GSM

Prepared by :
Eng. Mahmoud Yousry

Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning

Introduction

History of GSM system.

Wireless transmission
Multiple Access Techniques
Wireless Challenges

What is the GSM?

GSM is the Global System for Mobile telecommunications.
It is the European standard for the Mobile
telecommunications and it is considered as one of the most
popular standard worldwide.
It is known as the second generation mobile
telecommunications system 2G system.
It is used in Egypt by the three existing operators; Mobinil
and Vodafone; Etisalat.

Evolution of GSM
1982, CEPT conference is held to specify a common
European mobile telecommunications in the band of 900
MHz.
1987, GSM specs drafted TDMA+FDMA, Time multiplexing
of order 8 with slow frequency hopping.
1992, first GSM network was commercial in Finland .
Global System for Mobile (GSM) is a second-generation
digital cellular telephone system.

Evolution of GSM
1996, first GSM operator in Egypt owned by the
government.
1998, Egypt privatizes its GSM operator.
1998, Click GSM commercial launch.
GSM became the world's leading and fastest growing
mobile standard, spanning over 174 countries, serving
more than one in ten of the world's population.
2000, First GPRS roll out.

What is the GPRS?

GPRS is the General Packet Radio Service.
GPRS is defined by 3GPP (Third Generation Partnership
Project) and is employed to connect mobile cellular users to
PDN (Public Data Network).

Within the GSM network it shares the network databases

and radio access network.
It is known as the 2.5 generation mobile
telecommunications system 2G system.

GSM to GPRS development

GSM offers circuit-switched with good voice quality, but it is
providing data rates of 9.6 kbps which is too slow.
In 1999 General Packet Radio Service (GPRS) reuses the
existing GSM infrastructure to provide higher data rate
It was lunched to increase the data rate to 115 kbps by using
the packet-switched in data transmission and defining new
coding scheme.
In 2001 Evolved Data rate for GSM Evolution (EDGE) offers
data rate of 384 kbps by using new modulation scheme.
Now, EDGE is used in Mobinil the first operator in Egypt.

3G Systems

In 2000 the ITU-T was responsible for the IMT-2000

specification, which is meant to be a guideline for every 3G
standard
Universal Mobile Telecommunication Service (UMTS) is the
marketing name for the 3G has two standardization
bodies:
1- 3GPP which uses the W-CDMA technology.
2- 3GPP2 which uses the CDMA2000 technology.

WCDMA as a 3G Approach
The 3G solution for GSM is called WCDMA (Wideband
CDMA).

WCDMA requires a new radio spectrum as it operates in

ultra wide 5-MHz radio channels.
WCDMA meets the IMT-2000 requirements of 384 kbps
outdoors and 2 Mbps indoors.

Evolution Vs. Time

Functionality
& capabilities

UMTS
capable systems
EDGE
384
kbps

WCDMA
2 Mbps

GPRS
115 kbps

Circuit
data
9.6
kbps
Speech

HSCSD
57.6
kbps
Circuit

Packet
Switched

Switched

1997

1998
Fig 9-1

1999

2000

GSM Data Rates Evolution

2001

2002

Time

Introduction
History of GSM system.
Wireless transmission
Multiple Access Techniques

Wireless Challenges

Wireless transmission

source

Transmission medium

Destination

Wireless transmission
The kinds of transmission medium :
1- Twisted-pair:
It has very low bandwidth and it is easily tapped either
physically or by monitoring its electromagnetic radiation
2- Coaxial cable:
It has greater bandwidth than twisted-pair but it is very
expensive.
3- optical fibers:
It is very high bandwidth , very high bit rate and
inherently transmission medium.

Wireless transmission
4- Radio (wireless):
It is greatly depending on the particular frequency of
the electromagnetic wave
Some of their advantages :
a- They are very flexible and suitable for all terrain.
b- Portable system can be installed very quickly
c- There are often the most cost-effective solution

Wireless transmission

As a summary, the radio transmission is used in case of

mobile telecommunications. Also, the using of Radio
transmission is combined with many problems that
may cause the degradation of the service quality.

Introduction

History of GSM system.

Wireless transmission
Multiple Access Techniques

Wireless Challenges

Multiple Access Techniques

What is the Multiple Access Technique?

It is the use of the same transmission media by more than one user.

It is used because the limitation of transmission resources comparing with

the number of users

Multiple Access Techniques

Multiple Access Techniques

Three types of Multiple Access Technique are available:

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)

Multiple Access Techniques

Frequency Division Multiple Access (FDMA), divide the available bandwidth into
smaller bandwidths and allocated to the users.

Strength

f1

f2

f3

Multiple Access Techniques

Time Division Multiple Access (TDMA), allocate the whole available bandwidth to one
subscriber for a certain period of time
Strength
User
User 2,
1, time=t0
time=0

Multiple Access Techniques

Time Division Multiple Access (TDMA) and Frequency Division Multiple Access.

Strength
User 1
User 7
User 2

Multiple Access Techniques

Code Division Multiple Access (CDMA).

Strength

Frequency

Introduction

History of GSM system.

Wireless transmission
Multiple Access Techniques
Wireless Challenges

Wireless Challenges
Radio Communication applied to mobile telephony has the
following limitations :
Powerful transmitters & huge equipment are required.
Capacity is limited to the frequency band allocated.
So, in order to get rid of these shortages the cellular
system is being used in the mobile communications.

