordination.

1988/89 with the establishment of the European telecommunication

To Standards Institute (ETSI), the specification work was mooted to

1991/92 this international body.GSM becomes a technical committee within ETSI and

splits up to into GSM groups 1-4, later called Special Mobile Groups (SMG)

1-4, which are technical sub Committees. GSM finally stands for Global

system for Mobile Communications

1990 The GSM specifications for 900 MHz band are also applied to a Digital cellular

system on the 1800 MHz band (DCS1800), a PCN application initiated in the

United Kingdom.

1991 The GSM Recommendations comprise more than 130 single documents

including more than 5000 pages.

1992 Official commercial launch of GSM service in Europe.

1993 The GSM- MOU has 62 members (signatories) in 39 countries worldwide.

1993 The end of 1993 shows one million subscribers to GSM networks, however more

than 80% of them is to be found in Germany alone.

1993 First commercial services also start outside Europe: Australia, Hongkong.

The features and benefits expected in the new system were

Superior speech quality

Low terminal, operational, and service costs

A high level of security (confidentiality and fraud prevention)

International roaming

Support of low terminal hand portable terminals

A variety of new services and network facilities.


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1.7 CONSTRAINTS IN IMPLEMENTATION

A host of services viz., teleservices, supplementary services, and value added

services are being promised by GSM networks. There are certain impairments in

realizing an effective mobile communication system which has to meet the twin

objectives of quality and capacity. The following are the some of the problem areas in

deploying a GSM net work, which demand extensive planning and engineering.

(a) Radio frequency Utilization:

High spectrum efficiency should be achieved at reasonable cost .The bandwidth

on radio interface i.e. between the user equipment and the Radio transceiver, is to be

managed effectively to support ever increasing customer base with very limited number

of radio carriers. For high BW services e.g. MMS, as the GSM evolves towards 3G, more

spectrums is demanded. Bandwidth management is the key area, which decides the

success or otherwise of a mobile operator.

(b) Multipath radio environment:

The most significant problem in mobile radio systems is due to the channel itself.

In mobile radio systems, indeed, it is rare for there to exist one strong line of sight (LOS)

path between transmitter and receiver. Usually several significant signals are received by

reflection and scattering from buildings, etc...And then there are multiple paths from

transmitter to receiver.


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Figure 1:

The signals on these paths are subject to different delays, phase shifts, and Doppler shifts,

and arrives at the receiver in random phase relation to one another. The interference

between these signals gives rise to a number of deleterious effects. The most important of

these arefading and dispersion .Fading is due to the interference of multiple signals withrandom relative phase that causes variations in the amplitude of the received signal. This

will increase the error rate in digital systems, since errors will occur when the signal-to-

noise ratio drops below a certain threshold. Dispersion is due to differences in the delay

of the various paths, which disperses transmitted pulses in time. If the variation of the

delay is comparable with the symbol period, delayed signals from an earlier symbol may

interfere with the next symbol, causingInter-symbol interference (ISI).

The countermeasures for fading include diversity reception and equalization.

Mobility management:

The principal characteristic of mobile networks, which distinguishes them from

conventional fixed networks, is that the identity of calling and called subscribers is not

associated with a fixed geographical location. The subscribers establish a wireless

connection with the nearest base station, and can make or receive calls as they roam.

Radio

transceiver

Mobile eqpt

Multipath Radio environment


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Mobility management is concerned with how the network supports this function. When a

call is made to mobile customer, the network must be able to locate the mobile customer.

Network attachment process which includes a location updation process is the answer for

the mobility management. In the location update process, the network databases are

updated dynamically, so that the mobile can be reached to offer the services. If this

process is not done efficiently, it will result in poor call management and network

congestion.

(d) Services

International roaming shall be provided. Advanced PSTN services should be

provided consistent with ISDN services albeit at limited bit rates only. Encryption should

be used to improve security for both the operators and the customers.

(e) Network aspects:

ITU identification and numbering plans should be used an international signaling

system should be utilized. There should be a choice of charging structure and rates. No

modification shall be required to the PSTN due to its interconnection to GSM signaling

and control information should be protected.

(f) Cost:

The system parameters should be chosen to limit costs, particularly mobiles and

handsets. In a competitive environment, cost is the deciding factor for the survival of an

operator.


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CHAPTER-2

BANDWIDTH MANAGEMENT

2.1 INTRODUCTIONRadios move information from one place to another over channels, and radio

channel is an extraordinarily hostile medium to establish and maintain reliable

communications. The channel is particularly messy and unruly between mobile radios.

All the schemes and mechanisms we use to make communications possible on the mobile

radio channel with some measure of reliability between a mobile and its base radio

station are called physical layer, or the layer 1 procedures. The mechanisms include

modulation, power control, coding, timing, and host of other details that manage the

establishment and maintenance of the channel. The radio channel has to be fully

exploited for maximum capacities and optimum quality of service.

Band width is a scarce natural resource. The bandwidth has to be managed for

maximum capacity of the system and interference free communications. The spectrum

availability for an operator is very limited. The up link or down link spectrum is only

25 MHz, Out of this 25 MHz, 124 carriers of each 200 KHz are generated. These carriers

are to be shared amongst different operators. And as a result each operator gets only afew tens of carriers; making spectrum management a challenging area. The following

figure shows the radio connectivity between the mobile equipment and the Radio


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transmitter/receiver.

Figure 2: Radio Communication between mobile and Tx/Rx

For effective management of bandwidth, for conservation of spectrum and quality of

radio link; the following access techniques are implemented on the radio interface.

(1) Cellular structures and Frequency Reuse

(2) Multiple access Technologies

(3) Voice coding technologies

(4) Bandwidth effective Modulation scheme.

