GSM Fundamentals provides an overview of GSM bands, the general architecture of a GSM network including key components like the MS, BSS, NSS, HLR, VLR, EIR, AUC, and IWF. It also describes identification numbers used in GSM like IMSI, MSISDN, IMEI, and TMSI as well as logical channels and radio technology aspects.

1. GSM Fundamentals
Prepared By
Asem Mohammed Shams

2. GSM BANDS OVERVIEW
In GSM we have 3 bands used Here in the following table you will find the difference
between all of them
GSM 900
Frequency band
Up Link
Down link
Separation
Number of CH
Channel width
Cluster size
25 MHZ
890 – 915 MHZ
935 – 960 MHZ
45 MHZ
124
200 kHz
7 cells
GSM 1800
75 MHZ
1710 – 1785 MHZ
1805 – 1880 MHZ
95 MHZ
374
200 kHz
4 cells
GSM 1900
60 MHZ
1850 – 1910 MHZ
1930 – 1990 MHZ
80 MHZ
299
200 kHz
4 cells
Note: GSM in the 1800 band is often called Digital Cellular System (DCS), and GSM
in the1900 band used to be called PCS 1900. GSM is now more simply referred to
by the term “GSM”and the band. i.e.: GSM 450, GSM 900, GSM1800, and GSM
1900
Table – 1

3. General architecture of a GSM
network

4. MS:Mobile Station
It consist of 2 parts:
• ME:Mobile Equipment
Which contain (IMEI)
• SIM:Subscriber identification module
Which contain (IMSI,Ki,Kc)

5. BSS:Base Station Subsystem
• BSS is responsible for the system functions related to
radio and transmission of the GSM system
• It contains (BTS,BSC,TC)
• BTS:Base transceiver Station which is the first stage for
communication with MS and contains RF equipment for
transmit and receive to and from MS
• BSC: Base Station Controller which control a group of
BTS
• TC:Transcoder which is responsible for rate conversion
to save our BW resources (64Kbps to 16Kbps…and
16kbps to 64Kbps)

6. NSS:Network Switching Systems
•
The central component of the Network Subsystem is the Mobile
services Switching Center (MSC). It acts like a conventional
switching node of the PSTN or ISDN, and additionally, provides all
the functionality needed to handle a mobile subscriber, such as
registration, authentication, location updating, handovers, and call
routing to a roaming subscriber. These services are provided in
conjunction with several functional entities, which together form the
Network Subsystem.
•
•
•
•
•
•
•
It contains (HLR,VLR,EIR,AUC,EC,IWF)
HLR:Home location register
VLR:Visitor Loaction Register
EIR:Equipment Identity register
AUC:Authentication Center
EC:Echo Canceller
IWF:Inter-working function

7. HLR:Home LocationRegister
The HLR is the database for subscriber parameters. Various identification numbers
and addresses are stored, as well as authentication parameters. This
information is entered into the database by the network operator when a new
subscriber is added to the system.
The parameters stored in the HLR are listed below:
1.
2.
3.
4.
5.
Subscriber ID (IMSI and MSISDN)
Current subscriber VLR (Current location)
Supplementary service information (e.g. Current forwarding number )
Subscriber status (registered / de-registered )
Authentication key and AUC functionality
The HLR database contains the master database of all the subscribers in GSM
system. It’s data can be remotely accessed by all the MSCs and VLRs in the
network.
Although the network may contain more than one HLR, there is only one database
record per subscriber - each HLR is therefore handling a portion of the total
subscriber database.