Wireless Challenges
The Area to be covered is
divided into small cells.
So,
Low Transmission power.
Smaller equipment size.
Capacity of the system can be increased by
reuse the frequency.

Cell Geometry
The simplest shape is the circular shape
So, why not using this shape???????

Dead Spots

Cell Geometry
So, what are the possible shape and what is the used shape???

Types of Antennas
Two types of antennas are being commonly used; the Omni-directional antenna
and the directional antenna.
A site is the place where the transmission equipment is placed; this maybe on
the top of the buildings or on the top of the advertising.
In case of directional type antenna; one site is corresponding to 3 cells and in
case of Omni-directional antenna one site is corresponding to one cell and one
antenna.

Omni-Directional Cell

Sectorial Cells

Frequency Reuse

As the frequency resources that are given to an operator

Vodafone or Mobinil are limited; the frequency used in a
cell should be reused again in a cell that is away from this
cell in order to increase the capacity of the system.
Moreover, for a certain frequency 8 time slots are used for
more capacity of the system.
So, the access technique that is used is FDMA+TDMA with
frequency reused.

Cellular System Concepts

Reuse Pattern(Cluster):
Cells are grouped into Clusters
Available Band is distributed among
the cells of the cluster
N is the number of cells in a
cluster .
Each frequency is reused after the
same distance D
Reuse Plan = (D/R)= 3N
Where R is the cell radius

3
2

4
1
7

5
6

N=7 Cell Cluster

D
3
3

5
6

3
2

3
2

4
1

4
1
7

5
6

7
5

5
6

7 Cell Reuse Plan

Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning

Basic GSM Network Structure

Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications

Basic GSM Network Structure

PSTN
ISDN
PDN

BSC

MS

BTS
MSC
GMSC

BTS

BSC
VLR
MS

EIR

BTS

AUC

MS

HLR

Basic GSM Network Structure

Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications

Mobile Station (MS)

The Mobile Station (MS) is the interface between the user
and the network. The MS consists of two independent parts:
Subscriber Identity Module (SIM) card
Mobile Equipment (ME)

+
The Mobile Equipment is said to be a Mobile Station if the
Subscriber Identity Module ( SIM Card ) is added to it.

Mobile Station (MS)

MS features are defined as mandatory or optional
Mandatory features are:
Display of called number: Check the dialed number
before call setup.
Support of A5/1 and A5/2: Provisions are made for
support of up to seven algorithms as well as the support of
no encryption. It is mandatory for A5/1 and A5/2 and nonencrypted mode to be implemented, but other algorithms are
optional.
Short message indication and acknowledgment: This
feature allows the reception of the delivery report of short
messages to a MS from a service center.
Emergency call capabilities: It must be possible to make
an emergency call even without a valid SIM.

Mobile Equipment (ME)

The MS sends these features to the network when it
is powered on.
Optional features are:
On/Off switch: The MS can be equipped with the
means of switching its power supply on and off.
Short Message Service Cell Broadcast (SMSCB)
screening

Subscriber Identity Module

(SIM) card
A SIM card is an electronic smart card, which stores
information about the subscription.
Thus it contains a microprocessor and a memory
The SIM stores three types of subscriber related
information:
Fixed data stored before the subscription: e.g. IMSI,
authentication key and security algorithms.
Temporary network data: e.g. the location area of the
subscriber and other data of the subscriber.
Service data

Basic GSM Network Structure

Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications

Base Station Subsystem (BSS)

BTS

MS

BSC

BSS

The Base Station Sub-system (BSS) is responsible for all the

radio-related functions in the system.

Base Transceiver Station (BTS)

A BTS acts as the interface between MSs and the network, by
providing radio access to the network subscribers.
Converts the GSM radio signals into a format that can be
recognized by the BSC.
Ciphering using the ciphering key.
Channel coding and interleaving.
Records and passes to the BSC the Signal strength
measurements.

Base Station Controller (BSC)

The BSC is the central node within a BSS and co-ordinates the actions of
Base Stations. The BSC controls a major part of the radio network.
Its main functions can be divided into two types:
During Call Set Up:
Finding the called mobile station by paging.
Allocate the frequency for setting the call.
During Call Progress:
Monitoring the call quality.
Controlling the transmitted power to the MS depending
on the location of the MS.
Control the handover for the MS after receiving the
power measurements from the MS and from the BTS.

Base Station Controller (BSC)

TRC Handling
The TRC provides the BSS with rate adaptation
capabilities. This is necessary because the rate used over
the air interface and that used by MSC/VLRs are different 33.8 Kbit/s and 64 Kbit/s respectively. A device, which
performs rate adaptation is called a transcoder.

LAI and CGI

LAI: Location Area Identification.
A location area is determined by the operator and it consists
of one or more BSC coverage area. It is used to control the
subscribers within a coverage area larger than the cell area
and smaller than the MSC coverage area.
LAI consists of MCC (Mobile Country Code) + MNC (Mobile
Network Code) + LAC (Location Area Code 4 hex digit); for
example for Vodafone, Egypt one LAI can be as following:
602 02 8f65

LAI and CGI

CGI: Cell Global Identification.

