GSM

presentation:
Prsente par:
Sara HATTAN
Rania ARABYECH

Introduction
GSM system is a frequency- and timedivision cellular system, each physical
channel is characterized by a carrier
frequency and a time slot number
Cellular systems are designed to operate
with groups of low-power radios spread
out over the geographical service area.
Each group of radios serve MSs
presently located near them. The area
served by each group of radios is called a
CELL

GSM
Architecture

GSM Architecture

MS- Mobile Station

MS provides the air interface to the user in

GSM networks consists:

ME- Mobile
Equipement

IMEI- International
Identity

Mobile

Equipment

IMEI is a serial number unique to each mobile

stored in the ME.
IMEI can be used to identify MSs that are reported
stolen or operating incorrectly.

Mobiles are classified into five classes according to

their power rating.

SIM- Subscriber Identity Module

IMSI : International
Mobile Subscribers
Identity
Transmitted over the air
Initialising
IMSI can be maximum of 15
digits

TMSI: Temporary
Mobile Subscriber
Identity
Protect the subscriber from
being identified by someone
attempting to monitor the radio
interface.
The TMSI is a local number and
is always allocated by the VLR.

The TMSI is maximum of 4

MCC- Mobile Country Code (3 Digits) octets.
MNC- Mobile Network Code (2 Digits)
MSIN- Mobile Subscriber Identity
Number
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SIM- Subscriber Identity Module

LAI -Location Area
Identity

MSISDN- Mobile Station

International Standard Data
Number

Identifies the current location of the subscriber.

CC-Country code

MCC -Mobile country code lists all the

countries and assigned codes.

NDC-National destination code

countrys telecommunication
regulatory authority assigns an
NDC to each PLMN.

LAC : Location area code identifies a

location area within a GSM PLMN
MNC -Mobile network code is a unique
network code assigned to PLMN by the
countrys regulatory authority.

Subscriber number (SN)

variable-length field.

BSS Base Station

SubSystem:

The BSS is the fixed end of the radio interface

that provides control and radio coverage
functions for one or more cells and their
associated MSs.
The BSS consists of three basic elements:

BTS: Base Transceiver Stations

BSC: Base Station Controller

XCDR :Transcoder
transcoder.

or

RXCDR:

Remote

BSIC - Base
Station Identity
Code

BSIC allows a mobile

station to distinguish
between neighboring
base stations.

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NCC = National Colour

Code( Differs from
operator to operator )

BCC = Base Station Colour

Code( identifies the base
station to help
distinguish between
Cells using the same
BCCH frequencies)

CGI Cell Global

Identity

The CGI is a unique

international identification
for a cell.

MCC = Mobile Country Code (It

consists of 3 digits)

MNC = Mobile Network Code (It

consists of 2 digits )

LAC = Location Area Code (It is a

two bytes hex code)

LAI = Location Area Identity

(international code for a
location area)

CI = Cell Identity (This code

uses two bytes hex code to
identify the cells within an LAI)

LAC = Location Area Code (It is a

two bytes hex code)

The format is LAI+CI

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BTS Base Transceiver

Station:
The BTS network element consists of the hardware
components, such as radios, interface modules and
antenna systems that provide the Air Interface between
the BSS and the MSs.

The BTS provides radio channels (RF carriers) for a

specific RF coverage area.

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BSC Base Station

Controller:

The BSC network element provides the

control for the BSS.

The purpose of the BSC is to perform a variety of

functions:

Controls the BTS components

Controls and manages the associated BTSs

Performs Call Processing.

Performs Operations and Maintenance (O & M).

Provides the A Interface between the BSS and the MSC.

Manages the radio channels.

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XCDR - Transcoder
The speech transcoder is the interface between the 64 kbit/s PCM
channel in the land network and the 13 kbit/s vocoder channel used
on the Air Interface
The XCDR can multiplex 4 traffic channels into a single 64 kbit/s
timeslot. Thus a E1/T1 serial link can carry 4 times as many channels.
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NSS: Network
SubSystem

Performs the switching of calls as well as the

management of mobile services such as authentication

The switching system includes the following functional

elements:

MSC- Mobile Services Switching Center

EIR - Equipment Identity Register

HLR- Home Location Register

VLR- Visitor Location Register

AUC- Authentication Center

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MSC- Mobile Services

Switching Center

Co-ordinates the setting up of calls to and

from GSM users

Controls a number of Base Station Sites

(BSSs) within a specified geographical
coverage area

Provides radio subsystem access to the

subscriber and equipment databases

Gateway MSC: provides the interface

between PSTN and the BSS in the GSM
network
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EIR- Equipment Identity

Register

Contains a centralized database for

validating the international ME identity.

The EIR database is remotely accessed by

the MSCs in the Network and can also be
accessed by an MSC in a different PLMN.

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HLR- Home Location

Register

Contains the master database of all subscribers in

the PLMN.

The subscribers data may be accessed by the IMSI or the

MSISDN.

The parameters stored in HLR are:

Subscribers ID (IMSI and MSISDN )
Current subscriber VLR.
Supplementary services subscribed to.
Supplementary services information (eg. Current forwarding
address ).
Authentication key and AUC
18 functionality.
TMSI et MSRN

VLR- Visitor Location Register

Local subscriber database, holding details on

those subscribers who enter the area of the
network that it covers. The details are held in
the VLR until the subscriber moves into the area
serviced by another VLR.

The data stored in VLR are:

Mobile status ( Busy / Free / No answer etc. )
Location Area Identity ( LAI )
Temporary Mobile Subscribers Identity ( TMSI )
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Mobile Station Roaming Number ( MSRN )

AUC - Authentication Centre

The AUC is a processor system that perform

authentication function. It is normally co-located with
the HLR.

