LAB MANUAL # 10
STUDY A GSM MOBILE COMMUNICATION NETWORK
STRUCTURE
Cellular Generations
First Generation - 1G
• Advanced Mobile Phone Service (AMPS)
– US trials 1978; deployed in Japan (’79) & US (’83) in the 800 MHz band
– Still widely used in many parts of the world including Pakistan
• Nordic Mobile Telephony (NMT)
– Sweden, Norway, Demark & Finland
– Launched 1981; now largely retired
– 450 MHz; later at 900 MHz (NMT900)
• Total Access Communications System (TACS)
– British design; similar to AMPS; deployed 1985
– Some TACS-900 systems still in use in Europe
Second Generation – 2G
– Digital systems
– Increased capacity
– Speech compression; digital signal processing
– Utilize and extend “Intelligent Network” concepts
– Improve fraud prevention
– Add new services
• There is a wide diversity of 2G systems
– IS-136 North American TDMA; PDC (Japan)
– PHS
– IS-95 CDMA (CDMA One)
– GSM
– At present, GSM is the most widely used cellular technology.
Cellular System Definition
In a cellular system, the covering area of an operator is divided into cells. A cell
corresponds to the covering area of one transmitter or a small collection of transmitters.
The size of a cell is determined by the transmitter's power.
The concept of cellular systems is the use of low power transmitters in order to enable the
efficient reuse of the frequencies. In fact, if the transmitters used are very powerful, the
frequencies cannot be reused for hundreds of kilometers as they are limited to the covering
area of the transmitter.
The Cell
• Cellular Radio involves dividing a large service area into regions called “cells.”
• Each cell has the equipment to switch, transmit and receive calls.
• Cells - Reduce the need of High powered transmission
• Cells - Conventionally regarded as being hexagonal, but in reality they are irregularly
shaped.
• Cell shape is determined by the nature of the surrounding area e.g. Hills, tall building etc.
The hexagonal-shaped communication cells are artificial & are generated to simplify the planning
& design of a cellular network.
Need For Cellular structure:
Higher capacity & higher number of users.
Less transmission power needed.
Base station deals with interference, Transmission area etc.
Why Hexagonal Shaped Cells?
Fictitiou Idea Rea
The hexagonal shaped cells are artificial and cannot be generated in the real world. Engineers
draw these shaped cells because they approach the circular shape but get rid of the ambiguity and
unclearity produced from the overlap between the cells. Hexagonal shaped cells fit in a planned
area nicely and have no gaps or overlaps between them.
Frequency reuse concept
The frequency band allocated to a cellular mobile radio system is distributed over a group
of cells and this distribution is repeated in all the covering area of an operator
Frequencies used in a cell will be reused several cells away. The distance between the
cells using the same frequency must be sufficient to avoid interference. The frequency
reuse will increase considerably the capacity in number of users.
Cluster of cells
Adjacent cells that do not share frequencies is called a cluster
1-9 10-18
Mhz
19- 28-
27 36
6-cell cluster 4-cell cluster
The cells are grouped into clusters. The number of cells in a cluster must be determined so
that the cluster can be repeated continuously within the covering area of an operator.
The typical clusters contain 4, 7, 12 or 21 cells.
The number of cells in each cluster is very important. The smaller the number of cells per
cluster is, the bigger the number of channels per cell will be. The capacity of each cell will
be therefore increased.
The concept can be better understood by the previous example:
GSM Antennas
• Directional Antennas
• Vertically Polarized.
• Collinear Dipole Array with 8 to 12 elements.
• Beam Width 45o, 60o, 90o.
• High Gain Antennas with gain of 16 to 18 dbi.
• Mechanically/Electrically Downtiltable.
Features of GSM
• Compatibility
• Noise Robust
• Increased Capacity & Flexibility
• Use of Standard Open Interfaces
• Improved Security & Confidentiality
• Cleaner Handovers
• Subscriber Identification
• ISDN Compatibility
• Enhanced Range of Services
Noise Robust
• To combat the problems due to Noise Digital Interface is used.
• Digital Interface
• Analogue Air Interface
Standardized Open Interfaces
• Low Price
• Flexibility
Better Security & Confidentiality
• High Security risk for Analogue System operators.
