GSM System

Mobile Telecommunications and GSM

    MOBILE TELEPHONY
Mobile telecommunications is one of the fastest growing and
most demanding of all telecommunications technologies.
Currently, it represents an increasingly high percentage of all
new telephone subscriptions worldwide. In many cases, cellular
solutions successfully compete with traditional wire-line
networks and cordless telephones. In the future, cellular systems
employing digital technology will become the universal method
of telecommunication.

HISTORY OF Mobile COMMUNICATION
The origins of mobile communications followed quickly behind
the invention of radio in the late 1800s. The first applications of
mobile radio were related to the navigation and safety of ships at
sea. As radio concepts developed, so did it’s use as a
communications tool. The major milestones in the development
of wireless communications are summarized in the following:

 -1906 : Reginald Fesseden successfully transmits human
voice over radio. Up until that time, radio
communications consisted of transmissions of Morse
Code.

 -1915 :J. A. Fleming invents the vacuum tube making it
possible to build mobile radios.
 -1921 :The Detroit police department used a 2 MHz
frequency in the department's first vehicular mobile
radio. The system was only one way and police had
to find a wire-line phone to respond to radio
messages.
 -1930s :Amplitude Modulation (AM) two-way mobile
systems were in place in the U.S. that took
advantage of newly developed mobile transmitters
and utilized a "push-to-talk" or half-duplex
transmission. By the end of the decade channel
allocation grew from 11 to 40.
 -1935 :Invention of Frequency Modulation (FM) improved
audio quality. FM eliminated the need for large AM
transmitters and resulted in radio equipment which
required less power to operate. This made the use of
transmitters in vehicles more practical.

 -1940s :The Federal Communications Commission (FCC)
recognized a communication service it classified as
Domestic Public Land Mobile (DPLM) radio
service. The first DPLM system was established in
St. Louis in 1946 and it utilized the 150 MHz band.
The following year, a "highway" system was
developed along the New York - Boston corridor
using the 35-40 MHz band.
 -1947 :D.H. Ring, working at Bell Laboratories, envisions
the cellular concept.
 -1948 :Shockley, Bardeen and Brittain, at Bell Laboratories,
invent the transistor which enables electronic
equipment, including the radio to be miniaturized.
 -1949 :Radio Common Carriers (RCCs) were recognized.
 -1949,
 -1958 :Bell Systems made broadband proposals.
 -1964 :AT&T introduces Improved Mobile Telephone
System (IMTS).
 -1968 :The FCC began to address issue of new US spectrum
requirements.
 -1969 :Nordic countries of Denmark, Finland, Iceland,
Norway and Sweden agree to form a group to study
and recommend areas of cooperation in
telecommunication. This led to the standardization
of telecommunications for all members of the Nordic
Mobile Telephone (NMT) group, the first
comprehensive international standardization group.
 -1973 :The NMT group specifies a feature allowing mobile
telephones to be located within and across networks.
This feature would become the basis for roaming.
 -1979 :The FCC authorized the installation and testing of
the first developmental cellular system in the US
(Illinois Bell Telephone Company).

GSM NETWORK COMPONENTS
The GSM network is divided into two systems. Each of these
systems are comprised of a number of functional units which are
individual components of the mobile network. The two systems
are:
·  Switching SubSystem (SS)
·  Base Station subSystem (BSS)
In addition, as with all telecommunications networks, GSM
networks are operated, maintained and managed from
computerized centers.

Abbreviations of keywords

AUC :Authentication Center

BSC :Base Station Controller

BTS :Base Transceiver Station

EIR :Equipment Identity Register

HLR  :Home Location Register

MS :Mobile Station

MSC :Mobile Switching Center

NMC :Network Management Center

OMC :Operation and Maintenance Center

VLR :Visitor Location Register

The Function of Switching Subsystem

The SS is responsible for performing call processing and

subscriber related functions. It includes the following functional

units:

·  Mobile services Switching Center (MSC)

·  Home Location Register (HLR)

·  Visitor Location Register (VLR)

·  AUthentication Center (AUC)

·  Equipment Identity Register (EIR)

The Function of Base Station Subsystem

The BSS performs all the radio-related functions. The BSS is

comprised of the following functional units:

·  Base Station Controller (BSC)

·  Base Transceiver Station (BTS)

 The Function of OMC

The OMC performs all the operation and maintenance tasks for

the network such as monitoring network traffic and network

alarms. The OMC has access to both the SS and the BSS.