It is an identifier for the cell of GSM system.
CGI consists of LAI+CI (Cell Identity, 4 hex digit)

Basic GSM Network Structure

Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications

Network Switching Subsystem

(NSS)
HLR
AUC
BTS

BSC

MSC/VLR

NSS
BTS

Mobile Switching Center (MSC)

The primary node in a GSM network is the MSC. It is the
node which controls calls establishment. The primary
functions of an MSC include the following:
Switching and call routing to or from MS.
Charging.
Service provisioning.
Control of connected BSCs.
Access to PSTN.
Provides the gateway functionality to other networks.
One MSC controls more than one BSC.

Types of the MSC

There are three types of the MSC, the difference just in the function.

VMSC: Visited MSC

GMSC: Gateway MSC

TMSC: Transit MSC

Visited Mobile Switching Center

Its function is to switch in the level of BSCs and it is combined with a VLR.

MSC/VLR

BSC

BSC

Transit Mobile Switching Center

Its function is to switch between the different
VMSC. It is not combined with a VLR.

TMSC

VMSC

VMSC

Gateway Mobile Switching

Center (GMSC)
Its function is to connect the PLMN to the PSTN or to the
other PLMN existing in the country.
Vodafone

PSTN

Mobinil GMSC

TMSC

VMSC

TMSC

VMSC

VMSC

VMSC

Home Location Register (HLR)

The HLR is a centralized network database that stores and
manages all mobile subscriptions belonging to a specific
operator.
It acts as a permanent store for a persons subscription
information until that subscription is cancelled.
The primary functions of the HLR include:
Stores for each mobile subscriber:
Basic subscriber categories.
Supplementary services.
Current location.
Allowed/barred services.
Authentication data.
Subscription database management
Controls the routing of mobile terminated calls and SMS.

Visitor Location Register (VLR)

The role of a VLR in a GSM network is to act as a temporary
storage location for subscription information for MSs, which
are within a particular MSC service area.
Thus, there is one VLR for each MSC service area. This
means that the MSC does not have to contact the HLR
(which may be located in another country) every time the
subscriber uses a service or changes its status.

The VLR is always integrated with the MSC.

Visitor Location Register (VLR)

For the duration when the MS is within one MSC service
area, then the VLR contains a complete copy of the
necessary subscription details, including the following
information:
Identity numbers for the subscriber
Supplementary service information (e.g. Does the
subscriber has call waiting activated or not)
Activity of MS (e.g. idle or busy)
Current Location Area of MS

Authentication Center (AUC)

To protect GSM systems, the following security functions
have been defined:
Subscriber authentication: by performing authentication,
the network ensures that no unauthorized users can
access the network, including those that are attempting to
impersonate others.
Radio information ciphering: the information sent between
the network and an MS is ciphered. An MS can only
decipher information intended for it.

Authentication Center (AUC)

The information provided by the AUC to ensure authentication
processing is called a triplet and consists of:
A non predictable random number (RAND)
A Signed Response (SRES)
A ciphering Key (Kc)

Triplets
Authentication Center
RAND

SRES

Kc

Triplet
Home Location Register
RAND

SRES

Kc

Triplet
Visitor Location Register
RAND

SRES

Kc

The Ciphering Procedure

To the BSC

From the BSC

DATA

DATA

Base Transceiver Station

Kc

Encrypted
DATA

TDMA
Frame Number

Encryption
Algorithm A5
Encrypted
DATA

Decryption
Algorithm A5

Air Interface
Mobile Station

Encryption
Algorithm A5

Kc
TDMA
Frame Number

DATA

Decryption
Algorithm A5

DATA

The Authentication Procedure

Visitor
LocationSRES
Register
Rand
AUC

SRES AUC

Kc

VLR

Access

SRES MS

Barred

Base Station Subsystem

Rand

SRES MS

SRES MS

SIM Card

Rand

A3

Ki

Rand

A8

Kc

Equipment Identification
Register(EIR)
In order to block the stolen mobiles equipments; the EIR
equipment is used; also in case of the Mobile operator
wants to block a certain type of Mobile phones.
Example, In Turkey all the mobile phones bought from
outside Turkey are blocked and can not be used before
paying fees.
The Mobile equipment is identified by a number called
International Mobile Equipment Identity (IMEI). This
number is uniquely identifies the MS worldwide.

Equipment Identification
Register(EIR)
Because the subscriber and equipment are separate in
GSM, it is necessary to have a separate authentication
process for the MS equipment.
The equipment identification procedure uses the identity
of the equipment itself (IMEI) to ensure that the MS
terminal equipment is valid.

1. IMEI Request

4. Access/ Barring Data

EIR

2. IMEI

3. IMEI Check
MSC / VLR

Equipment Identification
Register(EIR)
International Mobile Equipment Identity (IMEI)
6 Digits

2 Digits

6 Digits

TAC

FAC

SN

IMEI
TAC: Type Approval Code,
The first two digits are the
code for the country
approval
FAC: Final Assembly Code
SN: Serial Number

Short Message Center (SMC)

An SMC is added to a GSM network to provide one or more
of the following messaging services:
Short Message Service (SMS) text messages.
Any service that combined with SMS; like the missed call
notification SMS and the voice mail notification SMS.

Basic GSM Network Structure

Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications

OSS

The operation and Maintenance center (OMC) is connected to all

equipment (the GMSC, MSC, HLR, VLR, AUC, EIR and the BSC).

It can be viewed as a computerized monitoring center were staff can

monitor and control the network remotely.

MSC
OMC
LAN

HLR

BSC

SMC

OSS
In the GSM network, the implementation of the OMC is
called the operation and support sub-system (OSS).

