GSM-Global System for Mobile Communication Thanks and Regards

GSM- Global System for Mobile Communication

Hi Dear Readers!

In this revision of the document, new data has been added with

white color of words and black shading to make the readers feel

comfortable with new information in the document without reading all

pages again. Further more, a chapter “GSM SUBSCRIBERS DATA” has been

added as Chapter # 6. I hope you will continue to enjoy the Document.

Thanks and Regards!

Engr. Syed Muhammad Munavvar Hussain

Engr. Syed Muhammad Munavvar Hussain 1


Chapter 1

INTRODUCTION TO GSM



1.1 Introduction

GSM is not only the acronym of Global System for Mobile Communication;

it stands for an extraordinary successful stage of development in the modern

information age. GSM stands for a new dimension for more than 50 million users

worldwide; it stands for more than 100 countries and above 220 network

operators; it stands for complexity; it stands for versatility. Wireless

communication has become possible with GSM in any town, any county, and

even on any continent of the world. [114]

Global System for Mobile Communication (GSM) is a set of ETSI standards

specifying the infrastructure for a digital cellular service. The standard is used in

approx. 85 countries in the world including such locations as Europe, Japan and

Australia. [001]

Figure 1.1: The Simple Representation of GSM Network [065]

GSM is worldwide standard that allows users of different operators to

connect and to shares the services simultaneously. GSM has been the backbone


http://www.etsi.org/


of the phenomenal success in mobile telecommunication over the last decade.

Now, at the dawn of the era of true broadband services, GSM continues to

evolve to meet new demands. One of GSM's great strengths is its international

roaming capability, giving consumers a seamless service in about 160 countries.

This has been a vital driver in growth, with around 300 million GSM subscribers

currently in Europe and Asia. In the Americas, today's 7 million subscribers are

set to grow rapidly, with market potential of 500 million in population, due to the

introduction of GSM 800, which allows operators using the 800 MHz band to

have access to GSM technology too. [002]

The Europeans realized this early on, and in 1982 the Conference of

European Posts and Telegraphs (CEPT) formed a study group called the Groupe

Spécial Mobile (GSM) to study and develop a pan-European public land mobile

system. The proposed system had to meet certain criteria:

• good subjective speech quality,

• low terminal and service cost,

• support for international roaming,

• ability to support handheld terminals,

• support for range of new services and facilities

• spectral efficiency, and

• ISDN compatibility. [003]

GSM is a standard for a Global System for Mobile communications. Global

System for Mobile communications, a mobile phone system based on multiple

radio cells (cellular mobile phone network). It has been agreed upon and is

completed by ETSI, the European Telecommunications Standards Institute .

GSM technology contains the essential “intelligent” functions for the supports

of the personal mobility, especially with regard to user identification and

authentication, and for the localization and administration of the mobile users.



GSM is mainly used for speech communication, but its use for mobile data

communication is growing steadily. The key technologies used in GSM are SMS,

General Packet Radio Service (GPRS), and Wireless Application Protocol

(WAP).

Here I am eager to mention the words of Mr. Chunk Parish, Founding

Member and Chairman (1998-1999) WAP Forum; “WAP is a major step in

building the Wireless Internet, where people on-the-go can access the internet

through their wireless devices to get information such as e-mails, news

headlines, stock reports, map directions, and sports scores when they need it

and where they need.” [115]

1.2 Two Main Standards of GSM

Two main standards are followed:

1. GSM 900 (global system for mobile communications in the 900 MHz band)

2. DCS 1800 (digital cellular system for the 1800 MHz band)

GSM 900 is a designed for extensive radio coverage even in rural areas.

DCS 1800 is designed for radio coverage in areas with very high subscriber

density.

GSM is a global standard, GSM 900 being used in most European, Asian and

pacific countries, GSM 1800 being used in the same place to increase the

capacity of the system, and GSM 1900 being used primarily in the US.

In its simplest form, Global System for Mobile Communication (GSM) is a set

of ETSI standards specifying the infrastructure for a digital cellular service. The

standard is used in approx. 85 countries in the world including such locations as

Europe, Japan and Australia.

The international designation of a public mobile radio network is PLMN

(Public Land Mobile Network), as opposed to the PSTN (Public Switched

Telephone Network).

Several PLMN, which are designed on the basis of same standards, are

compatible to each other. Therefore, a mobile subscriber can use the GSM/DCS


http://www.etsi.org/


specific mobile equipment and services in these compatible networks.[]

Hutchison Whampoa Limited. [004]

1.3 GSM frequency bands

System Band Uplink (MHz) Downlink (MHz)

E-GSM-900 900 880.0–915.0 925.0–960.0

R-GSM-900 900 876.0–915.0 921.0–960.0

T-GSM-900 900 870.4–876.0 915.4–921.0

DCS-1800 1800 1710.0–1785.0 1805.0–1880.0

PCS-1900 1900 1850.0–1910.0 1930.0–1990.0

Table 1.1: Frequency range for different GSM standards [005]

Pictorial representation of the uplink and downlink frequencies is given as

follows:

Figure 1.2: GSM Frequency Bands [108]



1.4 Specifications and Characteristics for GSM

Frequency band—the frequency range specified for GSM is 1,850 to

1,990 MHz (mobile station to base station).

Duplex distance—the duplex distance is 80 MHz. Duplex distance is the

distance between the uplink and downlink frequencies. A channel has two

frequencies, 80 MHz apart.

Channel separation—the separation between adjacent carrier

frequencies. In GSM, this is 200 kHz.

Modulation—Modulation is the process of sending a signal by changing

the characteristics of a carrier frequency. This is done in GSM via

Gaussian minimum shift keying (GMSK).

Transmission rate—GSM is a digital system with an over-the-air bit rate

of 270 kbps.

Access method—GSM utilizes the time division multiple access (TDMA)

concept. TDMA is a technique in which several different calls may share

the same carrier. Each call is assigned a particular time slot.

Speech coder—GSM uses linear predictive coding (LPC). The purpose of

LPC is to reduce the bit rate. The LPC provides parameters for a filter that

mimics the vocal tract. The signal passes through this filter, leaving behind

a residual signal. Speech is encoded at 13 kbps. [006], [007], [008], [009],

[010]



The next generation of the mobile communication is known as Universal Mobile

Telecommunication System (UMTS) in Europe and as International Mobile

Telecommunication System 2000 (IMT-2000) worldwide. [114]



Chapter 2

GSM NETWORK AREAS


PLMN Service Area

MSC/VLR Service Area

Location Area

Cell


2.1 GSM Network Areas

The GSM network is made up of geographic areas. As shown in Figure,

these areas include cells, location areas (LAs), MSC/VLR service areas, and

public land mobile network (PLMN) areas.

Figure 2.1: Network Areas [011]

2.1.1 Cell

Cell is the basic service area. The cell is the area given radio coverage by

one base transceiver station. The GSM network identifies each cell via the cell

global identity (CGI) number assigned to each cell.

2.1.2 Location Area

The location area is a group of cells. It is the area in which the subscriber

is paged. Each LA is served by one or more base station controllers, yet only by

a single MSC. Each LA is assigned a location area identity (LAI) number.



Figure 2.2: Location Areas

2.1.3 MSC/VLR Service Area

An MSC/VLR service area represents the part of the GSM network that is

covered by one MSC and which is reachable, as it is registered in the VLR of the

MSC (see Figure).

Figure 2.3: MSC/VLR service area [011], [012]

2.1.4 PLMN Service Area

The area covered by one network operator is called PLMN. A PLMN can

contain one or more MSCs. [013]



2.2 Cell-Detailed Description

In a cellular system, the communication area of the service provider is divided

into small geographical areas called cells. Each cell contains following

components:

• An antenna

• Solar or AC power network station

The solar or AC powered network station is called the Base Station (BS).

[109]

Figure 2.4: Cells [014]

A cell simply corresponds to the covering area of a transmitter or a small

collection of transmitters used for the area. The size of the cell may typically

range between 1 to 12 miles. The size of the cell is determined by many factors,

but the two important factors are:

• The transmitter power

• The population of the geographical region



2.2.1 Why Hexagonal Shaped Cells are better?

Cells are drawn in hexagonal shape because the hexagonal shaped cells

have no gaps or overlaps between them. It causes no interruption to the

communication of a mobile subscriber moving from one cell to another. It is

obvious from the figure that other shapes of the cells are leaving gaps where no

coverage is provided to the mobile users. On the other hand, there is no such

problem in hexagonal cells.

Figure 2.5: Types of Cells

2.2.2 Types of Cells

Due to the uneven changes in the population density of different countries

and regions in the world, there are different types of cells used according to the

best results in uninterruptible communication. These are listed as:

• Macro Cells

• Micro Cells

• Pico Cells

• Umbrella Cells

• Selective Cells [015], [016]



a) Macro Cells

Macro cells can be regarded as cells where the base station antenna is

installed on a mast or larger building structures that are taller than an average

roof-top level. [017]

A macro cell is a cell in a mobile phone network that provides radio

coverage served by a power cellular base station (tower). Generally, macro cells

provide coverage larger than micro cell such as rural areas or along highways.

The antennas for macro cells are mounted on ground-based masts, rooftops and

the other existing structures, at a height that provides a clear view over the

surrounding buildings and terrain. Macro cell base stations have power outputs of

typically tens of watts. [018], [019], [020]

b) Micro Cells

A micro cell is a cell in a mobile phone network served by a low power

cellular base station (tower), covering a limited area such as a mall, a hotel, or

a transportation hub. A micro cell is usually larger than a Pico cell, though the

distinction is not always clear. Typically the range of a micro cell is less than a

mile wide. [021]

The antennas for micro cells are mounted at street level. Micro cell

antennas are smaller than macro cell antennas and when mounted on existing

structures can often be disguised as building features. Micro cells provide radio

coverage over distances up to, typically, between 300m and 1000m. Micro cell

base stations have lower output powers than macro cells, typically a few watts.