• No Eavesdropping, High speech and data confidentiality, Digitized, Encoded and
Encrypted (A8 algorithm), Subscriber Authentication (A3 algorithm).
Cleaner Handovers
• The mobile measures up to 32 adjacent cells for
– Signal Strength (Rx Level)
– Signal Quality (Rx Quall)
– updated every 480mS and sends to BTS
• Sophisticated Handover based on
– Rx Level
– Interference
– Rx Quall
– Timing Advance
– Power Budget
Handovers
Base Station 2
Base Station 1
Base Station 3
ISDN Compatibility
• ISDN (Integrated Services Digital Network)
• 2B+D Signaling and information on ISDN line.
• The GSM Network is designed to operate within the ISDN System.
• GSM provides features compatible with ISDN.
Architecture of the GSM network
GSM Network Components
• Mobile Station consists of two parts
ME
• Hardware e.g. Telephone, Fax Machine, Computer.
SIM
• Smart Card which plugs into the ME.
Figure ( GSM Architecture)
MS
The MS is the equipment required to use the services provided by the GSM network
From a portability viewpoint the MS is classified in
A. Vehicle mounted station
B. Portable station
C. Hand-held station
All MS’s must be able to vary their emission power with a command driven by the BTS
From a functional viewpoint each MS can be identified as a whole of the
o Mobile Equipment (ME) or Mobile Termination (MT)
o Terminal Equipment (TE)
o Terminal Adapter (TA)
o Subscriber Identity Module (SIM)
SIM
It is basically
o a removable smart card in compliance with the ISO 7816 standard
o a plug-in module (25 x 15 mm)
It includes a microprocessor with all the subscriber-related information
The interface between the SIM and the other components of the ME (SIM-ME interface)
is fully defined in the Technical Specifications
SIM (and consequently MS) is protected by a Personal Identification Number (PIN)
It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card
Serial number
International Mobile Subscriber Identity (IMSI)
Security authentication and cyphering information
LAI (Location area identity)
TMSI ( Temporary Mobile Subscriber identity )
Temporary Network information (LAI, TMSI)
List of services subscribed by the user
Personal Identity Number (PIN)
Personal Unblocking Number (PUK)
MS-ISDN (Mobile Station International services digital network Number)
Access rights
Prohibited networks
Call messages
Phone numbers
Base Station System (BSS)
The BSS comprises:
Base Station Controller (BSC)
One or more Base Transceiver Stations (BTSs)
BTS contains:
Radio Transmitter/Receiver (TRX)
Signal processing and control equipment
Antennas and feeder cables
The BSC:
allocates a channel for the duration of a call
maintains the call:
controls the power transmitted by the BTS or MS
generates a handover to another cell when required
• BSC
– Controls up to 40 BTS
– Conveys information to/from BTS
– Connects terrestrial circuits & Air Interface Channels
– Controls handovers between BTSs under itself
• BTS
– Contains RF Hardware
– Limited control functionality
– 1 - 6 carriers in a BTS Cabinet
– 7 - 48 simultaneous calls per BTS
BTS
BTS is a network element with
transmission and reception devices
(transceivers) to and from the MS,
including
antennas
signal processing specific devices for the
Air interface management
It can be considered as a complex radio
modem controlled by the BSC
It is involved also in the transmission and
reception with the BSC through the A-bis
interface
It has just executive functions (no management)
Network Switching System (NSS)
• NSS (Network Switching System)
– MSC (Mobile Switching Centre)
– HLR (Home Location Register)
– VLR (Visitor Location Register)
– EIR (Equipment Identity Register)
– AUC (Authentication Centre)
– IWF (Interworking Function)
– EC (Echo Canceller)
• MSC
– Call Switching
– Operation & Management Support
– Internetwork Interworking
– Collects call billing data
• Gateway MSC
– MSC which provides interface between PSTN & BSS’s in the GSM Network.