OSS is the functional entity from which the network

operator can monitor and control the entire mobile
network.
Other functions provided by the OSS include provision of
network maintenance support as well as customer support.

Basic GSM Network Structure

Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications

Subscribers Identities in GSM

In order to identify the subscribers of the GSM system;
the standard defined identities to be assigned to the
subscribers uniquely worldwide.
MSISDN: Mobile Subscriber Integrated Service Digital Number
IMSI: International Mobile Subscriber Identity

MSRN: Mobile Subscriber Roaming number

TMSI: Temporary Mobile Subscriber Identity

GSM Identities
Mobile Station ISDN Number (MSISDN)
20

10

1100477

44

385

196099

CC

NDC

SN

CC

NDC

SN

Vodafone Egypt MSISDN

CC
NDC
SN

: Country Code
: National Destination Code
: Subscriber Number

Vodafone UK MSISDN

GSM Identities
International Mobile Subscriber Identity

(IMSI)

602

02

1234567890

234

15

1234567890

MCC

MNC

MSIN

MCC

MNC

MSIN

Vodafone Egypt IMSI

MCC
MNC
MSIN

: Mobile Country Code

: Mobile Network Code
: Mobile Station Identification Number

Vodafone UK IMSI

GSM Identities
Temporary Mobile Subscriber Identity Number

(TMSI)

The TMSI is allocated to the mobile subscriber in order to be

used instead of his IMSI during all radio communications.
The purpose is to keep subscriber information confidential
on the air interface. So that this make the call is so difficult
to be traced.
The VLR is the device responsible for the allocation of the
TMSI for the mobile subscriber.
This TMSI is changed at certain events or after a certain
time.
The TMSI consists of 4 bytes and determined by the telecom
operator.

GSM Identities
Mobile Station Roaming Number

(MSRN)

When a mobile terminating call is to be set-up, the HLR

of the called subscriber requests the MSC/VLR to allocate
an MSRN to the called subscriber.
This MSRN is returned via the HLR to the MSC where the
calling subscriber exists.
The routing is done using the MSRN. When the routing is
completed, the MSRN is released so it can be used in the
routing of another call.
The MSRN is in the same format like that of the MSISDN.

GSM Identities
On the SIM card the following data is included:
IMSI
TMSI
KI
KC
A3 Algorithm
A8 Algorithm

Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning

Communication Channels types

Channel Type

Properties

Applications

Simplex

One-way only

FM radio, television

Half duplex

Two-way, only one at a time

Police radio, push-to-talk

Full duplex

Two-way, both at the same

time

PSTN, Mobile systems

A duplex channel, such as that used during a mobile call, uses two
frequencies: one to the MS and one from the MS. The direction from the
MS to the network is referred to as uplink. The direction from the network
to the MS is referred to as downlink.
Because it requires less power to transmit at lower frequency over a
given distance, uplink frequencies in mobile systems are always the lower
band of frequencies this saves valuable battery power of the Mobile
station.

Frequency Allocation
GSM

900

GSM 1800

GSM 1900

Uplink

890-915 MHz

1710-1785 MHz

1850-1910 MHz

Downlink

935-960MHz

1805-1880 MHz

1930-1990MHz

Bandwidth

25 MHz

75 MHz

60 MHz

Duplex Distance

45MHz

95 MHz

80 MHz

Carrier Separation

200 KHz

200 KHz

200KHz

Radio Channels

124

374

299

Frequency

FDMA in GSM

Separation between carriers Frequency gap must be sufficient to eliminate

interference between adjacent channels.

Where The more the separation the less the co-channel interference but the
less the available channels suited in the bandwidth.

It is found that a 200 kHz channel separation is suitable for all systems.

Spectrum Allocation (GSM 900)

Downlink 935 960 MHz

Uplink 890 915 MHz

200 KHz
890.2

890

890.6
3

935

121
121 122 123 124

915

890.4

935.2
1

Uplink

935.6
3

F (MHz)

Downlink
121 122 123 124
121

960

935.4

GSM 900 Frequency Allocation

F (MHz)

TDMA in GSM

With TDMA, one carrier is used to carry a number of calls, each call using that
carrier at designated periods in time .

These periods of time are referred to as time slots .

Each MS on a call is assigned one time slot on the uplink frequency and one
on the downlink frequency, and both the same.

It is found that a 8 Time Slots per carrier, called physical channels is suitable
for all systems.

Information sent during one time slot is called a burst, and depending on
information sent we named what called logical Channels

Physical Channels
GSM band is divided into 124 RF channels, and each channel is
divided into 8 time slots using TDMA.
These time slots are called physical channels.
CH 124

CH 3

CH 2

CH 1

Logical Channels-Traffic Channels

TCH
Traffic Channels
Normal Burst

Speech
TCH/FS

TCH/HS

TCH Traffic Channel

TCH/FS Full rate Speech Channel
TCH/HS Half rate Speech Channel
TCH/9.6 Data Channel 9.6kb/s
TCH/4.8 Data Channel 4.8kb/s
TCH/2.4 Data Channel 2.4Kb/s

Data

TCH/9.6

TCH/2.4

TCH/4.8

Traffic Channels
Carries either encoded speech or user data up and down link between a
single mobile and a single BTS.

Types of traffic channel:

Full rate (TCH)
Transmits full rate speech (13 Kbits/s). A full rate TCH occupies one
physical channel.
Half rate (TCH/2)
Transmits half rate speech (6.5 Kbits/s).
Two half rate TCHs can share one physical channel, thus doubling the
capacity of a cell.