[022], [026]

c) Pico Cells

Pico cells are small cells whose diameter is only few dozen meters; they

are used mainly in indoor applications. It can cover e.g. a floor of


http://en.wikipedia.org/wiki/Picocell

http://en.wikipedia.org/wiki/Base_station

http://en.wikipedia.org/wiki/Mobile_phone

http://en.wikipedia.org/wiki/Microcell

http://en.wikipedia.org/wiki/Mobile_Phone


a building or an entire building, or for example in shopping centres or airports.

[023] Pico cells provide more localized coverage than micro cells, inside

buildings where coverage is poor or there are high numbers of users. [024], [026]

d) Umbrella Cells

A layer with micro cells is covered by at least one macro cell, and a micro

cell can in turn cover several pico cells, the covering cell is called an umbrella

cell. If there are very small cells and a user is crossing the cells very quickly, a

large number of handovers will occur among the different neighboring cells. The

power level inside an umbrella cell is increased compared to the micro cells with

which it is formed. This makes the mobile to stay in the same cell (umbrella cell)

causing the number of handovers to be decreased as well as the work to be

done by the network. [025], [026]

e) Selective Cells

The full coverage of the cells may not be required in all sorts of

applications, but cells with limited coverage are used with a particular shape.

These are named selective due to the selection of their shape with respect to the

coverage areas. For example, the cells used at the entrance of the tunnels are

selective cells because coverage of 120 degrees is used in them. [026]

2.2.3 Clusters

A rectangular repetition of frequencies results in a clustering of cells. The

clusters generated in this way can comprise the whole frequency band. [115]. A

cluster is a group of cells. No channels are reused within a cluster. Figure 4

illustrates a seven-cell cluster. [027]

For each cluster following holds true:



A cluster can contain all the frequencies of the mobile radio system.

Within a cluster, no frequency can be reused. The frequencies, however,

may be used at the earliest of the neighboring cluster.

The larger the cluster, the larger the values of frequency reuse factor ‘k’;

smaller the number of channels and the number of active subscribers per

cell.

Figure 2.6: cluster [028]

2.3 Frequency Reuse Concept

The concept of cellular systems is the use of low power transmitters in

order to enable’s the efficient reuse of the frequencies. If the transmitters of high

power are used, there will be interference between the user at the boundaries of



the cells. However, the set of available frequencies is limited and that is why

there is a need for the reuse of the frequencies. [029]

A frequency reuse pattern is a configuration of N cells, N being the reuse

factor, in which each cell uses a unique set of frequencies. When the pattern is

repeated, the frequencies can be reused. There are several different patterns,

but only two are shown below to clarify the idea.

Figure 2.7: Frequency Reuse [030]

The numbers in the cells define the pattern. The cells with the same

number in the pattern can use the same set of frequencies. In the pattern with

the reuse factor of 4, only one cell separates the cells using the same set of

frequencies. In the pattern with the reuse factor of 7, two cells separate the

reusing cells.

The distance between the cells using the same frequency must be

sufficient to avoid interference. The frequency reuse increases the capacity in the

number of users of a service provider.

For the proper function of the cellular system, following two conditions must

be satisfied:



• The power level of the transmitter within the single cell must be limited in

order to reduce the interference with the transmitters of the neighboring cells.

The distance of about 2.5 times the diameter of a cell must be maintained

between transmitters of the neighboring cells to avoid any damage to the

system.

• Neighboring cells can not share the same channels. In order to reduce the

interference, the frequencies must be reused only within a certain pattern.

It is required to maintain several radio channels for signaling in order to

exchange information needed to maintain the communication links within the

cellular networks. [031]



Chapter 3

HISTORY OF GSM



3.1 The Beginning of GSM

1980: In the early 1980s, as business was becoming increasingly international,

the communications industry focused exclusively on local cellular solutions, with

very few compatible systems. Nevertheless, it was clear there would be an

escalating demand for a technology that facilitated flexible and reliable mobile

communications. The problem was lack of capacity. By the early 1990s, it was

clear that analog technology would not be able to keep up with demand.

3.2 Groupe Speciale Mobile



1982: Conference Europeenne des Postes et Telecommunications began

specifying a European digital telecommunications standard; in the 900MHz

frequency band. The CEPT was formed in 1982 by the Conference Des

Administrations Europeans Des Posts et Telecommunications. In turn, the CEPT

established the Groupe Speciale Mobile (GSM) to develop the specification for a

pan-European mobile communications network capable of supporting the many

millions of subscribers likely to turn to mobile communications in the years

ahead. These standards later become known as Global System for Mobile

Communication (GSM).

3.3 Digital Technology

1985: West Germany, France and Italy signed an agreement for the

development of GSM. The United Kingdom joined in the following year, and the

group decided that digital technology would become the future of global wireless

communication. Digital technology offered an attractive combination of

performance and spectral efficiency. In addition, such a system would allow the

development of advanced features like speech security and data

communications. Digital also was compatible with Integrated Services Digital

Network (ISDN) technology, which was being developed by land-based.

Telecommunications systems throughout the world, and which would be

necessary for GSM to be successful

3.4 The ‘Memorandum of Understanding’ (MoU)

1986: the GSM Permanent Nucleus (headquartered in Paris) was formed to

assume overall responsibility for coordinating the development of GSM.

3.5 Vision becomes a reality



1986: the GSM Permanent Nucleus held a series of validation trials in Paris.

They tested eight or nine different designs in the quest for an appropriate radio

path, because at the heart of developing a new digital standard was the

resolution of questions relating to reliability and error correction. One of the most

important conclusions from the early tests of the new GSM technology was that

the new standard should employ Time Division Multiple Access (TDMA)

technology. The choice was TDMA or FDMA. [032]

1988: CEPT began producing GSM specifications for a phased implementation.

3.6 The European Telecommunication Standards Institute

1990: GSM responsibility was transferred to the European Telecommunication

Standards Institute (ETSI), and phase I of the GSM specifications were published

in 1990.

Phase I specifications were frozen to allow manufacturers to develop network

equipments.

3.7 Rapid Growth

1991: The GSM 1800 standard was released. Commercial service was started in

mid1991.

1993: Australia becomes the first non-European country to sign the MoU. First

commercial DCS 1800 system was launched in United Kingdom (UK).

There were 36 GSM networks in 22 countries, with 25 additional countries having

already selected or considering GSM. [033] This is not only a European standard

– South Africa, Australia, and many Middle and Far East countries have chosen

GSM.

1994: By the beginning of 1994, there were 1.3 million subscribers worldwide.

[034] the acronym GSM now (aptly) stands for Global System for Mobile

telecommunications. [035]



1995: The specification for the Personal Communication Services (PCS) was

developed in the USA. This version of GSM operates at 1900MHz

1996: The first GSM 1900 systems become available. Those comply with the

PCS 1900 standard. [036]

Finally, the history of GSM is summarized in the form of a simple table

given below:

Year Mobile System1981 Nordic Mobile Telephone (NMT) 450

1983 American Mobile Phone System (AMPS)

1985 Total Access Communication System (TACS)

1986 Nordic Mobile Telephony (NMT) 9001991 American Digital Cellular (ADC)1991 Global System for Mobile Communication (GSM)

1992 Digital Cellular System (DCS) 1800

1994 Personal Digital Cellular (PDC)

1995 PCS 1900-Canada1996 PCS-United States

Table 3.1: History of GSM, a quick overview [037]

Chapter 4GSM RADIO ASPECTS



4.1 Radio Transmission Aspects

For the GSM-900 system, two frequency bands have been made available:

• 890 - 915 MHz for the uplink (direction MS to BS)

• 935 - 960 MHz for the downlink (direction BS to MS).

The 25 MHz bands are then divided into 124 pairs of frequency duplex

channels with 200 kHz carrier spacing using Frequency Division Multiple Access


MS


(FDMA). Since it is not possible for a same cell to use two adjacent channels, the

channel spacing can be said to be 200 kHz interleaved.

Figure 4.1: Mobile Radio Propagation [039]

One or more carrier frequencies are assigned to individual Base Station (BS)

and a technique known as Time Division Multiple Access (TDMA) is used to split

this 200 kHz radio channel into 8 time slots (which creates 8 logical channels). A

logical channel is therefore defined by its frequency and the TDMA frame time

slot number. By employing eight time slots, each channel transmits the digitized

speech in a series of short bursts: a GSM terminal is only ever transmitting for

one eighth of the time. [038]

4.2 Aspects of Radio Propagation

Types of signal strength variations:



4.2.1 Macroscopic variations

Macroscopic variations are due to local mean, long term, or log-normal

fading. Its variation is due to the terrain contour between the BTS and the MS.

The fading effect is caused by shadowing and diffraction (bending) of the radio

waves.

4.2.2 Microscopic variations

Microscopic variations are due to multipath, short-term, or Rayleigh fading.

It is caused by the fact that as the MS moves, radio waves from many different

reflection paths will be received.

4.2.3 Digital TDMA Implementation

Advantages of digital transmission

The analog cellular system is known as the first-generation system.

Second generation cellular systems are digital. GSM systems are second-

generation systems.