The authentication process usually takes place each

time the subscriber initializes on the system.

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GSM Interfaces:
Air Interface.
Terrestrial Interface.
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Frequency
Allocation

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Frequency Band
GSM900

EGSM Extended GSM

DCS1800- Digital Cellular

System

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Frequency
Allocation

Mditel a une bande de 9Mhz (GSM 900) 45 canaux

ARFCN 900[80-124]

Mditel: ARFCN 1800[786-835]

BCCH [80-99]

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TCH[100-124]

Concept of Frequency
Reuse

GSM system is a cellular system and the

total channel resource is limited

The limited channel resources have to be

reused in different cells to expand system
capacity

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Reuse Density

Reuse density is the number of cells in a basic

reuse cluster.
For the n x m frequency reuse pattern,

f reuse n m

n: The number of BTSs in the reuse clusters

m: The number of the cells under each BTS.

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Frequency Reuse Problems

Co-channel interference
Interference between two cells using the same frequency

Adjacent channels
Interference
The template of a channel is not steep sides
so it will extend to the adjacent channels
interference due to the frequency
Multi-path signal Interference

Interference of useful signal itself which

falls outside the system delay equalizer

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Multiple Access
Technique:
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FDMA:
o FDMA uses different
frequency channels to
accomplish communication.
o The whole frequency
spectrum available is
divided into many individual
channels (for transmitting
and receiving) every
channel can support the
traffic for one subscriber or
some control information.

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TDMA:
o TDMA accomplishes the
communication in different
timeslot.
o A carrier is divided into
channels based on time.
Different signals occupy
different timeslots in
certain sequence , that is ,
many signals are
transmitted on the same
frequency in different time.

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Channel
Concept:

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Channel:
Physical
Channel:

Logical
Channel:
Variety of information is
transmitted between the MS
and BTS. There are different
logical channels depending
on the information sent.
The logical channels are of
two types:

Traffic channel

Control channel

Each timeslot on a
carrier is referred to as a
physical channel. Per
carrier there are 8
physical channels.

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Logical Chanel:
Many types of logical channels exists, eachdesigned to carry a
different message to or from an MS.
There are several types of burst. The relationship between
burstsand logical channels is shown
in this figure.
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Type of BURST:
Frequency
Correction
BURST:

Normal
BURST:

This GSM burst is used for

the standard
communications between
the BTS and the MS, and
typically transfers the
digitised voice data.

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Made up of 142 consecutive

zeros.

Enables MS to correst its

local oscillator locking it to
that of the BTS.

Type of BURST:
Synchronisati
on BURST:

Synchronisation for the

mobiles on the network.

Enables MS to synchronise
its timings with BTS.

Contains BSIC and TDMA

Frame number.

Access
BURST:

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Has a bigger guard period

since it is used during initial
access and the MS does not
know how far it is actually
from the BTS.

Dummy
BURST:

Transmitted on the

unused timeslots
of the BCCH
carrier in the
downlink.

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Speech
Processing:
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Voice Signal
Processing:
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Channel Coding:
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Interleaving:
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Ciphering:
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Call Flow:

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Mobile Originated Call:

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Mobile Terminated Call :

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Handover

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Types Handover

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Intra-cell
Handover
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Intra-BSC
Handover
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Intra-MSC
Handover
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Inter-MSC
Handover
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Interferences
Solutions

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Frequency
Hopping:
Hopping can be
implemented in two
ways:

Class according to
the min hopping
time unit:

Base-band hopping.

Timeslot hopping.

RF hopping.

Frame hopping.

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Hopping
Parameters:

Hopping mode: not

hopping, base band
hopping and RF hopping.

Mobile Allocation Index

Offset (MAIO): Determines
inside the hopping sequence
which frequency the mobile
starts to transmit on.

Mobile Allocation
(MA): Set of frequencies
the mobile is allowed to
hop over.

Mobile Allocation
Indicator (MAI): Defines a
hopping system made up by
an associated set of
frequencies (MA) to hop
over and a hopping
sequence (HSN) and
(MAIO).

Hopping Sequence
Number (HSN):
Determines the hopping
order used in the cell.
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Base Band Hopping:

The radio units transmit always the same frequency.
Number of frequencies for hopping = Number of carriers .
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Frame Hopping:
Frequency changes every TDMA frame. The different channel of
one TRX uses the same MAIO.
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Timeslot Hopping:
Frequency changes every timeslot. The different
channel of one TRX uses the different MAIO.
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Power Control:

WHY?

Prolong battery life

During the radio

transmission signals,
to reduce interference,
increase spectral
efficiency and prolong
battery life, the
transmit power can be
adjusted, which is
called the power
control.

Reduce network interference

Include both uplink power
control and downlink power
control
Level and quality are taken
into account
BSC or BTS is the final
adjudicator
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TA: Timing Advance

Before using
TA

After using
TA

Distance increase
Transmission delay
increase

The mobile station sent a

mesurement report from
mobile station to the base
station which carries a
delay value

Inter-code interference:
if the delay is too high, the
timeslots of the signal from
a certain mobile station and
that of the next signal from
another mobile station
received by the base station 59
will overlap each other

The BTS will monitor the

time when the call arrives
and send an instruction to
to the MS via downlink
channel evry 480 ms to
inform it the TA
transmission

DTX:
Discontinuous
Transmission

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DTX: Discontinuous

Transmission
Shut off the transmission at voice
intervals
Only transmit SID frames
The transcoder at the RX terminal
produces comfortable noise

VAD: Voice Activity

Detection
Implemented by the transcoder.
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