 Mobile Station(MS) : 

MSs do not belong to any of these systems.

      SWITCHING SUBSYSTEM (SS) COMPONENTS

1.Mobile Switching Center (MSC)

The MSC performs the telephony switching functions for the

mobile network. It controls calls to and from other telephony

and data systems, such as the Public Switched Telephone

Network (PSTN), Integrated Services Digital Network (ISDN),

public data networks, private networks and other mobile

networks.

 2.Gateway Functionality

Gateway functionality enables an MSC to interrogate a

network’s HLR in order to route a call to a Mobile Station (MS).

Such an MSC is called a Gateway MSC (GMSC). For example,

if a person connected to the PSTN wants to make a call to a

GSM mobile subscriber, then the PSTN exchange will access

the GSM network by first connecting the call to a GMSC. The

same is true of a call from an MS to another MS.

Any MSC in the mobile network can function as a gateway by

integration of the appropriate software.

  3.Home Location Register (HLR) function

The HLR is a centralized network database that stores and

manages all mobile subscriptions belonging to a specific

operator. It acts as a permanent store for a person’s subscription

information until that subscription is canceled. The information

stored includes:

·  Subscriber identity

·  Subscriber supplementary services

·  Subscriber location information

·  Subscriber authentication information

The HLR can be implemented in the same network node as the

MSC or as a stand-alone database. If the capacity of a HLR is

exceeded by the number of subscribers, additional HLRs may be

added.

 4.Visitor Location Register (VLR) Functions

The VLR database contains information about all the mobile

subscribers currently located in an MSC service area. Thus,

there is one VLR for each MSC in a network. The VLR

temporarily stores subscription information so that the MSC can

service all the subscribers currently visiting that MSC service

area. The VLR can be regarded as a distributed HLR as it holds

a copy of the HLR information stored about the subscriber.

When a subscriber roams into a new MSC service area, the VLR

connected to that MSC requests information about the

subscriber from the subscriber’s HLR. The HLR sends a copy of

the information to the VLR and updates its own location

information. When the subscriber makes a call, the VLR will

already have the information required for call set-up.

 5.Authentication Center (AUC) function

The main function of the AUC is to authenticate the subscribers

attempting to use a network. In this way, it is used to protect

network operators against fraud. The AUC is a database

connected to the HLR which provides it with the authentication

parameters and ciphering keys used to ensure network security.

Equipment Identity Register (EIR) functions

The EIR is a database containing mobile equipment identity

information which helps to block calls from stolen,

unauthorized, or defective MSs. It should be noted that due to

subscriber-equipment separation in GSM, the barring of MS

equipment does not result in automatic barring of a subscriber.

BASE STATION SYSTEM (BSS) COMPONENTS

  1.Base Station Controller (BSC) functions

The BSC manages all the radio-related functions of a GSM

network. It is a high capacity switch that provides functions such

as MS handover, radio channel assignment and the collection of

cell configuration data. A number of BSCs may be controlled by

each MSC.

  2.Base Transceiver Station (BTS) functions

The BTS controls the radio interface to the MS. The BTS

comprises the radio equipment such as transceivers and antennas

which are needed to serve each cell in the network. A group of

BTSs are controlled by a BSC.

   NETWORK MONITORING CENTERS functions

1.Operation and Maintenance Center (OMC)

An OMC is a computerized monitoring center which is

connected to other network components such as MSCs and

BSCs via X.25 data network links. In the OMC, staff are

presented with information about the status of the network and

can monitor and control a variety of system parameters. There

may be one or several OMCs within a network depending on the

network size.

 2.Network Management Center (NMC)

Centralized control of a network is done at a Network

Management Center (NMC). Only one NMC is required for a

network and this controls the subordinate OMCs. The advantage

of this hierarchical approach is that staff at the NMC can

concentrate on long term system-wide issues, whereas local

personnel at each OMC can concentrate on short term, regional

issues.

OMC and NMC functionality can be combined in the same

physical network node or implemented at different locations.

 3.MOBILE STATION (MS) functions

An MS is used by a mobile subscriber to communicate with the

mobile network. Several types of MSs exist, each allowing the

subscriber to make and receive calls. Manufacturers of MSs

offer a variety of designs and features to meet the needs of

different markets.

The range or coverage area of an MS depends on the output

power of the MS. Different types of MSs have different output

power capabilities and consequently different ranges. For

example, hand-held MSs have a lower output power and shorter

range than car-installed MSs with a roof mounted antenna.