The digital transmission over the air interface Um has a number of

advantages over analog transmission:

• Better speech quality

• Speech privacy and security (improved through encryption)

• High spectral efficiency (traffic density per MHz bandwidth, due to

extensive frequency reuse)

• Better resistance to interference (also by frequency hopping)

• Data services and ISDN compatibility

• Efficient use of battery power by RF power control [040]

4.2.4 Access methods



Cellular radio as a network does not specify how the individual subscribers

have access to the network. The two main access methods are: analog and

digital.

a) Analog access

Analog systems use the familiar single channel per user concept, known

as Frequency Division Multiple Access (FDMA). World-wide there are up to six

incompatible analog cellular standards, such as NMT. The available spectrum is

divided into channels A, B, C, D, and so on. During the call, a single user will

occupy completely one channel of e.g. 25 kHz bandwidth irrespective whether

the modulation is analog or digital. The signaling over the network is digital, the

speech is modulated analog narrow-band FM

b) Digital access

The aim of digital networks is to have:

• Better compatibility with the network supporting the cellular radio system

• Alternative access method to achieve a better spectral efficiency

Digital systems let each user have access to the frequency band for a short

time (traffic burst), during which time the user transmits data at a high rate.

4.2.5 Time division multiple access

Time Division Multiple Access (TDMA) is used in GSM-900 and GSM-

1800 digital cellular radio. In TDMA, the user's frequency allocation is shared with

other users (seven in case of GSM) who have time slots allocated at other times.

Hence, there are eight physical channels per frequency carrier.

In fact, the GSM system uses a mix of:



• TDMA (time slots on one carrier)

• FDMA (a number of carriers within the band), although frequency hopping

makes the FDMA somewhat more complex

4.2.6 Transmitting/ receiving processes

There are two major processes involved in transmitting and receiving

information over a digital radio link; coding and modulation.

From Source Data to Digital Radio Transmission:

Figure 4.2: Digital Radio Transmission System

4.3 Carrier Frequencies

GSM uses TDMA within a FDMA structure. As a result, different users can

transmit using the same frequency, but they can't transmit at the same time. A

25MHz frequency band is divided using an FDMA scheme into 124 one-way

carrier frequencies. Each base station is assigned one or more carriers to use in

its cell. A 200 kHz frequency band separates the carrier frequencies from each



other. Normally, a 25MHz band should be divisible into 125 carrier frequencies

but in GSM the 1st carrier frequency is used as a guard band between GSM and

other services that might be working on lower frequencies.

Figure 4.3: Frequency Division in the Uplink Spectrum

4.4 Bursts

Each carrier frequency is then divided according to time using a TDMA

scheme. Each of the carrier frequencies is divided into a 120ms multiframe. A

multiframe is made up of 26 frames. Two of these frames are used for control

purposes, while the remaining 24 frames are used for traffic.



Figure 4.4: Structure of a Multiframe

4.4.1 Burst Structure

In GSM, there are 4 different types of bursts. A normal burst is used to

carry speech and data information. The structure of the normal burst is shown

below. Each burst consists of 3 tail bits at each end, 2 data sequences of 57-bits,

a 26-bit training sequence for equalization, and 8.25 guard bits. There are 2

stealing bits (1 for each data sequence) that are used by Fast Access Control

Channels.

The frequency correction burst and synchronous burst have the same

length as normal burst. They have different internal structures to differentiate

them from normal bursts. The frequency correction burst is used in Frequency

Correction Channels (FCCH) and the synchronous burst is used in

Synchronization Channels (SCH). The random access burst is shorter than a

normal burst, and is only used on Random Access Channels (RACH).

Figure 4.5: Burst Structure

4.4.2 Burst types

The different types of bursts are defined in GSM

• Normal burst

• Dummy burst

• Access burst



• Synchronization burst

• Frequency correction burst [041]

Figure 4.6: The TDMA frame structure [042]



Figure 4.7: Organization of bursts, TDMA frames, and multiframes for speech

and data [046]

4.4.3 Burst parts

Examples of burst parts are: training sequence, encrypted bits, tail bits,

guard period and stealing flag bits.

a) Training sequence

A fixed bit pattern, called the TSC (training sequence code) is known by

both the MS and the BTS. It is used to train the MS in predicting and correcting

the signal distortions (due to Doppler and multipath effects) in the demodulation

process. The TSC has a 26, 41 or 64 bit pattern.

b) Encrypted bits

The encrypted bits represent the useful bits serving for speech, data

transmission, or signaling.

c) Tail bits

The tail bits (TB) at the beginning define ("flag") the start of a burst. The

tail bits at the end define the end of a burst.

d) Guard period

The guard period (GP) between to consecutive bursts is necessary for

switching the transmitter on and off. The transmitted amplitude is ramped up from

zero to a constant value over the useful period of a burst and then ramped down

to zero again. This is always required for the MS, and the BTS may do so if the



adjacent burst is not emitted. Switching off will reduce interference to other RF

channels.

e) Stealing flag bits

The network has the option to use the information bits in the normal burst

to send signaling data as needed. By setting a flag, using the stealing flag bits,

the receiver can distinguish between traffic (user data) and signaling information.

The stealing flag bits indicate whether the adjacent 57 bits in the

associated data field contain speech/data information or are "stolen" from the

traffic channel for carrying pre-emptive FACCH (fast associated control channel)

signaling information. The FACCH is used for sending signaling data if the

capacity of the SACCH (slow associated control channel) is not sufficient. [043]

4.5. Channels

A channel relates to the recurrence of one burst in every frame. The

channel is characterized by both its frequency and its position within the TDMA

frame. This characterization is cyclical, and the channel pattern repeats every 3

hours.

There are two major categories of channels in GSM: traffic channels, and

control channels. Channels can also be classified as being dedicated or

common. Dedicated channels are assigned to a mobile station, while common

channels are used by idle mobile stations.

4.5.1 Traffic Channels

Traffic channels transport speech and data information. A traffic channel

using a group of 26 TDMA frames called 26-Multiframe. In this standard, traffic

channels for uplinks and downlinks are separated by 3 bursts. Because of this,



the mobile station does not need to transmit and receive at the same time. A full

rate traffic channel uses 1 time slot in each of the traffic frames in a multiframe.

4.5.2 Control Channels

Control channels deal with network management messages and channel

maintenance tasks. These channels can be used by either idle or dedicated

mobile stations. Some of the common channel types are:

• Broadcast Control Channels

• Frequency Correction Channels

• Synchronization Channels

• Random Access Channels

• Paging Channels [044], [045]

4.5.3 Broadcast Control Channel (BCCH)

Continually broadcasts, on the downlink, information including base

station identity, frequency allocations, and frequency-hopping sequences.

4.5.4 Frequency Correction Channel (FCCH) and Synchronization Channel

(SCH)

It is used to synchronize the mobile to the time slot structure of a cell by

defining the boundaries of burst periods, and the time slot numbering. Every cell

in a GSM network broadcasts exactly one FCCH and one SCH, which are by

definition on time slot number 0 (within a TDMA frame).

4.5.5 Random Access Channel (RACH)

Slotted Aloha channel used by the mobile to request access to the network.

4.5.6 Paging Channel (PCH)

It is used to alert the mobile station of an incoming call. [047]



4.5.7 Access Grant Channel (AGCH)

It is used to allocate an SDCCH to a mobile for signaling.

4.6 Frequency hopping

The mobile station already has to be frequency agile, meaning it can move

between transmit, receive, and monitor time slot within one TDMA frame, which

normally are on different frequencies. GSM makes use of this inherent frequency

agility to implement slow frequency hopping, where the mobile and BTS transmit

each TDMA frame on a different carrier frequency. The frequency hopping

algorithm is broadcast on the Broadcast Control Channel. Since multipath fading

is dependent on carrier frequency, slow frequency hopping helps alleviate the

problem. In addition, co-channel interference is in effect randomized.

4.7 Discontinuous Transmission

Minimizing co-channel interference is a goal in any cellular system, since it

allows better service for a given cell size, or the use of smaller cells, thus

increasing the overall capacity of the system. Discontinuous transmission (DTX)

is a method that takes advantage of the fact that a person speaks less that 40

percent of the time in normal conversation. [048]

By turning the transmitter off during silence periods, an added benefit of

DTX is that power is conserved at the mobile unit.

The most important component of DTX is, of course, Voice Activity

Detection. It must distinguish between voice and noise inputs, a task that is not

as trivial as it appears, considering background noise. If a voice signal is

misinterpreted as noise, the transmitter is turned off and a very annoying effect

called clipping is heard at the receiving end. If, on the other hand, noise is

misinterpreted as a voice signal too often, the efficiency of DTX is dramatically

decreased. Another factor to consider is that when the transmitter is turned off,

there is total silence heard at the receiving end, due to the digital nature of GSM.

To assure the receiver that the connection is not dead, comfort noise is created



at the receiving end by trying to match the characteristics of the transmitting

end's background noise.

4.8 Discontinuous reception

Another method used to conserve power at the mobile station is

discontinuous reception. The paging channel, used by the base station to signal

an incoming call, is structured into sub-channels. Each mobile station needs to

listen only to its own sub-channel. In the time between successive paging sub-

channels, the mobile can go into sleep mode, when almost no power is used.

4.9 Power control

There are five classes of mobile stations defined, according to their peak

transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel

interference and to conserve power, both the mobiles and the Base Transceiver

Stations operate at the lowest power level that will maintain an acceptable signal

quality. Power levels can be stepped up or down in steps of 2 dB from the peak

power for the class down to a minimum of 13 dBm (20 milliwatts).

The mobile station measures the signal strength or signal quality (based

on the Bit Error Ratio), and passes the information to the Base Station Controller,

which ultimately decides if and when the power level should be changed. Power

control should be handled carefully, since there is the possibility of instability.

This arises from having mobiles in co-channel cells alternatively increase their

power in response to increased co-channel interference caused by the other

mobile increasing its power.