2.2 Cellular structures and Frequency Reuse

Traditional mobile service was structured similar to television broadcasting: One

very powerful transmitter located at the highest spot in an area would broadcast in a

radius of up to fifty kilometers. The scenario changes as the mobile density as well as the

coverage area grow. The answer to tackle the growth is coverage extensions based on

addition of new cells. The Cellular concept structured the mobile telephone network in a

Mobileswitch Radio

Controller RadioTransceiver

Mobile

Radio interface


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different way. Instead of using one powerful transmitter many low-powered transmitter

were placed through out a coverage area. For example, by dividing metropolitan region

into one hundred different areas (cells) with low power transmitters using twelve

conversations (channels) each, the system capacity could theoretically be increased from

twelve to thousands of conversations using one hundred low power transmitters while

reusing the frequencies.

The cellular concept employs variable low power levels, which allows cells to be sized

according to subscriber density and demand of a given area. As the populations grow,

cells can be added to accommodate that growth. Frequencies used in one cell cluster can

be reused in other cells. Conversations can be handed over from cell to cell to maintain

constant phone service as the user moves between cells.

Cells:

A cell is the basic geographic unit of cellular system. The term cellular comes

from the honeycomb areas into which a coverage region is divided. Cells are base

stations transmitting over small geographic areas that are represented as hexagons. Each

cell size varies depending upon landscape. Because of the constraint imposed by natural

terrain and man-made structures, the true shape of cell is not a perfect hexagon.

(a) Cellular System Characteristics

The distinguishing features of digital cellular systems compared to other mobile radio

systems are:

Small cells

A cellular system uses many base stations with relatively small coverage radii

(on the order of a 100 m to 30 km).

Clusters and Frequency reuse

The spectrum allocated for a cellular network is limited. As a result there is a limit

to the number of channels or frequencies that can be used. A group of cells is called a

cluster. All the frequencies are used in a cluster and no frequency is reused with in the

cluster. And the total set of frequencies is repeated in the adjacent cluster. Like that the

total service area, i.e may be a country or a continent, can be served with a small group of

frequencies. Frequency reuse is possible because the signal fades over the distance and


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hence it can be reused .For this reason each frequency is used simultaneously by multiple

base-mobile pairs; located at geographically distant cells. This frequency reuse allows a

much higher subscriber density per MHz of spectrum than other systems. System

capacity can be further increased by reducing the cell size (the coverage area of a single

base station), down to radii as small as 200 m.

Small, battery-powered handsets

In addition to supporting much higher densities than previous systems, this

approach enables the use of small, battery-powered handsets with a radio frequency that

is lower than the large mobile units used in earlier systems.

Performance of handovers

In cellular systems, continuous coverage is achieved by executing a handover

(the seamless transfer of the call from one base station to another) as the mobile unit

crosses cell boundaries. This requires the mobile to change frequencies under control of

the cellular network.

(b) Co channel cells and interference

Radio channels can be reused provided the separation between cells containing the same

channel set is far enough apart so that co-channel interference can be kept below

acceptable levels most of the time. Cells using the same channel set are called Co-

channel cells. Co-channel cells interfere with each other and quality is affected.

The following figure shows an example. Within the service area (PLMN), specific

channel sets are reused at a different location (another cell). In the example, there are 7

channel sets: A through G. Neighboring cells are not allowed to use the same frequencies.

For this reason all channel sets are used in a cluster of neighboring cells. As there are 7

channel sets, the PLMN can be divided into clusters of 7 cells each. The figure shows

three clusters.

Co-channel interference

Frequencies can be reused throughout a service area because radio signals typically

attenuate with distance to the base station (or mobile station). When the distance

between cells using the same frequencies becomes too small, co-channel Interference


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might occur and lead to service interruption or unacceptable quality of service.

As long as the ratio Frequency reuse distance = DCell radius R

is greater than some specified value, the ratio

Received radio carrier power = CReceived interferer radio carrier power I

will be greater than some given amount for small as well as large cell sizes; when all

signals are transmitted at the same power level. The average attenuation of radio signals

with distance in most cellular systems is a reduction to about 1/16 of the received power

for every doubling of distance (1/10000 per decade).

The frequency reuse distance known as separation distance is also known as the signal-

to-noise ratio. The figure on the opposite page shows the situation. At the base station,

both signals from subscribers within the cell covered by this base station and signals

from subscribers covered by other cells are received. Interference is caused by cells

using the same channel set. The ratio D/R needs to be large enough in order for the base

station to be able to cope with the interference. A co-channel interference factor Q is

defined

As Q=D/R = 3K where D is Frequency reuse distance ,Ris the cell radius and

Kis the reuse factor or the number of cells in a cluster.


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Figure 3: Illustration of cellular frequency concept

Capacity / performance trade-offs

When engineering a cellular network, the most important trade-off to make is the one

between call capacity and performance:

Relationship between K and Performance

The performance of a cellular network can be expressed in quality of service. That is

the value of Q shall be higher to achieve an acceptable quality of service. This means

a low (co-channel) interference level in the network.

The relationship between the reuse factor K and the network performance is: if K

increases, then the co-channel interference decreases, and so the performance

increases (note that there is a fixed relationship between K and ratio D/R).

Relationship between K and Cell Capacity

The other key relationship in cellular networks is the one between the reuse factor K

and call capacity. First of all, call capacity depends on the number of available

channels. In GSM, a limited number of frequencies is available (for GSM: 124

frequencies, and for GSM-1800: 374 frequencies). The frequencies are grouped into

cluster 1

Cluster 2

cluster3

R

D

K=reuse factor=No ofcells in a cluster

Q=D/R = 3K

Q is more Sys quality high-- K is more

-- No of cells in a

cluster more-- No of channels percell less

-- Traffic handlingcapacity low


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frequency sets. If K increases, the number of frequencies per set (and so per cell)

decreases, and so the call capacity per cell.

The value of K in GSM cellular networks varies between 4 and 21. Note that in real

networks, K is not a constant within the whole PLMN area, but varies depending on thetraffic capacity needed in certain regions. Typically, K is high in urban regions and low

in rural regions.