Logical Channels-Control Channels

CCH Control Channels
DCCH
SDCCH

FACCH

BCH
ACCH

BCCH
SACCH

Synch. CH.

CCCH
SCH
RACH

CBCH
PCH/AGCH

FCCH

Control Channels

These are used to carry signaling or synchronization data. They are

divided into three types:-

Broadcast CHannels (BCH)

Common Control CHannels (CCCH)
Dedicated Control CHannels (DCCH)

Broadcast Channels
From Single BTS to all the mobiles in the area

Frequency Correction Control CHannel (FCCH)

Carries information for frequency correction of the mobile

Synchronization CHannel (SCH)

Carries 2 important pieces of information
TDMA frame number (max = 2715684 )
Base station identity Code (BSIC)

Broadcast Control CHannel (BCCH)

Broadcasts some general cell information such as:
Location Area Identity (LAI),
maximum output power allowed in the cell
and the identity of BCCH carriers for neighboring cells.

Common Control Channels

To or from a certain BTS to a single mobile
Paging Channel (PCH)
BTS Transmits a paging message to indicate an incoming call or short message.
The paging message contains the identity number of the mobile subscriber that
the network wishes to contact.

Random Access Channel (RACH)

MS Answers paging message on the RACH by requesting a signaling channel of
SDCCH.

Access Grant Channel (AGCH)

Assigns a signaling channel (SDCCH) to the MS.

Dedicated Control Channels

Stand alone Dedicated Control Channel (SDCCH)
The BTS switches to the assigned SDCCH. The call set-up procedure is performed in
idle mode. The BSC assigns a TCH (carrier and time slot) and the MS switches to
the assigned SDCCH.
SDCCH is also used to Registration & Authentication

Slow Associated Control Channel (SACCH)

BTS Instructs the MS the transmitting power to use and gives instructions on
timing advance (TA).
MS Sends averaged measurements on its own BTS (signal strength and quality) and
neighboring BTSs (signal strength). The MS continues to use SACCH for this
purpose during a call.

Fast Associated Control Channel (FACCH)

Transmits handover information.
Transmits necessary handover information

Burst Structure Types

Normal Burst Structure
TDMA Frame

Tail
Bits

Training
Sequenc
e

Encrypted
Bits

Encrypted
Bits

Tail
Bit
s

156.25 bits in 577 u Sec

57

26

57

Guar
d
Perio
d
8.25

The tail bits help the equalizer to determine the start and stop points of the
transmitted bits. They are three bits at the beginning and at the end of the burst
and they are always zeros

Burst Structure Types

Normal Burst Structure
TDMA Frame

Tail
Bits

Training
Sequenc
e

Encrypted
Bits

Encrypted
Bits

Tail
Bit
s

Guar
d
Perio
d

156.25 bits in 577 u Sec

57

26

57

8.25

The subscriber speech or data is encrypted into 57 bit blocks. Each burst will
contain two 57 bits blocks from two different speech segments(20m Sec).

Burst Structure Types

Normal Burst Structure
TDMA Frame

Tail
Bits

Training
Sequenc
e

Encrypted
Bits

Encrypted
Bits

Tail
Bit
s

Guar
d
Perio
d

156.25 bits in 577 u Sec

57

26

57

One bit stealing flag will be added to each block to

indicate whether the burst is stolen for the FACCH
signaling or used as a normal traffic channel

8.25

Burst Structure Types

Normal Burst Structure
TDMA Frame

Tail
Bits

Training
Sequenc
e

Encrypted
Bits

Encrypted
Bits

Tail
Bit
s

Guar
d
Perio
d

156.25 bits in 577 u Sec

57

26

57

8.25

The guard period of 8.25 bits length, which is equivalent to about 30 s,

is left at the end of each burst, to prevent overlapping between
consecutive bursts and to facilitate burst synchronization

Burst Structure Types

Normal Burst Structure
TDMA Frame

Tail
Bits

Training
Sequenc
e

Encrypted
Bits

Encrypted
Bits

Tail
Bit
s

Guar
d
Perio
d

156.25 bits in 577 u Sec

57

26

57

8.25

It carries information of all logical channels except RACH, SCH and FCCH

Burst Structure Types

Frequency Correction Burst Structure
TDMA Frame

Tail
Bits

Fixed Bits

Tail
Bit
s

156.25 bits in 577 u Sec

142

Guar
d
Perio
d
8.25

This is the one used by the channel (FCH) for frequency correction of the mobile.
It consists of a long sequence of bits called the fixed bits which are all equal to
zeros, leading to a constant frequency output from the GMSK modulator

Burst Structure Types

Synchronization Burst Structure

TDMA Frame

Tail
Bits

Encrypted
Bits

Synchronization
Sequence

Encrypted
Bits

Tail
Bit
s

Guar
d
Perio
d

8.25

156.25 bits in 577 u Sec

39

64

39

The SCH burst consists of a long synchronization sequence along with the important
information being encrypted and divided into two blocks. The TDMA frame number is
sent on the SCH channel, which carries also the Base station Identity code (BSIC).
The TDMA frame number is used by the mobile to determine which control channels
will be transmitted on that frame.