4.10 From Speech to Radio Waves

Figure below depicts the sequence of operations from speech to radio

waves and from radio waves to speech. These operations are described in the

following sections.



Figure 4.8: Speech Conversion sequence of operations [049]

Chapter 5

GSM ARCHITECTURE



5.1 Basic Architecture

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

1. Radio subsystem (RSS)

2. Network (switching) subsystem (SSS)

3. Operation and maintenance subsystem (OMS) [050], [051]



Figure 5.1: Main Components of GSM Network

Before exploring the GSM architecture in depth, it is better to have a quick

overview of the above components, starting with the MS (the mobile station). As

we shall precede through our discussion on these components the architecture,

other parts of the entire network will elaborate automatically.

5.2 Radio Subsystem (RSS)

The Radio Subsystem (RSS) consists of:

• Mobile Station (MS)

• Base Station (BS)

• Radio Interface (Um)

5.2.1 Mobile station

Mobile station (MS) is a portable data and/or voice communications statio

which acts as a normal telephone whilst being able to move over a wide area. A

mobile station is typically made up of:

• an antenna



• an amplifier

• a receiver

• a transmitter and

• similar hardware and software for sending and receiving signals and

converting between RF waves and audio signals [052]

The mobile station (MS) comprises all user equipment and software needed

for communication with a Wireless telephone network. MS refers to the Mobile

Phone i.e. the handset held by the users in the mobile network. This is the

terminology of 2G systems like GSM. In the 3G systems, MS (mobile station) is

now referred as User Equipment UE. 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. [053], [064]

In GSM, the Mobile Station consists of four main components:

• Mobile Terminal (MT)- offers common functions that are used by all the

service the Mobile Station offers. It is equivalent to the network termination

of an ISDN access and is also the end-point of the radio interface.

• Terminal Equipment (TE) - is a peripheral device of the Mobile Station

and offers services to the user. It does not contain any functions specific in

GSM.

• Terminal Adapter (TA) - hides radio-specific characteristics.

• Subscriber Identity Module (SIM) - is a personalization of the Mobile

Station and stores user specific parameters (such as mobile number,

contacts etc). [054]


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Figure 5.2: components of the MS [055]

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.

5.2.1.1 Functions of a Mobile Station

The Mobile Station (MS) performs the following:

• Radio transmission termination

• Radio channel management

• Speech encoding/decoding

• Radio link error protection



• Flow control of data

• Mobility management

• Performance measurements of radio link [064], [065]

The MS has two very important entities, each with its own identity:

• Subscriber Identity Module (SIM)

• Mobile equipment [066]

5.2.1.2 Subscriber Identity Module (SIM)

Figure 5.3: Subscriber’s Identity Module

GSM subscribers are provided with a SIM (subscriber identity module)

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.

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

integrated circuit chip with a microprocessor, random access memory (RAM),

and read only memory (ROM). The subscriber inserts it in the MS unit 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 subscriber’s 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 service provider if requested by the subscriber

can also permanently bypass the PIN. Disabling the PIN code simplifies the call

setup but reduces the protection of the user’s account in the event of a stolen

SIM. [067], [068]

5.2.1.3 Functions of a SIM

The functions of the Subscriber Identity Module (SIM) are:

• Authentication of the validity of the MS when accessing the network

• User authentication

• Storage of subscriber-related information, which can be: data fixed during

administrative phase (e.g., subscriber identification), and temporary

network data (e.g., cell location identity).

5.2.1.4 Mobile Equipment (ME)

The mobile equipment is also called the terminal and is responsible for

communication with the GSM system and converting the radio signals in to

human voice and reverse is also true.

According to the power and applications of it, M.E. is divided into different

types:

• Fixed Terminals

• Portable Terminals

• Handheld terminals

a) Fixed Terminals

These MEs are installed in cars having the maximum power output of 20 W.



b) Portable Terminals

Portable terminals are also installed in the vehicles. Their maximum

allowed output power is 8 W.

c) Handheld terminals

The handheld terminals are most popular because of their smaller size

and weight, which are decreasing continuously. These terminals can emit up to 2

W of power. With evolution in technology, the maximum allowed power is

reduced to 0.8 W.

5.1.2.5 Mobile subscriber identities in GSM

It would be better to discuss some of the important subscriber identities in

the GSM, which make the use of this technology safer for every person whether

he/she is a subscriber of GSM or not.

1) International Mobile Subscriber Identity (IMSI)

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

country code (MCC), mobile network code (MNC) (to identify the PLMN), and a

PLMN unique mobile subscriber identification number (MSIN). The IMSI is the

only absolute identity that a subscriber has within the GSM system. The IMSI

consists of the MCC followed by the MNC and MSIN and shall not exceed 15

digits. It is used in the case of system-internal signaling transactions in order to

identify a subscriber. The first two digits of the MSIN identify the HLR where the

mobile subscriber is administrated. [069]

2) Temporary 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 subscriber’s 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 MSC with their IMSI.

3) Mobile Station ISDN Number

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 Destination Code

(NDC), Subscriber Number (SN), which shall not exceed 15 digits. Here too the

first two digits of the SN identify the HLR where the mobile subscriber is

administrated.

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.

5) International Mobile Equipment Identity

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.

The Base Station (BS) terminates the radio interface (Um) on the stationary

network side. The BS has a modular design and includes the:



• Base Transceiver Station (BTS)

• Base Station Controller (BSC)

• Transcoding and Rate Adaptation Unit (TRAU) [070], [071]

5.2.2 Base Station System (BSS)

In GSM, the Base Station System is a term given to a BSC (Base Station

Controller) and the BTS (Base Transceiver Station) associated with it. The

number of BTS associated with a BSC is dependent on the manufacturer.

Although not mandatory, through interpretation of the Abis interface standard

BTS and BSC employed within a BSS will always be supplied by the same

manufacturer. [072], [073]

Figure 5.4: BSS Structure

The BSC, the TRAU and BTS form a unit, which is called Base Station

System (BSS) in the GSM terminology. A BSC can control several BTS. Every

BSC contained in the network controls one BSS. The interface between BSC and

BTS is called Abis - interface. An interface is 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 BS.

The base station subsystem (BSS) is the section of a traditional cellular

telephone network which is responsible for handling traffic and signaling between


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a mobile phone and the network switching subsystem. The BSS carries out

transcoding of speech channels, allocation of radio channels to mobile phones,

paging, quality management of transmission and reception over the air interface

and many other tasks related to the radio network. [074]

Figure 5.5: Detailed BSS Components

5.2.2.1 Base Station Controller (BSC)

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. BSC within a mobile network is a key component for handling and

routing information. The BSC provides all the control functions and physical links

between the MSC and BTS. It is a high-capacity switch that provides functions

such as handover, cell configuration data, and control of radio frequency (RF)


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power levels in base transceiver stations. A number of BSCs are served by an

MSC.

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

transmission lines from the BSC to its BTSs. [074]

A BSC is often based on a distributed computing architecture, with

redundancy applied to critical functional units to ensure availability in the event of

fault conditions. Redundancy often extends beyond the BSC equipment itself and

is commonly used in the power supplies and in the transmission equipment

providing the A-ter interface to PCU.

The databases for all the sites, including information such as carrier

frequencies, frequency hopping lists, power reduction levels, receiving levels for

cell border calculation, are stored in the BSC. This data is obtained directly from

radio planning engineering which involves modeling of the signal propagation as

well as traffic projections. [075], [076]


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5.2.2.2 Packet control unit

The packet control unit (PCU) is a late addition to the GSM standard. It

performs some of the processing tasks of the BSC, but for packet data. The

allocation of channels between voice and data is controlled by the base station,

but once a channel is allocated to the PCU, the PCU takes full control over that

channel.

The PCU can be built into the base station, built into the BSC or even, in

some proposed architectures, it can be at the SGSN site. In most of the cases,

the PCU is a separate node communicating extensively with the BSC on the

radio side and the SGSN on the Gb side.

5.2.2.3 Base Terminal Station (BTS)

The BTS handles the radio interface to the mobile station. The BTS is the

radio equipment (transceivers and antennas) needed to service each cell in the

network. A group of BTSs are controlled by a BSC.

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.



Figure 5.6: BTS with its antennae [076]

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.

There are two categorize in which, BTS may be arranged in the cells depending

upon the circumstances of the region in which they are to be used. The two

arrangements are shown in figure below.

Collocated BTS



Remote BTS

Star BTS

Figure 5.7: Two types of BTS arrangements [077]

5.2.2.4 Functions of BTS

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. Transcoding 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 channel measurement corresponding to the downlink

measurements made by MS has to be made by BTS. [078], [079]

5.2.2.5 Sectorization

By using directional antennas on a base station, each pointing in different

directions, it is possible to sectorize the base station so that several different cells

are served from the same location. Typically these directional antennas have a

beam width of 65 to 85 degrees. This increases the traffic capacity of the base

station (each frequency can carry eight voice channels) whilst not greatly

increasing the interference caused to neighboring cells (in any given direction,

only a small number of frequencies are being broadcast). Typically two antennas

are used per sector, at spacing of ten or more wavelengths apart. This allows the

operator to overcome the effects of fading due to physical phenomena such as

multipath reception. Some amplification of the received signal as it leaves the

antenna is often used to preserve the balance between uplink and downlink

signal.


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Figure 5.8(a): Site Sectorization-Omni Cell site [056]

Figure 5.8(b): Site Sectorization-Tri Cell site [056]

Figure 5.8(c): Site Sectorization-Hex Cell site [056]

5.2.2.6 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 rural 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 urban

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

either a collocated or remote BSC. In rural areas, most BTSs are installed to

provide maximum coverage rather then maximum capacity.