If K increases, then performance increases

If K increases, then call capacity decreases per cell

The number of sites to cover a given area with a given high traffic density, and hence

the cost of the infrastructure, is determined directly by the reuse factor and the number

of traffic channels that can be extracted from the available spectrum. These two factors

are compounded in what is called spectral efficiency of the system. Not all systems

allow the same performance in this domain: they depend in particular on the robustness

of the radio transmission scheme against interference, but also on the use of a number of

technical tricks, such as reducing transmission during the silences of a speech

communication. The spectral efficiency, together with the constraints on the cell size,

determines also the possible compromises between the capacity and the cost of the

infrastructure. All this explains the importance given to spectral efficiency.

2.3 DIGITAL MODULATION OF GSM RADIO : GMSK

The radio connectivity between the mobile station and the Radio transceiver is

made by transmitting carrier .The digital information generated by the system or the

network is to be imparted to the radio carrier by suitable digital modulation technique.

If the amplitude of a carrier is shifted with binary information, it is said ASK is

employed, wherein the amplitude of the carrier is switched between their full-on and

full-off conditions. If the carrier frequency is shifted with the binary information, this is

equivalent to shifting between two or more carriers of diff frequencies. This is FSK and

is widely used in analog cellular systems for signaling functions. There is no limit to the

number of carrier frequencies that can be shifted, but the use of two frequencies, quite

close together, is the universal implementation of FSK. As with FSK ,the shift between

various carriers differing from each other only in their relative phase(PSK).There are


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many varieties of PSK ,and each is broadly distinguished from the others by the number

of allowed phases .

Gaussian Minimum Shift Keying (GMSK)

The modulation specified for GSM is GMSK with BT=0.3 and rate 270 5/6

kbauds. GMSK is a type of constant envelope FSK, where the frequency modulation is a

result of a carefully contrived phase modulation .The most important feature of GMSK

is that it is a constant envelope variety of modulation. This means there is a distinct

lack of AM in the carrier with a constant limiting of the occupied bandwidth.

The constant amplitude of the GMSK signal makes it suitable for use with high

efficiency amplifiers. An easy way to understand the GMSK signal is to first investigate

its precursor, MinimumShift Keying(MSK).The following figure indicates the steps in

generating an MSK signal. How the data is treated in GMSK is explained below:

The waveforms are all aligned together in phase. Little scales are placed are placed

in the figure to help make the phase relationships between the waveforms clearer.

10 bits of the data stream {1101011000} is considered for analysis. The data stream is

divided into odd and even bit streams:(odd bits and even bits).In creating odd bits

and even bits ,each alternate odd and even bit in data is hold for two bit times.

Staggering odd bits and even bits already helps to create a waveform with minimal AM.

For convenience odd bits and even bits are made to take the values 1or -1. In GSMcase ,if the data rate (in waveform data) is 270.833 kbps, then the staggered odd bits

and even bits will have half the rate135.4 kbps .The fourth and fifth wave forms in the

following figure are the high freq and the low freq versions, respectively ,of the carrier.

Since MSK is a form of FSK, finally modulated carrier needs two diff. frequency

components (low and high).the MSK signal is created by shifting between these two

frequencies. The MSK signal is created starting with bit number 2, with the help of the

truth table given below along with the waveforms. At any instant the odd and even bit

values are taken from the table and follow the rules as given in the truth table to obtain

the MSK waveform at that instant. Either the high or the low freq versions of the carrier

is picked corresponding to the instant under consideration and also according to the

sense instructions(+or-) as given in the table ,the wave form is to be turned up or down.


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The resulting MSK waveform appears in waveform as MSK; which is the fifth

waveform in the figure. Smooth phase transitions can be noticed, as the MSK waveform

1 2 3 4 5 6 7 8 9 10

data

odd

bits

even

bits

high

freq

low

freq

MSK

wave

Generating Minimum Shift Keying

MSK truth tableDigital inputMSK OutputBit ValueFrequency

senseOdd bitEven BitHigh or Low+ or -11High--11Low-1-1Low+-

1-1High-


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changes its frequency one from the other. These high and low frequencies shall be as

close together as possible in the freq domain.

To make a GMSK signal from an MSK signal ,the stretched data waveforms (each135.4

kbps) have to be filtered with a Gaussian filter of an appropriate bandwidth defined by

the BT product(Bandwidth*Time).In GSM case ,BT is 0.3,which makes B=81.3 kHz

when T is 3.7 micro sec (T=1/270.833).

2.4 SPEECH CODING IN GSM

Due to the restricted transmission capacity on the radio channel, it is desirable

to minimize the number of bits we need to transmit. The information is transmitted

within pulses, so that the content, the representation of the originally continuous audio

signal, is compressed in the time domain when it is transmitted over the radio path.

Inside the receiver, the information is decompressed, or expanded, in order to regenerate

the continuous audio signal. The device that transforms the human voice into a digital

stream of data suitable for transmission over the radio interface and regenerates an

audible analog representation of the received data (voice) is called a speech codec.

2.4.1 How the speech coding works in GSM

Sound (human voice) is converted to an electrical signal by the microphone. To

digitize this analog signal, it is sampled at 8 KHz rate. The signal is sampled after

filtering. Every 125 micro seconds, a value is sampled from the analog signal and

quantized by a 13 bit word. The 125 micro sec sampling intervals are derived from a

sampling frequency of 8 KHz, which is 8000 samples per second. A sampling rate of

8000 samples per second means that the output of Analog to Digital converter delivers a

data rate of 8000x 13bps=104 Kbps.104 Kbps data is far too high to be economically

transmitted over the radio interface; considering the Bandwidth scarcity. Band width has

to be shared by number of users for costing advantages. The speech coder will have to

do something to significantly reduce this rate by extracting irrelevant components in thedata stream. The speech coder has to search for excess baggage we can safely remove

from the bit stream scheduled for transport over the radio path. GSM uses to processes

to strip redundant fat from the data representing voice traffic. The compression

algorithm used in GSM is a procedure called RPE-LTP ,explained below.