 GSM MSs consist of:

·  A mobile terminal

·  A Subscriber Identity Module (SIM)

Unlike other standards, in GSM the subscriber is separated from

the mobile terminal. Each subscriber’s information is stored as a

"smart card" SIM. The SIM can be plugged into any GSM

mobile terminal. This brings the advantages of security and

portability for subscribers. For example, subscriber A’s mobile

terminal may have been stolen. However, subscriber A’s own

SIM can be used in another person’s mobile terminal and the

calls will be charged to subscriber A.

  GSM NETWORK STRUCTURE

Every telephone network needs a specific structure to route

incoming calls to the correct exchange and then on to the

subscriber. In a mobile network, this structure is very important

because the subscribers are mobile. As subscribers move

through the network, these structures are used to monitor their

location.

   CELL

A cell is the basic unit of a cellular system and is defined as the

area of radio coverage given by one BS antenna system. Each

cell is assigned a unique number called Cell Global Identity

(CGI). In a complete network covering an entire country, the

number of cells can be quite high.

    LOCATION AREA (LA)

A Location Area (LA) is defined as a group of cells. Within the

network, a subscriber’s location is known by the LA which they

are in. The identity of the LA in which an MS is currently

located is stored in the VLR.

When an MS crosses a boundary from a cell belonging to one

LA into a cell belonging to another LA, it must report its new

location to the network1. When an MS crosses a cell boundary

within an LA, it does need to report its new location to the

network. When there is call for an MS, a paging message is

broadcast within all cells belonging to an LA.

   MSC SERVICE AREA

An MSC service area is made up of a number of LAs and

represents the geographical part of the network controlled by

one MSC. In order to be able to route a call to an MS, the

subscriber’s MSC service area is also recorded and monitored.

The subscriber’s MSC service area is stored in the HLR.

   PLMN SERVICE AREA

A Public Land Mobile Network (PLMN) service area is the

entire set of cells served by one network operator and is defined

as the area in which an operator offers radio coverage and access

to its network. In any one country there may be several PLMN

service areas, one for each mobile operator’s network.

  GSM FREQUENCY

As GSM has grown worldwide, it has expanded to operate at

three frequency bands: 900, 1800 and 1900.

   1.GSM 900

The original frequency band specified for GSM was 900 MHz.

Most GSM networks worldwide use this band. In some

countries and extended version of GSM 900 can be used, which

provides extra network capacity. This extended version of GSM

is called E-GSM, while the primary version is called P-GSM.

   2.GSM 1800

In 1990, in order to increase competition between operators, the

United Kingdom requested the start of a new version of GSM

adapted to the 1800 MHz frequency band. Licenses have been

issued in several countries and networks are in full operation.

By granting licenses for GSM 1800 in addition to GSM 900, a

country can increase the number of operators. In this way, due to

increased competition, the service to subscribers is improved.

   3.GSM 1900

In 1995, the Personal Communications Services (PCS) concept

was specified in the United States. The basic idea is to enable

"person-to-person" communication rather than "station-tostation".

PCS does not require that such services be

implemented using cellular technology, but this has proven to be

the most effective method. The frequencies available for PCS

are around 1900 MHz. As GSM 900 could not be used in North

America due to prior allocation of the 900 MHz frequencies,

GSM 1900 MHz is seen as an opportunity to bridge this gap.

The main differences between the American GSM 1900

standard and GSM 900 is that it supports ANSI signaling.

   KEY TERMS

During the development of mobile systems, many terms arose

which are used to describe the call cases and situations

involving MSs. The primary terms used are described below.

An MS can have one of the following states:

·  IDLE : the MS is ON but a call is not in progress

·  Active :the MS is ON and a call is in progress

·  Detached: the MS is OFF

    MS Registration and Roaming

When an MS is powered off it is detached from the network.

When the subscriber switches power on, the MS scans the GSM

frequencies for special channels called control channels. When it

finds a control channel, the MS measures the signal strength it

receives on that channel and records it. When all control

channels have been measured, the MS tunes to the strongest one.

When the MS has just been powered on, the MS must register

with the network which will then update the MS’s status to idle.

If the location of the MS is noticed to be different from the

currently stored location then a location update will also take

place.

As the MS moves through the network, it continues to scan the

control channels to ensure that it is tuned to the strongest

possible channel. If the MS finds one which is stronger, then the

MS retunes to this new control channel2. If the new control

channel belongs to a new LA, the MS will also inform the

network of its new location.

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