5.2.2.7 Transcoder and Rate Adaptation Unit (TRAU)

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

architecture as a part of the BTS is the Transcoder/Rate Adaptation Unit (TRAU).

The TRAU is the equipment in which coding and decoding is carried out as well

as rate adaptation 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 TRAU adapts the 64 Kbps from the MSC to

the comparatively low transmission rate of the radio interface of 16 Kbps.

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 BTS likewise is a

standard the Abis. 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.

5.2.2.8 Transcoder (XCDR)

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.

Figure 5.9: Transcoder Interfacing

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-Kbps circuit. Thus, the TRAU output data rate is

64 Kbps. Then, up to 30 such 64-Kbps channels are multiplexed onto a 2.048

Mbps if a CEPT1 channel is provided on the Abis 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-

Kbps channels, or 120- Kbps by 16-Kbps channels. [080]

5.2.3 The Interfaces in the GSM

5.2.3.1 Um-interface

It is the air interface between the mobile station (MS) and the BTS. This

interface uses LAPDm protocol for signaling, to conduct call control,

measurement reporting, handover, power control, authentication, authorization,

location update and so on. Traffic and signaling are sent in bursts of 0.577 ms at

intervals of 4.615 ms, to form data blocks each 20 ms.

Figure 5.10: Simple description of Interfaces [057]

5.2.3.2 Abis-interface


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It is the interface between the BTS and BSC, generally carried by a DS-1, ES-1,

or E1 TDM circuit. Uses TDM sub-channels for traffic (TCH), LAPD protocol for

BTS supervision and telecom signaling, and carries synchronization from the

BSC to the BTS and MS.

The Abis interface uses multiplexing (Mult) or rate adaptation (RA) on its

links. The first option means that four 16 kbit/s links are multiplexed into one 64

kbit/s channel. The latter option means that no multiplexing of the 16 kbit/s

channels takes place. [058]

5.2.3.3 A-interface

It is the interface between the BSC and MSC. It is used for carrying traffic

channels and the BSSAP user part of the SS7 stack. Although there are usually

transcoding units between BSC and MSC, the signaling communication takes

place between these two ending points and the transcoder unit doesn't touch the

SS7 information, only the voice or CS data are transcoded or rate adapted.

5.2.3.4 Ater-interface

It is the interface between the BSC and transcoder. It is a proprietary

interface whose name depends on the vendor (for example Ater by Nokia), it

carries the A interface information from the BSC leaving it untouched.

5.2.3.5 Gb-interface

It connects the BSS to the SGSN in the GPRS core network . [059]


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Figure 5.11: Signaling protocol structure in GSM

In the figure: Layer 1 is the physical layer, Layer 2 is the data link layer, and Layer 3 is the GSM

signaling protocol. [060]

5.3 Switching Subsystem (SSS)

The Switching Subsystem (SSS) comprises of:

• Mobile services Switching Centre (MSC)

• Home Location Register (HLR)

• Visitor Location Register (VLR)

• Authentication Centre (AuC)

• Equipment Identification Register (EIR) [061]

Figure 5.12: The NSS [081], [083]



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 function is 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 Network Switching Subsystem, also referred to as the GSM core

network, usually refers to the circuit-switched core network, used for traditional

GSM services such as voice calls, SMS, and circuit switched data calls.

There is also an overlay architecture on the GSM core network to provide

packet-switched data services and is known as the GPRS core network. This allows

mobile phones to have access to services such as WAP, MMS, and Internet

access.

All mobile phones manufactured today have both circuit and packet based

services, so most operators have a GPRS network in addition to the standard

GSM core network. [062]

5.3.1 Mobile Switching Center (MSC)

An MSC is the point of connection to the network for mobile subscribers of

a wireless telephone network. It connects to the subscribers through base

stations and radio transmission equipment that control the air interface, and to

the network of other MSCs and wireless infrastructure through voice trunks and


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SS7. An MSC includes the procedures for mobile registration and is generally co-

sited with a visitor location register (VLR) that is used to temporarily store

information relating to the mobile subscribers temporarily connected to that MSC.

The MSC performs the telephony switching functions of the system. It controls

calls to and from other telephone and data systems. It also performs such

functions as toll ticketing, network interfacing, common channel signaling, and

others.

5.3.1.1 Other GSM core network elements connected to MSC

The MSC connects to the following elements:

• The home location register (HLR) for obtaining data about the SIM and mobile

services ISDN number (MSISDN; i.e., the telephone number).

• The UMTS terrestrial radio access network (UTRAN) which handles the radio

communication with 3G mobile phones.

• The visitor location register (VLR) for determining where other mobile

subscribers are located.

• Other MSCs for procedures such as handover.

5.3.1.2 Other network elements of MSC

a) Billing Center

Each MSC writes call accounting records to local disk memory. Billing

Center periodically polls the disk records of each MSC to collect the billing data

for the PLMN.

b) Service Center

The Service Center interfaces with the MSCs to provide special services,

such as the Short Message Service (SMS), to mobile subscribers in the PLMN.



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The Billing Center and Service Center are not a basic part of the GSM system.

[083]

c) Tasks of the MSC

The main tasks of the MSC include:

• Delivering calls to subscribers as they arrive based on information from the

VLR.

• Connecting outgoing calls to other mobile subscribers or the PSTN.

• Delivering SMSs from subscribers to the short message service centre (SMSC)

and vice versa.

• Arranging handovers from BSC to BSC.

• Carrying out handovers from this MSC to another.

• Supporting supplementary services such as conference calls or call hold.

• Generating billing information. [084]

5.3.1.3 Functions-based Types

There are various different names for MSCs in different contexts which

reflects their complex role in the network, all of these terms though could refer to

the same MSC, but doing different things at different times.

1) Gateway MSC (G-MSC)

The gateway MSC (G-MSC) is the MSC that determines which visited

MSC the subscriber who is being called is currently located. It also interfaces

with the PSTN. All mobile to mobile calls and PSTN to mobile calls are routed

through a G-MSC. The term is only valid in the context of one call since any MSC

may provide both the gateway function and the Visited MSC function; however,

some manufacturers design dedicated high capacity MSCs which do not have


http://en.wikipedia.org/w/index.php?title=Supplementary_services&action=edit&redlink=1

http://en.wikipedia.org/wiki/SMSC

http://en.wikipedia.org/wiki/GSM_services#Call_set_up_process_for_incoming_calls


any BSSs connected to them. These MSCs will then be the Gateway MSC for

many of the calls they handle.

2) Visited MSC (V-MSC)

The visited MSC (V-MSC) is the MSC where a customer is currently

located. The VLR associated with this MSC will have the subscriber's data in it.

3) Anchor MSC

The anchor MSC is the MSC from which a handover has been initiated.

The target MSC is the MSC toward which a Handover should take place. A

mobile switching centre server is a part of the redesigned MSC concept starting

from 3GPP Release 5. [084]

4) Short Message Service Center (SMSC)

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

The SMSC is a store-and-forward device used to provide peer-to-peer text

messaging services in mobile networks. Any text message issued from a

mobile handset is forwarded to the SMSC, where the location of the called

subscriber is determined by consulting the appropriate HLR. If the subscriber is

currently connected to a reachable network, the location is determined and the

text message is transmitted. If not, the message is stored for later transmission

once the subscriber becomes available. The SMSC also includes back-end

interfaces for the connection of enhanced service platforms that can be used to


http://en.wikipedia.org/w/index.php?title=3GPP_Release_5&action=edit&redlink=1

http://en.wikipedia.org/wiki/Mobile_switching_centre_server


http://en.wikipedia.org/wiki/Visitors_Location_Register

http://en.wikipedia.org/wiki/Base_Station_Subsystem


implement a variety of SMS services such as televoting and premium rate data

services (e.g., weather, traffic, sports, and news). [085]

5.3.2 Home location register (HLR)

'Home Location Register' The home location register (HLR) is a central

database that contains details of each mobile phone subscriber that is authorized

to use the GSM core network. There can be several logical, and physical, HLRs

per public land mobile network (PLMN), though one international mobile subscriber identity

(IMSI)/MSISDN pair can be associated with only one logical HLR (which can

span several physical nodes) at a time.

The HLR stores details of every SIM card issued by the mobile phone operator.

Each SIM has a unique identifier called an IMSI which is the primary key to each

HLR record.

The next important items of data associated with the SIM are the

MSISDNs, which are the telephone numbers used by mobile phones to make

and receive calls. The primary MSISDN is the number used for making and

receiving voice calls and SMS, but it is possible for a SIM to have other

secondary MSISDNs associated with it for fax and data calls. Each MSISDN is

also a primary key to the HLR record. The HLR data is stored for as long as a

subscriber remains with the mobile phone operator.

5.3.2.1 Data stored in the HLR

• GSM services that the subscriber has requested or been given.

• GPRS settings to allow the subscriber to access packet services.

• Current location of subscriber (VLR and serving GPRS support node/SGSN).

• Call diverts settings applicable for each associated MSISDN.


http://en.wikipedia.org/wiki/Call_forwarding

http://en.wikipedia.org/wiki/SGSN

http://en.wikipedia.org/wiki/GPRS

http://en.wikipedia.org/wiki/Primary_key

http://en.wikipedia.org/wiki/Fax

http://en.wikipedia.org/wiki/Telephone_number

http://en.wikipedia.org/wiki/Primary_key

http://en.wikipedia.org/wiki/SIM_card

http://en.wikipedia.org/wiki/IMSI

http://en.wikipedia.org/wiki/Public_Land_Mobile_Network


The HLR is a system which directly receives and processes MAP transactions

and messages from elements in the GSM network, for example, the location

update messages received as mobile phones roam around.