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2.4.2 REGULAR PULSE EXCITATION AND LONG TERM

PREDICTION (RPELTP)

Every 20ms, 160 sampled values from the ADC are taken and stored in an

intermediate memory. An analysis of a set of data samples produces eight filtercoefficients and an excitation signal for a time-invariant digital filter. This filter can be

regarded as a digital imitation of the human vocal tract, where the finer coefficients

represent vocal modifiers(e.g., teeth, tongue, pharynx)and the excitation signal

represents the sound(e.g., pitch , loudness) or the absence of sound that we pass through

the vocal tract(filter). A correct setting of filter coefficients and an appropriate excitation

signal yields a sound typical of the human voice.

The procedure, so far, has not performed any data reductions. The reductions

come in further steps, which take advantage of certain attributes of the human ear and

vocal tract .The 160 samples, transformed into filter coefficients, are divided into four

blocks of 40 samples each. Each block represents a 5-ms period of voice. These blocks

are sorted into four sequences. Where each sequence contains very forth sample from

the original 160 samples. Sequence number 1 contains samples 1, 5, 9, 13., 37,

sequence number 2 contains samples 2, 6, 10, 14, .38, Sequence number 3 contains

samples3, 7, 11, 15,39, and Sequence number 4 contains samples 4, 8, 12, 1640. The

first reduction in data comes when the speech encoder selects the sequence with the

most energy.

This linear predictive coding (LPC) and regular pulse excitation (RPE) analysis

has a very short memory of approximately 1ms. A more long-term consideration of

neighboring (or adjacent) blocks in time is not performed here, There are numerous

correlations in the human voice, especially in long vowels such as the in car, where the

same sound recurs in succeeding 5-ms samples. Taking the similarity of sounds between

adjacent samples (Adjacent 5-ms blocks) into account can significantly reduce the

amount of data required to describe the human voice. This second reduction task is

performed by a LTP Function.

2.4.3LONG-TERM PREDICTION ANALYSIS (LTP)


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The LTP function accepts a sequence selected by the LPC/RPE analysis. Upon

accepting sequence, it then looks among all the previous sequences passed to it (which

will reside in another intermediate memory for 15ms) for the earlier sequence that has

the highest correlation to ( bears the greatest resemblance to ) the current sequence. It

can be said that the LTP function looks for the one sequence from among those already

received that is most similar to the sequence just received from the LPC/RPE. Now it is

only necessary to transmit a value representing the difference between the two

sequences, along with a pointer to tell the receiver on the other end of the radio channel,

which sequence it should select among its recently received sequences for comparison.

The receiver knows which differential values it has to apply to which sequences. The

transmission of the whole sequence is not necessary, only the difference between

sequences, This further reduces the data on the channel.

The speech coder issues a block of 260bits (a speech frame) once every

20ms. This corresponds to net data rate of 13kbps, a data reduction of a factor of eight.

Speech transcoding is a task that requires a large number of calculations at high speeds.

It is, therefore, an ideal application for digital signal processing (DSP) techniques.

CHAPTER-3


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GSM NETWORK ARCHITECTURE

3.1 INTRODUCTION

A GSM system is basically designed as a combination of three major subsystems:

the network subsystem, (NSS)

the radio subsystem, (RSS) and

The operation support subsystem. (OSS)

In order to ensure that network operators will have several sources of cellular

infrastructure equipment, GSM decided to specify not only the air interface, but also the

main interfaces that identify different parts. There are three dominant interfaces, namely,

an interface between MSC and the base Transceiver Station (BTS), and an Um interface

between the BTS and MS.

3.2 GSM NETWORK STRUCTURE

Every telephone network needs a well-designed structure in order to route

incoming called to the correct exchange and finally to the called subscriber. In a mobile

network, this structure is of great importance because of the mobility of all its

subscribers [1-4]. In the GSM system, the network is divided into the following

partitioned areas.

GSM service area;

PLMN service area;

MSC service area;

Location area;

Cells.

The GSM service is the total area served by the combination of all member countries

where a mobile can be serviced. The next level is the PLMN service area. There can be

several within a country, based on its size. The links between a GSM/PLMN network


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and other PSTN, ISDN, or PLMN network will be on the level of international or

national transit exchange. All incoming calls for a GSM/PLMN network will be routed

to a gateway MSC. A gateway MSC works as an incoming transit exchange for the

GSM/PLMN. In a GSM/PLMN network, all mobile-terminated calls will be routed to a

gateway MSC. Call connections between PLMNs, or to fixed networks, must be routed

through certain designated MSCs called a gateway MSC. The gateway MSC contains

the interworking functions to make these connections. They also route incoming calls to

the proper MSC within the network. The next level of division is the MSC/VLR service

area. In one PLMN there can be several MSC/VLR service areas. MSC/VLR is a role

controller of calls within its jurisdiction. In order to route a call to a mobile subscriber,

the path through links to the MSC in the MSC area where the subscriber is currently

located. The mobile location can be uniquely identified since the MS is registered in a

VLR, which is generally associated with an MSC.

The next division level is that of the LAs within a MSC/VLR combination.

There are several LAs within one MSC/VLR combination. A LA is a part of the

MSC/VLR service area in which a MS may move freely without updating location

information to the MSC/VLR exchange that control the LA. Within a LA a paging

message is broadcast in order to find the called mobile subscriber. The LA can be

identified by the system using the Location Area Identity (LAI). The LA is used by the

GSM system to search for a subscriber in a active state.

Lastly, a LA is divided into many cells. A cell is an identity served by one BTS.

The MS distinguishes between cells using the Base Station Identification code (BSIC)

that the cell site broadcast over the air.