Burst Structure Types

Access Burst Structure
TDMA Frame

Tail
Bits

Synchronization
Sequence

Encrypte
d Bits

Tail
Bits

41

36

Guard Period
68.25

156.25 bits in 577 u Sec

The Access Burst is used by the RACH channel. The mobile sends this burst when
it does not know the distance to its serving BTS, which is the case when the
mobile is switched on or after it makes a handover to a new cell. So this burst
must be shorter in order to prevent it from overlapping with the burst on the next
time slot.

Burst Structure Types

Dummy Burst Structure
0

Tail
Bits

Encrypted
Bits

TDMA Frame

Training
Sequenc
e

Encrypted
Bits

Tail
Bit
s

Guar
d
Perio
d

8.25

156.25 bits in 577 u Sec

58

26

58

The dummy burst is sent from the BTS when there is nothing else to be sent. It
carries no information and it has the same structure of a normal burst with the
encrypted bits replaced by a known bit pattern to the mobile.

1 Hyper frame = 2048 Super frames = 2,715,648 TDMA Frames = 3hrs 28 min and 53.76 s
Hyper Frame
2

e.g. TCH

e.g. BCCH

e.g. TCH

26

57

26

1 Super frame = 51 TCH Multi frames

1 Super frame = 26 BCCH Multi frames

1 BCCH Multi frame = 51 TDMA Frames

1 TCH Multi frame = 26 TDMA Frames

51

26

57

Normal burst

156.25 bits in 577 m Sec

e.g. BCCH

8.25

51

Power Measurements in Active Mode

1.

2.
3.

To enable the mobile from making power measurements during a call, the
uplink time slot will be delayed by an offset of three time slots from the down
link time slot. (The mobile will try to measure the signal strength of these
carriers one by one during the time between transmission and reception of the
allocated traffic channel)
The mobile is informed on the SACCH channel which BCCH frequencies to be
measured.
To make sure that the measured carriers do not belong to co-channel cells, the
mobile will have to check the identity of the adjacent cells by reading the BSIC
value sent on the SCH of each cell. This will take place during the idle frame
number 26.

4.

(Note) The signal strength of the serving cell is measured during reception of the
allocated traffic channel.

The mobile will make a list of the strongest six carriers and their BSIC values
along with the signal strength of its cell, and reports this list to the BSC via the
uplink SACCH channel which is repeated once every 26 frame.

Radio Transmission problems

As it was stated before that the mobile telecommunications will use radio
transmission as the transmission technique; the radio transmission is suffering
from many problems which causing unacceptable degradation of the service
quality.
We will discuss these problems in details during our course.

Radio Transmission problems

1.

Path Loss

2.

Shadowing (Normal Fading)

3.

Multipath Fading

a. Rayleigh Fading
b. Time Dispersion
4.

Time Alignment

Radio Transmission problems

1. Path Loss
Cause

Due to Increasing distance between MS, and BTS.

Solution
Handover

Radio Transmission problems

2. Shadowing (Normal Fading)

Cause

Due to Obstacles between MS, and BTS.

Solution

increase the fading margin for the receiver in the mobile

station and at the Transceiver station of the mobile network; so that how
weak the signal received is the receiver will still can receive and regenerate
this signal.

Radio Transmission problems

3. Multipath Fading
a. Rayleigh Fading
Cause

Due to different paths of signals between MS, and BTS.

Which cause fading dips as a result of different in phase and amplitude
Solution
Diversity

a. Space Diversity
b. Polarization Diversity
c. Frequency Diversity ( Frequency Hopping)

Radio Transmission problems

3. Multipath Fading
b. Time Dispersion
Cause

Due to different paths of signals between MS, and BTS.

Which cause Intersymbol Interference (ISI)
Solution
Adaptive Equalizer, by using training sequence in bursts

Radio Transmission problems

4. Time Alignment

Cause
Due to different distance of different MSs from BTS (Near-Far)
Solution

Time Advance

Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning

Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS

Location Update
Why do we need to update our location data ?
Actually, the location update process is done in aim to exactly identify
the location of the subscribers within the network so that any incoming
call goes directly to the called subscriber.
To fulfill this aim, one can say that we may update the system with the cell ID each
time the subscriber changes his serving cell. The MSC/VLR will now know the exact
cell you are roaming in. This will result in a huge amount of location update
messages.

An extreme is never to make a location update and to be paged in all the network.
This will cause huge amount of paging messages.

Location Update

Types of Location Update

1. Normal Location update within same MSC/VLR service area
2. Normal Location update between 2 different MSC/VLR service areas

3. IMSI attach/detach
4. Periodic Location Update

Normal Location within the same

MSC/VLR Service area
4
MSC/VLR

Updates
LA Record

1. The Mobile sends an SDCCH allocation

request message to the BTS on RACCH
2. The BTS responds with the SDCCH
allocation message on AGCH .

BSC

3. The mobile sends a location update

request message with its IMSI to the
MSC/VLR on SDCCH
4. The MSC/VLR updates the location
information and sends a Location Update
confirmation message on SDCCH
5. SDCCH is released

Normal Location Update between 2

different MSC/VLR service areas
1. The mobile sends a location
update request to the MSC.

VLR Address
=
New
Old MSC
VLR

HLR

IMSI to MGT
translation

2. The new MSC/VLR receives the IMSI

and conclude the its HLR address.
3. The MSC/VLR sends a subscriber
information request with the IMSI
to the proper HLR

Old MSC/VLR

New MSC/VLR

Old BSC

NEW BSC

4. The HLR stores the address of

the new MSC/VLR

5. The HLR sends the data to the

new MSC/VLR and it is kept there
6. The HLR sends a location
cancellation message to the old
MSC/VLR to remove the data

7. The new MSC/VLR sends a location

updating confirmation message to
the mobile

LA 2
LA 1

IMSI Detach
1. At power off, the MS asks for a signaling SDCCH channel.