Home Location Register (HLR)
• Reference database for the Subscriber profiles-
– Subscriber ID (IMSI & MSISDN)
– Current VLR Address
– Supplementary Services subscribed
– Supplementary Service Information
– Subscriber Status (Registered/deregistered)
– Authentication Key and AUC functionality
– TMSI
– MSRN
The HLR data base contains the master database of GSM
Whenever a new subscriber is joins, the information is saved in the HLR
It has one record for each user
If there are more than one HLR’s in the system then each will have a portion of the total
database
Visitor Location Register (VLR)
• Temporary Data, which exists as long as the subscriber is active in a particular Coverage
area.
• Contains the following-
– Mobile Status (Busy/ Free/ No Answer/etc.)
– Location Area Identity (LAI)
– TMSI
– MSRN (Mobile Station Roaming Number)
When a subscriber enters a new MSC,
the VLR associated to this MSC will request information about the new subscriber to its
corresponding HLR.
It copies user data temporarily from HLR when user is roaming in its region
The VLR is always implemented
together with a MSC
Equipment Identity Register (EIR)
• Contains Database for validating IMEI
– White List (valid ME)
– Black List (Stolen ME)
– Grey List (Faulty ME)
Operation & Maintenance Centre
• Event & Alarm Management
• Fault Management
• Performance Management
• Configuration Management
• Security Management
o The OMC has access to the (G) MSC, BSC.
o Handles error messages being reported from the Network
o Controls the traffic load of the BSC, and the BTS.
NETWORK MANAGEMENT CENTRE
o NETWORK MANAGEMENT CENTRE (NMC)
– Offers Hierarchical Regionalized Network Management of a complete GSM
system.
NETWORK MANAGEMENT CENTRE
NMC
OMC OMC
OMC
REGION 3
REGION 2
REGION 1
NETWORK
Basic Processes
• AUTHENTICATION
• CIPHERING
• REGISTRATION
• CALL ESTABLISHMENT
• HANDOVER / HANDOFF
MSC
The MSC main scope consists in performing switching functions
It co-ordinates the setting-up of the call to and from the GSM users located in the area of
its competence
It controls more BSCs
MSC has interfaces with BSS on one side and with the external networks on the other side
o the interface with external networks requires a gateway (GMSC) for adaptation
GMSC
The Gateway MSC is able to route calls coming from
o MSCs of other PLMN
o PSTN and ISDN switching exchanges
VLR
VLR is charge of temporarily storing subscription data for those MSs currently present
within its coverage area
o International Mobile Subscriber Identity (IMSI)
o Mobile Subscriber ISDN (MSISDN)
o supplementary services subscribed
o authentication and ciphering parameters
o Location Area Identity (LAI)
VLR keeps location registrations and updates as long as subscriber is within its coverage
area
It is always associated with one or more MSCs
HLR
It stores the static subscriber information relevant to the provision of the
telecommunication services
o independently of the current location of the MS
These data are permanently stored
The only temporary data regards the dynamic data, variable in real time
o LAC identifying the LA where is currently the MS
o parameters of the new subscribed supplementary services
It is able to handle roughly a hundred thousand subscribers’ data
Mobile Station Roaming Number
The MSRN format is the same as MSISDN, but it is temporary
MSRN = CC + NDC + SN
o CC = Country Code
o NDC = National Destination Code
o SN = Subscriber Number
SN points to a database
MSRN includes sufficient information to enable the GMSC to route the call to the target
MSC
AUC
It is the GSM functional unit managing the authentication and ciphering procedures of the
information broadcasted through the radio channel
It creates for each subscriber the required triplet for the ciphering
o Random number (RAND)
o Signed Response (SRES)
o ciphering key Kc
AUC stores the authentication key Ki (32 hexadecimal digits) protecting with an
encryption algorithm
EIR
The Equipment Identification Register main goal consists in storing the International
Mobile Equipment Identity (IMEI)
EIR is a database installed in the NSS allowing at the GSM network to verify the
authorisation of the active MEs
o White list
o Grey list
o Black list
Transmission Media
• Access Network
• Backbone Network
Handovers
Occurs when signal strength of the serving cell gets greater then neighboring cell.
Handovers occurs to improve the quality of voice.
The relevant data is fed back to the system
Figure : GSM Call Establishment
Comments and Discussion
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
__________________________________________________________________________