3.3 MOBILE STATION (MS)

The MS includes radio equipment and the man machine interface (MMI) that a

subscribe needs in order to access the services provided by the GSM PLMN. MS can be

installed in Vehicles or can be portable or handheld stations. The MS may include

provisions for data communication as well as voice. A mobile transmits and receives

message to and from the GSM system over the air interface to establish and continue

connections through the system.


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Different type of MSs can provide different type of data interfaces. To provide a

common model for describing these different MS configuration, reference

configuration for MS, similar to those defined for ISDN land stations, has been

defined. Each MS is identified by an IMEI that is permanently stored in the mobile unit.

Upon request, the MS sends this number over the signaling channel to the MSC. The

IMEI can be used to identify mobile units that are reported stolen or operating

incorrectly.

Just as the IMEI identities the mobile equipment, other numbers are used to

identity the mobile subscriber. Different subscriber identities are used in different phases

of call setup. The Mobile Subscriber ISDN Number (MSISDN) is the number that the

calling party dials in order to reach the subscriber. It is used by the land network to route

calls toward an appropriate MSC. The international mobile subscribe identity (IMSI) is

the primary function of the subscriber within the mobile network and is permanently

assigned to him. The GSM system can also assign a Temporary Mobile Subscriber

Identity (TMSI) to identity a mobile. This number can be periodically changed by the

system and protect the subscriber from being identified by those attempting to monitor

the radio channel.

3.3.1 Functions of MS

The primary functions of MS are to transmit and receive voice and data over theair interface of the GSM system. MS performs the signal processing function of

digitizing, encoding, error protecting, encrypting, and modulating the transmitted

signals. It also performs the inverse functions on the received signals from the BS.

In order to transmit voice and data signals, the mobile must be in synchronization

with the system so that the messages are the transmitted and received by the mobile at

the correct instant. To achieve this, the MS automatically tunes and synchronizes to the

frequency and TDMA timeslot specified by the BSC. This message is received over a

dedicated timeslot several times within a multiform period of 51 frames. We shall

discuss the details of this in the next chapter. The exact synchronization will also

include adjusting the timing advance to compensate for varying distance of the mobile

from the BTS.


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Figure 4: Network Architecture

The MS monitors the power level and signal quality, determined by the BER for

known receiver bit sequences (synchronization sequence), from both its current BTS and

up to six surrounding BTSs. This data is received on the downlink broadcast control

channel. The MS determines and send to the current BTS a list of the six best-received

BTS signals. The measurement results from MS on downlink quality and surrounding

BTS signal levels are sent to BSC and processed within the BSC. The system then uses

this list for best cell handover decisions.

BTS

MSC/VL

R

HL

PSTN

ISDN

Dat

aNetworks

Air

OSS

BTS

BTS

MSCVLR

BSBS

AA-bis

interface


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MS keeps the GSM network informed of its location during both national and

international roaming, even when it is inactive. This enables the system to page in its

present LA.

The MS includes an equalizer that compensates for multi-path distortion on the

received signal. This reduces inter-symbol interference that would otherwise degrade the

BER.

Finally, the MS can store and display short received alphanumeric messages on the

liquid crystal display (LCD) that is used to show call dialing and status information.

These messages are limited to 160 characters in length.

Power Levels

These are five different categories of mobile telephone units specified by the

European GSM system: 20W, 8W, 5W, 2W, and 0.8W. These correspond to 43-dBm,

39-dBm, 37-dBm, 33-dBm, and 29-dBm power levels. The 20-W and 8-W units (peak

power) are either for vehicle-mounted or portable station use.

The MS power is adjustable in 2-dB steps from its nominal value down to 20mW

(13 dBm). This is done automatically under remote control from the BTS, which

monitors the received power and adjusts the MS transmitter to the minimum power

setting necessary for reliable transmission.

3.3.2 SIM Card

As described in the first chapter, GSM subscribers are provided with a SIM card with

its unique identification at the very beginning of the service. By divorcing the subscriber

ID from the equipment ID, the subscriber may never own the GSM mobile equipment

set. The subscriber is identified in the system when he inserts the SIM card in the mobile

equipment. This provides an enormous amount of flexibility to the subscribers since

they can now use any GSM-specified mobile equipment. Thus with a SIM card the idea

of Personalize the equipment currently in use and the respective information used by

the network (location information) needs to be updated. The smart card SIM is portable

between Mobile Equipment (ME) units. The user only needs to take his smart card on a

trip. He can then rent a ME unit at the destination, even in another country, and insert

his own SIM. Any calls he makes will be charged to his home GSM account. Also, the


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GSM system will be able to reach him at the ME unit he is currently using.

The SIM is a removable SC, the size of a credit card, and contains an integrated

circuit chip with a microprocessor, random access memory (RAM), and read only

memory (ROM). It is inserted in the MS unit by the subscriber when he or she wants to

use the MS to make or receive a call. As stated, a SIM also comes in a modular from

that can be mounted in the subscribers equipment.

When a mobile subscriber wants to use the system, he or she mounts their SIM card

and provide their Personal Identification Number (PIN), which is compared with a PIN

stored within the SIM. If the user enters three incorrect PIN codes, the SIM is disabled.

The PIN can also be permanently bypassed by the service provider if requested by the

subscriber. Disabling the PIN code simplifies the call setup but reduces the protection of

the users account in the event of a stolen SIM.

3.4 IDENTIFICATION NUMBERS

3.4.1 International Mobile Subscriber Identity.(IMSI)

An IMSI is assigned to each authorized GSM user. It consists of a mobile country code

(MSC), mobile network code (MNC), and a PLMN unique mobile subscriber

identification number (MSIN). The IMSI is not hardware-specific. Instead, it is

maintained on a SC by an authorized subscriber and is the only absolute identity that a

subscriber has within the GSM system. The IMSI consists of the MCC followed by the

NMSI and shall not exceed 15 digits.