5.3.2.2 Other GSM core network elements connected to HLR

The HLR connects to the following elements:

• The G-MSC for handling incoming calls

• The VLR for handling requests from mobile phones to attach to the network

• The SMSC for handling incoming SMS

• The voice mail system for delivering notifications to the mobile phone that a

message is waiting

• The AUC for authentication and ciphering and exchange of data (triplets)

5.3.2.3 Functions of HLR

The main function of the HLR is to manage the fact that SIMs and phones

move around a lot. The following procedures are implemented to deal with this:

• Manage the mobility of subscribers by means of updating their position in

administrative areas called 'location areas', which are identified with a LAC.

The action of a user of moving from one LA to another is followed by the HLR

with a Location area update while retrieving information from BSS as base

station identity code (BSIC).

• Send the subscriber data to a VLR or SGSN when a subscriber first roams

there.

• Broker between the G-MSC or SMSC and the subscriber's current VLR in

order to allow incoming calls or text messages to be delivered.

• Remove subscriber data from the previous VLR when a subscriber has

roamed away from it. [086], [087], [088]


http://en.wikipedia.org/wiki/GSM_services#Call_set_up_process_for_incoming_calls

http://en.wikipedia.org/wiki/Base_station_identity_code

http://en.wikipedia.org/wiki/Base_station_identity_code

http://en.wikipedia.org/wiki/Voice_mail

http://en.wikipedia.org/wiki/Mobile_Application_Part


5.3.3 Visitor location register (VLR)

The visitor location register is a temporary database of the subscribers

who have roamed into the particular area which it serves. Each base station in the

network is served by exactly one VLR; hence a subscriber cannot be present in

more than one VLR at a time.

The data stored in the VLR has either been received from the HLR, or

collected from the MS. In practice, for performance reasons, most vendors

integrate the VLR directly to the V-MSC and, where this is not done, the VLR is

very tightly linked with the MSC via a proprietary interface.

5.3.3.1 Data stored in VLR

• IMSI (the subscriber's identity number).

• Authentication data.

• MSISDN (the subscriber's phone number).

• GSM services that the subscriber is allowed to access.

• Access point (GPRS) subscribed.

• The HLR address of the subscriber.

5.3.3.2 Other GSM core network elements connected to VLR

The VLR connects to the following elements:

• The V-MSC to pass needed data for its procedures; e.g., authentication or

call setup.

• The HLR to request data for mobile phones attached to its serving area.

• Other VLRs to transfer temporary data concerning the mobile when they roam

into new VLR areas. For example, the temporal mobile subscriber identity (TMSI).

5.3.3.3 Functions of VLR


http://en.wikipedia.org/wiki/TMSI

http://en.wikipedia.org/w/index.php?title=GPRS_Access_Points&action=edit&redlink=1

http://en.wikipedia.org/wiki/IMSI

http://en.wikipedia.org/wiki/Base_Transceiver_Station


The primary functions of the VLR are:

• To inform the HLR that a subscriber has arrived in the particular area covered

by the VLR.

• To track where the subscriber is within the VLR area (location area) when no

call is ongoing.

• To allow or disallow which services the subscriber may use.

• To allocate roaming numbers during the processing of incoming calls.

• To purge the subscriber record if a subscriber becomes inactive whilst in the

area of a VLR. The VLR deletes the subscriber's data after a fixed time period

of inactivity and informs the HLR (e.g., when the phone has been switched off

and left off or when the subscriber has moved to an area with no coverage for

a long time).

• To delete the subscriber record when a subscriber explicitly moves to

another, as instructed by the HLR. [086], [087], [088]

5.3.4 Authentication centre (AUC)

The authentication centre (AUC) is a function to authenticate each SIM card

that attempts to connect to the GSM core network (typically when the phone is

powered on). Once the authentication is successful, the HLR is allowed to

manage the SIM and services described above. An encryption key is also

generated that is subsequently used to encrypt all wireless communications

(voice, SMS, etc.) between the mobile phone and the GSM core network.

If the authentication fails, then no services are possible from that particular

combination of SIM card and mobile phone operator attempted. There is an

additional form of identification check performed on the serial number of the

mobile phone described in the EIR section below, but this is not relevant to the

AUC processing.


http://en.wikipedia.org/wiki/Encryption_key

http://en.wikipedia.org/wiki/SIM_card

http://en.wikipedia.org/wiki/Authentication


Proper implementation of security in and around the AUC is a key part of

an operator's strategy to avoid SIM cloning.

The AUC does not engage directly in the authentication process, but

instead generates data known as triplets for the MSC to use during the

procedure. The security of the process depends upon a shared secret between

the AUC and the SIM called the Ki. The Ki is securely burned into the SIM during

manufacture and is also securely replicated onto the AUC. This Ki is never

transmitted between the AUC and SIM, but is combined with the IMSI to produce

a challenge/response for identification purposes and an encryption key called Kc

for use in over the air communications. [088]

5.3.5 Equipment Identity Register (EIR)

The EIR is a database that contains information about the identity of

mobile equipment that prevents calls from stolen, unauthorized, or defective

mobile stations. The AUC and EIR are implemented as stand-alone nodes or as

a combined AUC/EIR node.

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 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

MS’s. This is the category of genuine equipment.

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


http://en.wikipedia.org/wiki/Shared_secret

http://en.wikipedia.org/wiki/SIM_cloning


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. [088]

5.4 Operation and Maintenance Subsystem (OMS)

The Operations and Maintenance Center (OMC) is the centralized

maintenance and diagnostic heart of the Base Station System (BSS). It allows

the network provider to operate, administer, and monitor the functioning of the

BSS. An OMS consists of one or more Operation & Maintenance Centre (OMC).

Figure 5.13: The OMC [089]

The operations and maintenance center (OMC) is connected to all

equipment in the switching system and to the BSC. The implementation of OMC

is called the operation and support system (OSS). The OSS is the functional

entity from which the network operator monitors and controls the system. The

purpose of OSS is to offer the 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.

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. [090], [091]



Chapter 6

GSM SUBSCRIBERS

DATA



GSM Subscribers’ Data

Different addresses and service-specific data exist in GSM network.

Addresses serve to identify, authenticate, and localize subscribers, or switch

connections to subscribers. Service-specific data are used to parameterize and

personalize the supplementary services. The association of most important

identifiers and their storage locations are summarized in figure below.



SPC: Signaling Point Code

Figure 6.1: “Subscribers Data in GSM”

Further data about the subscribers and their contractual agreement with

the service provider is tabulated on next page.

Subscribers and Subscription Data Tracking and Routing Information1. International Mobile Subscriber’s

Identity (IMSI)

2. International Mobile Subscriber’s

ISDN number (MSISDN)

3. Bearer and Teleservice

Subscription

4. Service

Restrictions (e.g. Roaming

1. Mobile Station Roaming Number

(MSRN)

2. current VLR Address (if

available)

3. current MSC Address (if

available)

4. Local Mobile Service Identity

(LMSI), (if available)



Restrictions)

5. Parameters for additional

services

6. Information on Subscriber’s

Equipment (if available)

7. Authentication Data (Subject to

Implementation)Table 6.1: “Mobile Subscribers’ Data in HLR”

Subscribers and Subscription Data Tracking and Routing Information1. International Mobile Subscriber’s

Identity (IMSI)

2. International Mobile Subscriber’s

ISDN number (MSISDN)

3. Parameters for Supplementary

Services

4. Information on Subscriber-used

Equipment (if available)

5. Authentication Data (Subject to

Implementation))

1. Mobile Station Roaming Number

(MSRN)

2. Temporary MS Identity (TMSI)

3. LMSI

4. Local Area Identity of Location

Area, where Ms was Registered

(used for paging and cell setup)

Table 6.2: “Mobile Subscribers’ Data in VLR”

Chapter 7

GSM SERVICES



7.1 GSM Subscriber Services

There are two basic types of services offered through GSM:

1. Telephony (also referred to as teleservices)

2. Data (also referred to as bearer services)

Telephony services are mainly voice services that provide subscribers with

the complete capability (including necessary terminal equipment) to



communicate with other subscribers. Data services provide the capacity

necessary to transmit appropriate data signals between two access points

creating an interface to the network.

GSM Teleservices include:

• Speech (telephone, emergency call)

• Fax transmission

• SMS (short messaging service)

• MHS access (access to Message Handling System)

• Videotext access

• Teletext transmission [115]

In addition to normal telephony and emergency calling, the following

subscriber services are supported by GSM:

• Dual-tone multi-frequency (DTMF) —DTMF is a tone signaling scheme

often used for various control purposes via the telephone network, such as

remote control of an answering machine. GSM supports full-originating

DTMF.

• Facsimile group III —GSM supports CCITT Group 3 facsimile. As

standard fax machines are designed to be connected to a telephone using

analog signals, a special fax converter connected to the exchange is used

in the GSM system. This enables a GSM–connected fax to communicate

with any analog fax in the network.



• Short message services —a convenient facility of the GSM network is

the short message service. A message consisting of a maximum of 160

alphanumeric characters can be sent to or from a mobile station. This

service can be viewed as an advanced form of alphanumeric paging with

a number of advantages. If the subscriber's mobile unit is powered off or

has left the coverage area, the message is stored and offered back to the

subscriber when the mobile is powered on or has reentered the coverage

area of the network. This function ensures that the message will be

received.

• Cell broadcast —a variation of the short message service is the cell

broadcast facility. A message of a maximum of 93 characters can be

broadcast to all mobile subscribers in a certain geographic area. Typical

applications include traffic congestion warnings and reports on accidents.

• Voice mail —this service is actually an answering machine within the

network, which is controlled by the subscriber. Calls can be forwarded to

the subscriber's voice-mail box and the subscriber checks for messages

via a personal security code.