2. The MS uses this signaling channel to send the IMSI detach message to the
MSC/VLR.
3. In the VLR, an IMSI detach flag is set for the subscriber. This is used to reject
incoming calls to the MS.
4. The subscribers record is kept in the VLR for a certain time; i.e. 24 hour then
it is removed if the subscriber doesnt switch on his mobile.

IMSI Attach
IMSI attach is a complement to the IMSI detach procedure. It is used by the
mobile subscriber to inform the network that it has re-entered an active state
and is still in the same location area. If the MS changes location area while
being switched off, a normal location update takes place.
1. The MS requests a signaling SDCCH channel.
2. The MSC/VLR receives the IMSI attach message from the MS.
3. The MSC/VLR sets the IMSI attach in the VLR. The mobile is now ready
for normal call handling.

4. The VLR returns an acknowledgment to the MS.

IMSI Attach

BSC

MSC/VLR
3

Periodic Location Update

Periodic location update is a routine task performed by the network if the

MS doesnt make any network action (sets a call, sends SMS, location
update, receives a call,. etc)
If the MS doesnt respond to this periodic location update, it will be marked
as implicitly detached. ( Temporarily out of service )

Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS

Handover

Handover is to keep continuity of the call when the subscriber is

roaming along the network moving from one cell to another and
moving between different nodes in the network.

During call, the MS is continuously measuring transmission quality

of neighboring cells and reports this results to the BSC through the
BTS.

The BSC, being responsible on supervising the cells, is responsible

of handover initiation.

Good neighbor relations between cells is an important factor in

keeping the network performance in the accepted level.

Types of Handover
1. Intra BSC Handover:
When the cell to which the call will be handed over belongs to the same BSC of the
serving cell.

2. Inter BSC / Intra MSC Handover:

When the cell to which the call will be handed over belongs to the different
BSCs but to the same serving MSC.

3. Inter MSC
When the cell to which the call will be handed over belongs to the different BSC
and different MSC.

1.

BSC
7

8
2

The BSC decides from the power measurement

reports that the call must be handed over to
another cell
2. The BSC checks for an vacant traffic TCH in the
new cell and orders this cell to activate the TCH

3. The BSC orders the serving cell to send

a message to the MS on FACCH telling the
information
of new TCH

4. The MS tunes to the new frequency and

3

Sends handover access burst

New Cell

6
5

5. The new cell detects the handover burst

and sends information about the suitable
timing advance to the MS

6. The MS sends a HO complete message to the new cell

7. The new cell sends a message to the BSC that
the handover is successful

8. The BSC orders the old Cell to release the TCH

MSC/VLR

Old BSC

New BSC

Old MSC

Old BSC

New MSC

New BSC

Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS

MSC

BSC

1. The MS uses RACH to ask for a signaling channel.

2. The BSC/TRC allocates a signaling channel, using
AGCH.
3. The MS sends a call set-up request via SDCCH to the
MSC/VLR. Over SDCCH all signaling preceding a
call takes place. This includes:

Marking the MS as active in the VLR

The authentication procedure

Start ciphering

Equipment identification

Sending the B-subscribers number to the network

Checking if the subscriber has the service Barring

of outgoing calls activated
4. The MSC/VLR instructs the BSC/TRC to allocate an
idle TCH. The BTS and MS are told to tune to the
TCH.
5. The MSC/VLR forwards the Bnumber to an exchange
in the PSTN, which establishes a connection to the
subscriber.
6. If the B-subscriber answers, the connection is
established.

Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS

1. Calling a GSM
subscribers
2. Forwarding call to
GMSC
3. Signal Setup to HLR
4. 5. Request MSRN from
VLR
6. Forward responsible
MSC to GMSC
7. Forward Call to current
MSC
8. 9. Get current status of
MS
10.11. Paging of MS
12.13. MS answers
14.15. Security checks
16.17. Set up connection

Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS

Short Messages

The Short Message Service (SMS) allows a mobile subscriber to send and
receive text messages composed of 160 characters at most.
These messages can be read on the phones LCD panel and they are delivered
using the signaling channel SACCH; so short messages can be received while a
call is in progress.
The short messages sent or received are handled by the Short Message Service
Center (SMSC), which consists of three parts:
Service Center
SC
Short Message Service GMSC
SMS-GMSC
Short Message Service Inter-working MSC
SMS-IWMSC

Short Message Delivery

Process
SC
4

SMS-IWMSC

SMS-GMSC

6
7
MSC1

MO

HLR

MSC2

MT

MT

Unsuccessful delivery
SC

SMS-IWMSC

SMS-GMSC

5
MSC2
3

4
HLR
6

Retry method
Usage of Alert Service Center in the HLR

MT

Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning

GSM Coverage Plan

To provide coverage for a large service area of a mobile network

we have two Options:

(A) Install one transceiver with high radio power at the center of
the service area
Drawbacks:
The mobile equipments used in this network should have
high output power in order to be able to transmit signals
across the coverage area.
The usage of the radio resources would be limited.