3.4.2Temporary Mobile Subscriber Identity (TMSI)

A TMSI is a MSC-VLR specific alias that is designed to maintain user confidentiality. It

is assigned only after successful subscriber authentication. The correlation of a TMSI to

an IMSI only occurs during a mobile subscribers initial transaction with an MSC (for

example, location updating). Under certain condition (such as traffic system disruption

and malfunctioning of the system), the MSC can direct individual TMSIs to provide the


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MSC with their IMSI.

3.4.3 Mobile Station ISDN Number (MSISDN)

The MS international number must be dialed after the international prefix in order to

obtain a mobile subscriber in another country. The MSISDN numbers is composed of

the country code (CC) followed by the National Significant Number (N(S)N), which

shall not exceed 15 digits.

3.4.4 The Mobile Station Roaming Number (MSRN)

The MSRN is allocated on temporary basis when the MS roams into another numbering

area. The MSRN number is used by the HLR for rerouting calls to the MS. It is assigned

upon demand by the HLR on a per-call basis. The MSRN for PSTN/ISDN routing shall

have the same structure as international ISDN numbers in the area in which the MSRN

is allocated. The HLR knows in what MSC/VLR service area the subscriber is located.

At the reception of the MSRN, HLR sends it to the GMSC, which can now route the call

to the MSC/VLR exchange where the called subscriber is currently registered.

3.4.5 International Mobile Equipment Identity (IMEI)

The IMEI is the unique identity of the equipment used by a subscriber by each PLMN

and is used to determine authorized (white), unauthorized (black), and malfunctioning

(gray) GSM hardware. In conjunction with the IMSI, it is used to ensure that only

authorized users are granted access to the system. An IMEI is never sent in cipher mode

by MS.

3.5 BASE STATION SYSTEM (BSS)


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The BSS is a set of BS equipment consisting of a Radio transmitter/receiver

called BTS (Base Transceiver Station)and a controller called BSC (Base Station

Controller)The BSS is viewed by the MSC through a single A interface as being the

entity responsible for communicating with MSs in a certain area. The radio equipment

of a BSS may be composed of one or more cells. A BSS may consist of one or more

BTS. The interface between BSC and BTS is designed as an A-bis interface. The BSS

includes two types of machines: the BTS in contact with the MSs through the radio

interface and the BSC, the latter being in contact with the MSC. The function split is

basically between transmission equipment, the BTS, and managing equipment at the

BSC. A BTS compares radio transmission and reception devices, up to and including the

antennas, and also all the signal processing specific to the radio interface. A single

transceiver within BTS supports eight basic radio channels of the same TDM frame. A

BSC is a network component in the PLMN that function for control of one or more

BTS. It is a functional entity that handles common control functions within a BTS.

A BTS is a network component that serves one cell and is controlled by a BSC.

BTS is typically able to handle three to five radio carries, carrying between 24 and 40

simultaneous communication. Reducing the BTS volume is important to keeping down

the cost of the cell sites.

An important component of the BSS that is considered in the GSM architecture

as a part of the BTS is the Transcoder/Rate Adapter Unit (TRAU). The TRAU is the

equipment in which coding and decoding is carried out as well as rate adoption in case

of data. Although the specifications consider the TRAU as a subpart of the BTS, it can

be sited away from the BTS (at MSC), and even between the BSC and the MSC.

The interface between the MSC and the BSS is a standardized SS7 interface (A-

interface) that, as stated before, is fully defined in the GSM recommendations. This

allows the system operator to purchase switching equipment from one supplier and radio

equipment and the controller from another. The interface between the BSC and a remote


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BTS likewise is a standard the A-bis. In splitting the BSS functions between BTS and

BSC, the main principle was that only such functions that had to reside close to the radio

transmitters/receivers should be placed in BTS. This will also help reduce the

complexity of the BTS.

3.5.1 Functions of BTS

As stated, the primary responsibility of the BTS is to transmit and receive radio signals

from a mobile unit over an air interface. To perform this function completely, the signals

are encoded, encrypted, multiplexed, modulated, and then fed to the antenna system at

the cell site. Trans-coding to bring 13-kbps speech to a standard data rate of 16 kbps and

then combining four of these signals to 64 kbps is essentially a part of BTS, though it

can be done at BSC or at MSC. The voice communication can be either at a full or half

rate over logical speech channel. In order to keep the mobile synchronized, BTS

transmits frequency and time synchronization signals over frequency correction channel

(FCCH and BCCH logical channels. The received signal from the mobile is decoded,

decrypted, and equalized for channel impairments.

Random access detection is made by BTS, which then sends the message to BSC. The

channel subsequent assignment is made by BSC. Timing advance is determined by BTS.

BTS signals the mobile for proper timing adjustment. Uplink radio channelmeasurement corresponding to the downlink measurements made by MS has to be made

by BTS.

3.5.2 BTS-BSC Configurations

There are several BTS-BSC configurations: single site; single cell; single site; multicell;

and multisite, multicell. These configurations are chosen based on the rular or urban

application. These configurations make the GSM system economical since the operation

has options to adapt the best layout based on the traffic requirement. Thus, in some

sense, system optimization is possible by the proper choice of the configuration. These

include omni directional rural configuration where the BSC and BTS are on the same

site; chain and multidrop loop configuration in which several BTSs are controlled by a

single remote BSC with a chain or ring connection topology; rural star configuration in

which several BTSs are connected by individual lines to the same BSC; and sectorized


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urban configuration in which three BTSs share the same site and are controlled by either

a collocated or remote BSC.

In rural areas, most BSs are installed to provide maximum coverage rather then

maximum capacity.

3.6 Transcoder (TXCDR)

Depending on the relative costs of a transmission plant for a particular cellular operator,

there may be some benefit, for larger cells and certain network topologies, in having the

transcoder either at the BTS, BSC or MSC location. If the transcoder is located at MSC,

they are still considered functionally a part of the BSS. This approach allows for the

maximum of flexibility and innovation in optimizing the transmission between MSC

and BTS.