• Fax mail —with this service, the subscriber can receive fax messages at

any fax machine. The messages are stored in a service center from which

they can be retrieved by the subscriber via a personal security code to the

desired fax number. [091], [92], [93], [097]



Figure 7.1: “Bearer and Teleservices”

In this figure, note the following aspects carefully:

All components from TE (MS) to TE (MS) are named Teleservices

GSM Network and Interworking Function form the PLMN Section

Components between the two TE (MS) makes the Bearer Services [115]

7.2 Supplementary Services

GSM supports a comprehensive set of supplementary services that can

complement and support both telephony and data services. Supplementary

services are defined by GSM and are characterized as revenue-generating

features. A partial listing of supplementary services follows.

• Call forwarding —this service gives the subscriber the ability to forward

incoming calls to another number if the called mobile unit is not reachable,

if it is busy, if there is no reply, or if call forwarding is allowed

unconditionally.

• Barring of outgoing calls —this service makes it possible for a

mobile subscriber to prevent all outgoing calls.



• Barring of incoming calls —this function allows the subscriber to prevent

incoming calls. The following two conditions for incoming call barring exist:

baring of all incoming calls and barring of incoming calls when roaming

outside the home PLMN.

• Advice of charge (AoC) —The AoC service provides the mobile

subscriber with an estimate of the call charges. There are two types of

AoC information: one that provides the subscriber with an estimate of the

bill and one that can be used for immediate charging purposes. AoC for

data calls is provided on the basis of time measurements.

• Call hold —this service enables the subscriber to interrupt an ongoing call

and then subsequently reestablish the call. The call hold service is only

applicable to normal telephony.

• Call waiting —this service enables the mobile subscriber to be notified of

an incoming call during a conversation. The subscriber can answer, reject,

or ignore the incoming call. Call waiting is applicable to all GSM

telecommunications services using a circuit-switched connection.

• Multiparty service —the multiparty service enables a mobile subscriber

to establish a multiparty conversation—that is, a simultaneous

conversation between three and six subscribers. This service is only

applicable to normal telephony.

• Calling line identification presentation/restriction —these services

supply the called party with the integrated services digital network (ISDN)

number of the calling party. The restriction service enables the calling

party to restrict the presentation.



• Closed user groups (CUGs) —CUGs are generally comparable to a

PBX. They are a group of subscribers who are capable of only calling

themselves and certain numbers. [091], [93], [94], [95], [097]

Note: Bearer and teleservices are carried under the umbrella term “telecommunication

services”.

Chapter 8

GSM FUNCTIONS



8.1 Network operations GSM functions

In this chapter, the description of the GSM network is focused on the different

functions to fulfill by the network and not on its physical components. In GSM,

five main functions can be defined:

• Transmission



• Radio Resources Management (RRM).

• Mobility Management (MM).

• Communication Management (CM).

• Operation, Administration and Maintenance (OAM).

8.2 Transmission

Transmission means sending and receiving of data and signaling bits. Not

all the components of the GSM network are strongly related with both types of

types of Tx. While the MSC, BTS and BSC, among others, are involved with data

and signaling, components such as HLR, VLR or EIR registers, are only

concerned with signaling. The GSM standard also provides separate facilities for

transmitting digital data. This allows a mobile phone to act like any other

computer on the Internet, sending and receiving data via the Internet Protocol.

[097]

8.3 Radio Resources Management (RRM)

The role of the RR function is to establish, maintain and release

communication links between mobile stations and the MSC. The elements that

are mainly concerned with the RR function are the MN and the BTS. However,

since the RR component performs connection management also during cell

handoffs, it also affects the MSC which is the handoff management component.

The RR is also responsible for the management of frequency resources as well

as varying radio interface conditions. Main component operations are:

• Channel assignment, change and release.

• Handoff

• Frequency hopping.

• Power-level control.


http://en.wikipedia.org/wiki/Internet_Protocol

http://en.wikipedia.org/wiki/Internet


• Discontinuous transmission and reception.

• Timing advance. [099], [100]

8.3.1 Handoff

The user movements may result a change in the channel/cell, when the

quality of the communication is degrading; this is known as handoff. Handoffs

occur between:

• between channels within a cell

• between cells controlled by the same BSC

• between cells under the same MSC but controlled by different BSCs

• between cells controlled by different MSCs.

Handoffs are mainly controlled by the MSC. However to avoid unnecessary

signalling, the first two types of handoffs are managed by the respective BSC

(thus, the MSC is only notified of the handoff).



Figure 8.1: Inter-BTS, Intra-BSC [101]

To perform the handoff the mobile station controls continuously its own signal

strength and the signal strength of the neighboring cells. The list of cells that

must be monitored by the mobile station is given by the base station. Power

measurements allow deciding which the best cell is in order to maintain the

quality of the communication link. Two basic algorithms are used for handoffs:

• The `minimum acceptable performance' algorithm. When the quality of the

transmission degrades, the power level of the mobile is increased, until

the increase of the power level has no effect on the quality of the signal.

Upon this link layer hint, a handoff is initiated.

The `power budget' algorithm: Here the handoff pre-empts the power

increase, to obtain a good SIR.



8.4 Mobility Management (MM)

The MM component handles:

• Location Management: Location is managed through periodically or on-

demand. At power-on time, the MH signals an IMSI attach. On-demand

location updates are signaled when the MN moves to a different PLMN or

new location area (LA). The signal is sent to the new MSC/VLR, which

forwards it to the subscriber's HLR. Upon authorization in the new

MSC/VLR, the subscriber's HLR removes the registration entry of the MN

at the old MSC/VLR. If after the update time interval, the MN has not

registered, it is then deregistered. On power-off, the MN performs an IMSI

detach.

8.5 Security and authentication

Authentication involves the SIM card and the Authentication Center. A

secret key, stored in the SIM card and the AuC together with a ciphering

algorithm called A3, are used to authenticate the user. The MN and the AuC

computes a SRES through A3 using the secret key and a nonce generated by

the AuC. If the two computed SRES are the same, the subscriber is

authenticated.

The different services to which the subscriber has access are also

checked. Next the security check is performed in the equipment identity (IMEI). If

the IMEI number of the mobile is authorized in the EIR, the mobile station is

allowed to connect the network. To assure user confidentiality, the user is

registered with a Temporary Mobile Subscriber Identity (TMSI) after its first

location update procedure. Enciphering is another option to guarantee a very

strong security. [100], [102]



8.6 Communication Management (CM)

The CM component manages:

1. Call control (CC): it controls call setup, management and tear-down in

relation to management of type of service. Call routing is the primary task

for this component. To reach a mobile subscriber, a user dials the Mobile

Subscriber ISDN (MSISDN) number which includes:

• a country code

• a national destination code; this identifies the subscriber's operator

• a code mapping to the subscriber's HLR.

• The call is then passsed to the GMSC (if the call is originated from a

fixed network) that 'knows' the HLR corresponding to the particular

MSISDN number. The GMSC signals the HLR for call routing

information. The HLR requests this information from the subscriber's

current VLR. This VLR allocates temporarily a Mobile Station Roaming

Number (MSRN) for the call. The MSRN number is the information

returned by the HLR to the GMSC. It is latter that routes the call

through the MSRN number, to the subscriber's current MSC/VLR. In

the subscriber's current LA, the mobile is paged.

2. Supplementary Services management: This involves the MN and the

HLR.

SMS management: Here the GSM network contacts the Short Message

Service Center through the two following interfaces:

• SMS-GMSC for Mobile Terminating Short Messages (SMS-MT/PP). It

has the same role as the GMSC.

• SMS-IWMSC for Mobile Originating Short Messages (SMS-MO/PP).

[103], [104]



8.7 Operation, Administration and Maintenance (OAM)

The OAM component allows the operator to monitor and control the

system as well as modify the configuration of the elements of the system. Not

only the OSS is part of the OAM, but also the BSS and NSS participate in

functions such as:

• Provide the operator with all the information it needs. This information is

forwarded to the OSS to control the network.

• Perform self-test tasks in addition to the OAM functions.

• Control of multiple BTSs by the BSS. [105]

8.8 Mobile Subscriber Roaming

When a mobile subscriber roams into a new location area (new VLR), the

VLR automatically determines that it must update the HLR with the new location

information, which it does using an SS7 Location Update Request Message. The

Location Update Message is routed to the HLR through the SS7 network, based

on the global title translation of the IMSI that is stored within the SCCP Called

Party Address portion of the message. The HLR responds with a message that

informs the VLR whether the subscriber should be provided service in the new

location.



Figure 8.2: Call routing for a mobile terminating call [106]

8.9 Mobile Subscriber ISDN Number (MSISDN) Call Routing

When a user dials a GSM mobile subscriber's MSISDN, the PSTN routes

the call to the Home MSC based on the dialed telephone number. The MSC must

then query the HLR based on the MSISDN, to attain routing information required

to route the call to the subscribers' current location.

The MSC stores global title translation tables that are used to determine

the HLR associated with the MSISDN. When only one HLR exists, the translation

tables are trivial. When more than one HLR is used however, the translations

become extremely challenging; with one translation record per subscriber (see

the example below). Having determined the appropriate HLR address, the MSC

sends a Routing Information Request to it.



When the HLR receives the Routing Information Request, it maps the

MSISDN to the IMSI, and ascertains the subscribers' profile including the current

VLR at which the subscriber is registered. The HLR then queries the VLR for a

Mobile Station Roaming Number (MSRN). The MSRN is essentially an ISDN

telephone number at which the mobile subscriber can currently be reached. The

MSRN is a temporary number that is valid only for the duration of a single call.