8. VLR:Visitor Location Register
The VLR contains a copy of most of the data stored at the HLR. However, It is a
temporary data that exists for only as long as the subscriber is "active" within the
VLR coverage.
So, the VLR provides a temporary local database for the subscriber. This function
reduces the need for excessive and time-consuming references to the "home"
HLR database.
The additional data stored in the VLR is listed below:
1.
2.
3.
4.
Mobile status (busy /free/no answer etc.)
Location Area Identity (LAI)
Temporary Mobile Subscriber Identity (TMSI)
Mobile Station Roaming Number(MSRN)

9. EIR:Equipment Identity Register
The EIR contains a centralized database for validating the International Mobile
Equipment Identity (IMEI).
This database is concerned solely with MS equipment and not with the subscriber.
The EIR database consists of lists of IMEIs organized as follows:
1. WHITE LIST contains those IMEIs that have been assigned to valid mobile
Equipment.
2. BLACK LIST Contains IMEIs of mobiles which have been reported stolen or which
are have forbidden service for some other reason.
3. GREY LIST Contains IMEIs of mobiles that have problems (e.g. faulty software).
These are not sufficiently significant to enter into a "black list".

10. AUC:Aurhentication center

11. The AUC is a processor system. It performs the "authentication " function. It will
normally be co -located with the HLR. The authentication process will usually take
place each time the subscriber "initializes "on the system.
In the authentication process, secure data stored on the SIM card is calculated and
compared with the data held in the HLR database. The Authentication Process is as
follows:
A random number is sent to the Mobile from the AUC.
This number is calculated together with Authentication Key (Ki) stored in the SIM
card by authentication algorithms, which is held in the SIM card.
The calculation of the random number and Ki will get two results. One is a response
called as SRES, which are returned to the AUC. Another is an Encryption Key
called as Kc which is stored in the SIM card. The Encryption key is used to encrypt
data that is sent over the air interface in order to make the interface more secure.
1. While the mobile is carrying out these calculations, the AUC carries out exactly
the same calculations using the random number and ki stored in the HLR. Then
AUC also gets a response.
2. The AUC compares it with the response from the subscriber. If the responses
produced by the AUC and the subscriber are the same, the subscriber is permitted
to access the network.
3. The Encryption Key produced by the AUC is stored and sent to the BTS to
enable ciphering to take place.
The first time a subscriber attempts to make a call, the full authentication process
takes place. However, for subsequent calls, authentication may not be necessary.

12. IWF: Inter-Working Function
The IWF provides the function to enable the GSM system to connect with the
various forms of data networks.
The basic features of the IWF are listed below:
1. Rate Conversion function
2. Protocol Adaptation function
GSM system may require IWF capability or not. This depends upon the network
to which it is being connected.

14. There are two types of OMC as below:
1. OMC (R) - It is assigned specifically to the Base Station System.
2. OMC (S) – It is assigned specifically to the Network Switching System.
These two parts can be located together or work independently according to the
practical requirements.

16. PCU
Pb Interface
BSC
BTS
MS
MSC
Ater Interface
Um Interface
Abis
Interface
TC
A interface

22. The IMSI is the unique international code for the mobile subscriber within the
GSM system area. But it is not known to the mobile subscriber. IMSI is stored in
SIM card and is assigned to the MSISDN in the HLR. It is also stored in HLR and
VLR.
The structure of IMSI is shown in this diagram

23. MSISDN is used in the following cases:
1. By the caller to set up a connection to this mobile subscriber
2. By the MSC/Gateway MSC to address the HLR when inquiring the visitor
location of the mobile subscriber.
MSISDN is stored as semi-permanent data in the database of HLR and VLR. It is
transferred on MAP interface.

24. IMEI means International Mobile Station Equipment Identification. It is the
unique number for mobile equipment. Please note the difference between
IMSI and IMEI. IMSI is the identification for mobile subscriber, IMEI is the
identification for mobile equipment.

25. 1. TMSI means Temporary Mobile Subscriber Identification.
2. The TMSI is assigned only after successful subscriber authentication.
3. The VLR controls the allocation of new TMSI numbers and notifies them to
the HLR.
4. TMSI is used to ensure that the identity of the mobile subscriber on the air
interface is kept secret. The TMSI will be updated frequently; this makes it
very difficult for the call to be traced and therefore provides a high degree
of security for the subscriber.
5. The TMSI consists of 4 bytes (8 HEX numbers) and determined by the
telecom operator.

29. Logical channels

32. Radio Technology
1.
2.
3.
4.
5.
GMSK modulation
Frequency Hopping
Diversity
Power Control
VAD,DTX,DRX