The transcoder is the device that takes 13-Kbps speech or 3.6/6/12-Kbps data

multiplexes and four of them to convert into standard 64-Kbps data. First, the 13 Kbps

or the data at 3.6/6/12 Kbps are brought up to the level of 16 Kbps by inserting

additional synchronizing data to make up the difference between a 13-Kbps speech or

lower rate data, and then four of them are combined in the transcoder to provide 64

Kbps channel within the BSS. Four traffic channels can then be multiplexed on one 64-

Kpbs circuit. Thus, the TRAU output data rate is 64 Kbps. Then, up to 30 such 64-Kpbs

channels are multiplexed onto a 2.048 Mbps if a CEPT1 channel is provided on the A-

bis interface. This channel can carry up to 120-(16x 120) traffic and control signals.

Since the data rate to the PSTN is normally at 2 Mbps, which is the result of combining

30-Kbps by 64-Kbph channels, or 120- Kbps by 16-Kpbs channels.

3.6.1 BASE STATION CONTROLLER (BSC)The BSC, as discussed, is connected to the MSC on one side and to the BTS on

the other. The BSC performs the Radio Resource (RR) management for the cells under

its control. It assigns and release frequencies and timeslots for all MSs in its own area.

The BSC performs the intercell handover for MSs moving between BTS in its control. It

also reallocates frequencies to the BTSs in its area to meet locally heavy demands


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during peak hours or on special events. The BSC controls the power transmission of

both BSSs and MSs in its area. The minimum power level for a mobile unit is broadcast

over the BCCH. The BSC provides the time and frequency synchronization reference

signals broadcast by its BTSs. The BSC also measures the time delay of received MS

signals relative to the BTS clock. If the received MS signal is not centered in its

assigned timeslot at the BTS, The BSC can direct the BTS to notify the MS to advance

the timing such that proper synchronization takes place. The functions of BSC are as

follows.

The BSC may also perform traffic concentration to reduce the number of

transmission lines from the BSC to its BTSs, as discussed in the last section.

3.7 SWITCHING SUBSYSTEMS:

3.7.1 MOBILE SWITCHING CENTER ( MSC) and

GATEWAY SWITCHING CENTER (GMSC)

The network and the switching subsystem together include the main switching

functions of GSM as well as the databases needed for subscriber data and mobility

management (VLR). The main role of the MSC is to manage the communications

between the GSM users and other telecommunication network users. The basic

switching functions of performed by the MSC, whose main function is to coordinate

setting up calls to and from GSM users. The MSC has interface with the BSS on one

side (through which MSC VLR is in contact with GSM users) and the external networks

on the other (ISDN/PSTN/PSPDN). The main difference between a MSC and an

exchange in a fixed network is that the MSC has to take into account the impact of the

allocation of RRs and the mobile nature of the subscribers and has to perform, in

addition, at least, activities required for the location registration and handover.

The MSC is a telephony switch that performs all the switching functions

for MSs located in a geographical area as the MSC area. The MSC must also handle

different types of numbers and identities related to the same MS and contained in

different registers: IMSI, TMSI, ISDN number, and MSRN. In general identities are

used in the interface between the MSC and the MS, while numbers are used in the fixed


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part of the network, such as, for routing.

3.7.2 Functions of MSC

As stated, the main function of the MSC is to coordinate the set up of calls

between GSM mobile and PSTN users. Specifically, it performs functions such as

paging, resource allocation, location registration, and encryption.

Specifically, the call-handling function of paging is controlled by MSC. MSC

coordinates the set up of call to and from all GSM subscribers operating in its areas. The

dynamics allocation of access resources is done in coordination with the BSS. More

specifically, the MSC decides when and which types of channels should be assigned to

which MS. The channel identity and related radio parameters are the responsibility of

the BSS, The MSC provides the control of interworking with different networks. It is

transparent for the subscriber authentication procedure.

The MSC supervises the connection transfer between different BSSs for MSs,

with an active call, moving from one call to another. This is ensured if the two BSSs are

connected to the same MSC but also when they are not. In this latter case the procedure

is more complex, since more then one MSC in involved. The MSC performs billing on

calls for all subscribers based in its areas. When the subscriber is roaming elsewhere, the

MSC obtains data for the call billing from the visited MSC. Encryption parameters

transfers from VLR to BSS to facilitate ciphering on the radio interface are done byMSC. The exchange of signaling information on the various interface toward the other

network elements and the management of the interface themselves are all controlled by

the MSC. Finally, the MSC serves as a SMS gateway to forward SMS messages from

Short Message Service Centers (SMSC) to the subscribers and from the subscribers to

the SMSCs. It thus acts as a message mailbox and delivery system.

3.7.3 VLR (VISITOR LOCATION REGISTER)

The VLR is collocated with an MSC. A MS roaming in an MSC area is

controlled by the VLR responsible for that area. When a MS appears in a LA, it starts a

registration procedure. The MSC for that area notices this registration and transfers to

the VLR the identify of the LA where the MS is situated. A VLR may be in charge of

one or several MSC LAs. The VLR constitutes the databases that support the MSC in


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the storage and retrieval of the data of subscribers present in its area. When an MS

enters the MSC area borders, it signals its arrival to the MSC that stores its identify in

the VLR. The information necessary to manage the MS is contained in the HLR and is

transferred to the VLR so that they can be easily retrieved if so required.

Data Stored in VLR

The data contained in the VLR and in the HLR are more or less the same.

Nevertheless the data are present in the VLR only as long as the MS is registered in the

area related to that VLR. Data associated with the movement of mobile are IMSI,

MSISDN, MSRN, and TMSI. The terms permanent and temporary, in this case, are

meaningful only during that time interval. Some data are mandatory, others are optional.