The HLR generates a response message, which includes the MSRN, and

sends it back across the SS7 network to the MSC. Finally, the MSC attempts to

complete the call using the MSRN provided. [107]



Chapter 9

ADVANTAGES &

DISADVANTAGES OF

GSM



9.1 Advantages of GSM

• GSM is mature; this maturity means a more stable network with robust

features

• Less signal deterioration inside buildings

• Ability to use repeaters

• Talk-time is generally higher in GSM phones due to the pulse nature of

transmission

• The availability of Subscriber Identity Modules allows users to switch

networks and handsets at will

• GSM covers virtually all parts of the world so international roaming is not a

problem.

• The subscriber can enjoy the broadest international coverage. It is

possible with the GSM roaming service. [110]

• Good coverage indoors on 850/900 MHz. Repeaters possible. 35 km hard limit.

• Very good due to simple protocol, good coverage and mature, power-efficient chipsets.

[111], [112]

9.2 Disadvantages of GSM

• Pulse nature of TDMA transmission used in 2G interferes with some

electronics, especially certain audio amplifiers. 3G uses W-CDMA now.

• Intellectual property is concentrated among a few industry participants,

creating barriers to entry for new entrants and limiting competition among

phone manufacturers.

• GSM has a fixed maximum cell site range of 35 km, which is imposed by

technical limitations. [110], [111



Chapter 10

CONCLUSIONS



From our discussion on the GSM technology in this report, it may be

concluded that GSM is a very complex standard for the telecommunications. it

may be considered as the first attempt to create a global and universal

communication system involving all the countries of the world. Then the GSM

technology was used as the basis for developing the Universal Mobile

Telecommunication System (UMTS). Today, 160 different countries are using the

GSM, and the growth is much rapid with increase in the GSM subscriber in

millions a year. The GSM provides continuous and uninterruptible communication

to all its subscribers with strong signal quality. The marvelous and awe-inspiring

feature of roaming in GSM increases the attraction of this technology, making it

more popular among people than any other technology. Actually, GSM has many

features for its users which require much space to cover in depth.

The GSM architecture is quite amazing as it employs the modular

structure. The advantage associated with this type of modular structure is that it

becomes easy to work with and understand each of the modules separately

without causing the functionality of remaining modules to be interrupted. The

main three parts involved are; Radio subsystem (RSS), Network (switching)

subsystem (SSS), and Operation and maintenance subsystem (OMS). The RSS

consists of; Mobile Equipment (ME), Base Station (BS), and Radio Interface

(Um). The SSS has five main parts; Mobile services Switching Centre (MSC),

Home Location Register (HLR), Visitor Location Register (VLR), Authentication

Centre (AuC), Equipment Identification Register (EIR). Finally, an OMS, the heart

of the BSS, consists of one or more Operation & Maintenance Centre (OMC).

The services provided by the GSM are divided into three categories; the

teleservices, Bearer services, and supplementary services. Its functions include;

Transmission, Radio Resources Management (RRM), Mobility Management

(MM), Communication Management (CM), Operation, Administration and

Maintenance (OAM).



There are many advantages of GSM technology, but few of them are;

GSM more stable network with robust features, there is less signal deterioration

inside buildings etc., the availability of SIMs allows users to switch networks and

handsets at will, GSM covers virtually all parts of the world so international

roaming is not a problem, the subscriber can enjoy the broadest international

coverage. It is possible with the GSM roaming service, Very good due to simple

protocol, good coverage and mature, power-efficient chipsets.

Nothing in this world is ideal, drawbacks are always there. The

disadvantage associated with the GSM is that pulse nature of TDMA

transmission used interferes with some electronics, especially certain audio

amplifiers. GSM has a fixed maximum cell site range of 35 km, which is imposed

by technical limitations.

Overall, GSM is really a great and efficient technology bringing world

together and making every place as our homes due to a communication with

people anywhere anytime.



Appendix A

FDMA and TDMA

There are three common technologies used by 2G cell-phone networks for

transmitting information (we'll discuss 3G technologies in the 3G section):

• Frequency division multiple access (FDMA)

• Time division multiple access (TDMA)

• Code division multiple access (CDMA)

Although these technologies sound very intimidating, you can get a good

sense of how they work just by breaking down the title of each one. The first

word tells you what the access method is. The second word, division, lets you

know that it splits calls based on that access method.

FDMA puts each call on a separate frequency.

TDMA assigns each call a certain portion of time on a designated frequency.

CDMA gives a unique code to each call and spreads it over the available

frequencies.

The last part of each name is multiple access. This simply means that

more than one user can utilize each cell.

FDMA

FDMA separates the spectrum into distinct voice channels by splitting it into

uniform chunks of bandwidth. To better understand FDMA, think of radio stations:

Each station sends its signal at a different frequency within the available band.


http://electronics.howstuffworks.com/question326.htm


FDMA is used mainly for analog transmission. While it is certainly capable of

carrying digital information, FDMA is not considered to be an efficient method for

digital transmission.

Figure A1: In FDMA, each phone uses a different frequency. [113]

TDMA

TDMA is the access method used by the Electronics Industry Alliance and

the Telecommunications Industry Association for Interim Standard 54 (IS-54) and

Interim Standard 136 (IS-136). Using TDMA, a narrow band that is 30 kHz wide

and 6.7 milliseconds long is split time-wise into three time slots. Narrow band

means "channels" in the traditional sense. Each conversation gets the radio for

one-third of the time. This is possible because voice data that has been

converted to digital information is compressed so that it takes up significantly less


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http://electronics.howstuffworks.com/framed.htm?parent=cell-phone.htm&url=http://eia.org


transmission space. Therefore, TDMA has three times the capacity of an analog

system using the same number of channels. TDMA systems operate in either the

800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

Figure A2: TDMA splits a frequency into time slots [113]



Appendix B

Difference between GSM and CDMA

In cellular service, there are two chief competing network technologies:

Global System for Mobile Communications (GSM) and Code Division Multiple

Access (CDMA). Understanding the difference between GSM and CDMA will

permit you to choose a carrier that uses the preferable network technology for

your needs.

The GSM Association is an international organization founded in 1987,

dedicated to providing, developing, and overseeing the worldwide wireless

standard of GSM. CDMA, a proprietary standard designed by Qualcomm in the

United States, has been the dominant network standard for North America and

parts of Asia. However, GSM networks continue to make inroads in the United

States, as CDMA networks make progress in other parts of the world. There are

camps on both sides that firmly believe either GSM or CDMA architecture is

superior to the other. The fanatical reader who simply wants bottom line

information to make a choice, the following considerations may be helpful.

Coverage

The most important factor is getting service in the areas you will be using

your phone. Upon viewing competitors' coverage maps you may discover that

only GSM or CDMA carriers offer cellular service in your area. If so, there is no

decision to be made, but most people will find that they do have a choice.



Data Transfer Speed

With the advent of cellular phones doing double and triple duty as

streaming video devices, podcast receivers and email devices, speed is

important to those who use the phone for more than making calls. CDMA has

been traditionally faster than GSM, though both technologies continue to rapidly

leapfrog along this path. Both boast "3G" standards, or 3rd generation

technologies.

EVDO, also known as CDMA2000, is CDMA's answer to the need for

speed with a downstream rate of about 2 megabits per second, though some

reports suggest real world speeds are closer to 300-700 kilobits per second

(kbps). This is comparable to basic DSL.

GSM's answer is EDGE (Enhanced Data Rates for GSM Evolution), which

boasts data rates of up to 384 kbps with real world speeds reported closer to 70-

140 kbps. With added technologies still in the works that include UMTS

(Universal Mobile Telephone Standard) and HSDPA (High Speed Downlink

Packet Access), speeds reportedly increase to about 275—380 kbps. This

technology is also known as W-CDMA, but is incompatible with CDMA networks.

An EDGE-ready phone is required.

In the case of EVDO, theoretical high traffic can degrade speed and

performance, while the EDGE network is more susceptible to interference. Both

require being within close range of a cell to get the best speeds, while

performance decreases with distance.

Subscriber Identity Module (SIM) cards

In the United States only GSM phones use SIM cards. The removable SIM

card allows phones to be instantly activated, interchanged, swapped out and


http://www.wisegeek.com/what-is-evdo.htm


upgraded, all without carrier intervention. The SIM itself is tied to the network,

rather than the actual phone. Phones that are card-enabled can be used with any

GSM carrier. The CDMA equivalent, a R-UIM card, is only available in parts of

Asia but remains on the horizon for the U.S. market. CDMA carriers in the U.S.

require proprietary handsets that are linked to one carrier only and are not card-

enabled.

To upgrade a CDMA phone, the carrier must deactivate the old phone

then activate the new one. The old phone becomes useless.

Roaming

For the most part, both networks have fairly concentrated coverage in

major cities and along major highways. GSM carriers, however, have roaming

contracts with other GSM carriers, allowing wider coverage of more rural areas,

generally speaking, often without roaming charges to the customer. CDMA

networks may not cover rural areas as well as GSM carriers, and though they

may contract with GSM cells for roaming in more rural areas, the charge to the

customer will generally be significantly higher.

International Roaming

If you need to make calls to other countries, a GSM carrier can offer

international roaming, as GSM networks dominate the world market. If you travel

to other countries you can even use your GSM cell phone abroad, providing it is

a quad-band phone (850/900/1800/1900 MHz). By purchasing a SIM card with

minutes and a local number in the country you are visiting, you can make calls

against the card to save yourself international roaming charges from your carrier

back home. CDMA phones that are not card-enabled do not have this capability.

According CDG.org, CDMA networks support over 270 million subscribers

worldwide, while GSM.org tallies up their score at over 1 billion.



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