GSM Coverage Plan

(B) Divide the service area into smaller areas (cells)
Advantages:
Each cell as well as the mobile handsets will have
relatively small power transceivers.
The frequency spectrum might be reused in two far
separated cells. This yields:
1- Unlimited capacity of the system.

2- Good interference characteristics

Cell Geometry
Problem of omni directional antennas

Dead Spots

Cell Geometrical Shape

Differentiation between these three shapes will be in order to
optimize the number of cells required to cover a given service area
against the cell transceiver power. By some calculations, you will find
that using hexagonal shaped cells achieves the optimum.

Cell Geometrical Shape

Umbrella Cell

Normal Cell

Normal Cell

Cell Geometrical Shape

Umbrella cell

Fast moving
subscribers

Pico
cell

Macro cell

Slow moving
subscribers

In building
coverage

Clusters
Cluster is a set of cells where the
frequency is not being reused
within this cluster.
Cluster can be 3, 4, 7 and 9 cells.

Sectorization

Omni-Directional Cell

sectroized Cells

3/9 Cluster
A3

B3
A2

A1
A3

B3
A2

A1

C1
A3

A2

B3
A2

A1

C3

B1

C1
A3

C2
C1

A2
A1

C3
C2
C1

C3
C2
C1

B2
B1

C3
B2

B1

B2
B1

A2

B3

A1

B3

A1

A2

B3

C1

C2

A3

C2

A3

C1

C2

C3

B1

B2

B2
B1

B2

C3

B1

B2

C3

A1
B3

A1

B3
A2

C2

A3

C2

A1
A3

C1

B3

A2

B1

B2

C3

B2

C3

B1

A3

C2
C1

4/12 Cluster
A3

B3
A2

A1

A3
B2

B1
C3

A1

C2
A3

B3
A2

A1

A3
B2

B1
C3

C1
B3

A1

A3
B2

B1
C3

A2
B1

C1

D2
D1

B2
B1

C3

D3
C2

D2
D1

D1

A2

D3

D2

B3

A1

C2

C1

C2

A3

B1

D3

C1

B2

C3

D3

C3

D1

A2

B2
B1

D2

B3

A1

C2

D2
D1

A3

B2

C3

D3

C1

D1
B3

A1

C2
C1

C1

D1

A2

D3

D2

B3

A1

C2

D2

C2

A3

B1

D3

C1

B2

C3

D3

C3

D1

A2

B2
B1

D2

B3

A1

C2

D2
D1

A2

B1

A1

C2

A3

B3

A2

D3

C1

B2

C3

D3
C2

A3

A2

B2

C3

D1

A3

B1

D2

B3

A1

A2

D3

C1

B3

C1

D2
D1

A3

E3
A2

A1
B3

E2
E1

D3
B2

B1

7/21 Cluster

D2
D1

C3

B3

A3
A2
A1

B1

D1

F1

C2
C1

A1

B2

G2
G1

B1

D2

C3

F2
F1

G3
C2

C1

C1

G2
G1

F2
F1

G3
C2

F3

D1

D2

C3

E1

F3

D1

E2

D3

E1

B2

E3

E2

D3

B1

A2

B3

A1

G2

A3

E3

A2

B3

G1

F2

G3

F1

C2

F3

A3
F2

G3

C1

D2

C3

D1

E2

D3

F3
D2

C3

E1

B2

D3

B1
E3

E2
E1

B2

G2
G1

B3

A1

F1

C2

E3

A2

F2

G3

C1

A3

F3

G2
G1

Frequency Reuse
If the GSM900 system has 124 Absolute Radio Frequency Traffic
Channels, and if we are using only in our network 60 of them, then
we can only serve 8 x 60 = 480 Calls if we only use the frequency
once.
However, a cellular network overcome this constraint and
maximizes the number of subscribers that it can serve by using
frequency re-use.

The frequency reuse is performed by dividing the whole available

frequencies between a group of neighboring cells which is called
frequency reuse pattern or a Cluster, and then repeat this cluster
over the whole network

Co- Channel Interference

Co-channel interference is caused by short distance between the cell
and other cell that use the same frequency.

To overcome this type of interference. Each frequency is reused after

the same distance D
Reuse Plan = (D/R)2 = 3N.
cluster

Where N is the number of cells per

Adjacent Channel Interference

Adjacent frequencies, that are frequencies shifted 200kHz from the
carrier frequency, must be avoided in the same cell and preferably in
neighboring cells also .

To overcome this type we must make good planning for the frequencies
in the cluster

Frequency Planning
A3

B3
A2

A1

A3

B1

B3
A2

A1

C3
B2

C1
B3

A3
A2

C2

C1

A3
C2

B1
C3

B2

A1

B3

A2
A1

B2
B1

C3
B2

B1

C2
C1

C3
C2
C1

Frequency
group
Channels

A1

B1

C1

A2

B2

C2

A3

B3

C3

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

84

85

86

87

Frequency Planning

In a real network the allocation of channels to cells will not be

as uniform as in table, as some cells will require more channels
and some will require less.
In this case, a channel may be taken from a cell with low
traffic load and moved to one with a higher traffic load.
However, if doing so, it is important to ensure that
interference is still minimized.

Which Cluster Size to use?

Carrier to interference ratio

Its the difference in power level between the carrier in a given cell and the same
carrier received from the nearest cell that reuses the same frequency.

Number of frequencies
per site

Traffic Channels

3/9

High

High

Low

4/12

Medium

Medium

Medium

7/21

Low

Low

High

C/I Ratio

END