3.7.4 HOME LOCATION REGISTER (HLR)

The HLR is a database that permanently stores data related to a given set of

subscribers. The HLR is the reference database for subscriber parameters. Various

identification numbers and addresses as well as authentication parameters, services

subscribed, and special routing information are stored. Current subscriber status

including a subscribers temporary roaming number and associated VLR if the mobile is

roaming, are maintained.

The HLR provides data needed to route calls to all MS-SIMs home based in itsMSC area, even when they are roaming out of area or in other GSM networks. The HLR

provides the current location data needed to support searching for and paging the MS-

SIM for incoming calls, wherever the MS-SIM may be. The HLR is responsible for

storage and provision of SIM authentication and encryption parameters needed by the

MSC where the MS-SIM is operating. It obtains these parameters from the AUC.

The HLR maintains record of which supplementary service each user has subscribed to

and provides permission control in granting services. The HLR stores the identification

of SMS gateways that have messages for the subscriber under the SMS until they can be

transmitted to the subscriber and receipt is knowledge.

Some data are mandatory, other data are optional. Both the HLR and the VLR

can be implemented in the same equipment in an MSC (collocated). A PLMN may


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contain one or several HLRs.

3.7.5 AUTHENTICATION CENTER (AUC)

The AUC stores information that is necessary to protect communication through

the air interface against intrusions, to which the mobile is vulnerable. The legitimacy of

the subscriber is established through authentication and ciphering, which protects the

user information against unwanted disclosure. Authentication information and ciphering

keys are stored in a database within the AUC, which protects the user information

against unwanted disclosure and access.

In the authentication procedure, the key Ki is never transmitted to the mobile

over the air path, only a random number is sent. In order to gain access to the system,

the mobile must provide the correct Signed Response (SRES) in answer to a random

number (RAND) generated by AUC.

Also, Ki and the cipher key Kc are never transmitted across the air interface

between the BTS and the MS. Only the random challenge and the calculated response

are transmitted. Thus, the value of Ki and Kc are kept secure. The cipher key, on the

other hand, is transmitted on the SS7 link between the home HLR/AUC and the visited

MSC, which is a point of potential vulnerability. On the other hand, the random number

and cipher key is supposed to change with each phone call, so finding them on one call

will not benefit using them on the next call.The HLR is also responsible for the authentication of the subscriber each time

he makes or receives a call. The AUC, which actually performs this function, is a

separate GSM entity that will often be physically included with the HLR. Being

separate, it will use separate processing equipment for the AUC database functions.

3.7.6 EQUIPMENT IDENTIFY REGISTER (EIR)

EIR is a database that stores the IMEI numbers for all registered ME units. The

IMEI uniquely identifies all registered ME. There is generally one EIR per PLMN. It

interfaces to the various HLR in the PLMN. The EIR keeps track of all ME units in the

PLMN. It maintains various lists of message. The database stores the ME identification

and has nothing do with subscriber who is receiving or originating call. There are three


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classes of ME that are stored in the database, and each group has different

characteristics.

White List: -contains those IMEIs that are known to have been assigned to

valid MSs. This is the category of genuine equipment.

Black List: - contains IMEIs of mobiles that have been reported stolen.

Gray List: - contains IMEIs of mobiles that have problems (for example,

faulty software, and wrong make of the equipment). This list contains all

MEs with faults not important enough for barring.

INTERWORKING FUNCTION

GSM provided a wide range of data services to its subscribers. The GSM

system interface with the various forms of public and private data networks

currently available. It is the job of the IWF to provide this interfacing

capability.

The IWF, which in essence is a part of MSC, provides the subscriber with access to data

rate and protocol conversion facilities so that data can be transmitted between GSM

Data Terminal Equipment (DTE) and a land-line DTE.

ECHO CANCELER (EC)

EC is used on the PSTN side of the MSC for all voice circuits. The EC is

required at the MSC PSTN interface to reduce the effect of GSM delay when the mobile is

connected to the PSTN circuit. The total round-trip delay introduced by the GSM system,

which is the result of speech encoding, decoding and signal processing, is of the order of

180 ms. Normally this delay would not be an annoying factor to the mobile, except when

communicating to PSTN as it requires a two-wire to four-wire hybrid transformer in the

circuit. This hybrid is required at the local switching office because the standard local loop

is a two-wire circuit. Due to the presence of this hybrid, some of the energy at its four-wire

receive side from the mobile is coupled to the four-wire transmit side and thus

retransmitted to the mobile. This causes the echo, which does not affect the land subscriber

but is an annoying factor to the mobile. The standard EC cancels about 70 ms of delay.


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During a normal PSTN (land-to-land call), no echo is apparent because the delay

is too short and the land user is unable to distinguish between the echo and the normal

telephone side tones However, with the GSM round-trip delay added and without the EC,

the effect would be irritating to the MS subscriber.

3.8 OPERATION AND MAINTENANCE CENTER (OMC)

The OMC provides alarm-handling functions to report and log alarms generated

by the other network entities. The maintenance personnel at the OMC can define that

criticality of the alarm. Maintenance covers both technical and administrative actions to

maintain and correct the system operation, or to restore normal operations after a

breakdown, in the shortest possible time.

The fault management functions of the OMC allow network devices to be

manually or automatically removed from or restored to service. The status of network

devices can be checked, and tests and diagnostics on various devices can be invoked. For

example, diagnostics may be initiated remotely by the OMC. A mobile call trace facility

can also be invoked. The performance management functions included collecting traffic

statistics from the GSM network entities and archiving them in disk files or displaying

them for analysis. Because a potential to collect large amounts of data exists, maintenance

personal can select which of the detailed statistics to be collected based on personal

interests and past experience. As a result of performance analysis, if necessary, an alarm

can be set remotely.

The OMC provides system change control for the software revisions and

configuration data bases in the network entities or uploaded to the OMC. The OMC also

keeps track of the different software versions running on different subsystem of the GSM.

Documents

1-Introduction,Bw Mangmnt,Gsm Architecture