GSM Fundamentals

Prepared By

Asem Mohammed Shams

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 GSM 1800 GSM 1900

Frequency bandUp LinkDown linkSeparationNumber of CH Channel widthCluster size

25 MHZ890 – 915 MHZ

935 – 960 MHZ45 MHZ

124200 kHz7 cells

75 MHZ1710 – 1785 MHZ1805 – 1880 MHZ

95 MHZ374

200 kHz4 cells

60 MHZ1850 – 1910 MHZ1930 – 1990 MHZ

80 MHZ299

200 kHz4 cells

Table – 1

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

General architecture of a GSM network

MS:Mobile Station

It consist of 2 parts:

• ME:Mobile Equipment

Which contain (IMEI)

• SIM:Subscriber identification module

Which contain (IMSI,Ki,Kc)

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)

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

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. Subscriber ID (IMSI and MSISDN)2. Current subscriber VLR (Current location)3. Supplementary service information (e.g. Current forwarding number )4. Subscriber status (registered / de-registered )5. 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.

HLR:Home LocationRegister

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. Mobile status (busy /free/no answer etc.)2. Location Area Identity (LAI)3. Temporary Mobile Subscriber Identity (TMSI)4. Mobile Station Roaming Number(MSRN)

VLR:Visitor Location 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".

EIR:Equipment Identity Register

AUC:Aurhentication center

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.

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 function2. Protocol Adaptation function

GSM system may require IWF capability or not. This depends upon the network to which it is being connected.

IWF: Inter-Working Function

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.

MS BTSBSC

PCU

MSC

A interface

Abis Interface

Um Interface

Pb Interface

TC

Ater Interface

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

MSISDN is used in the following cases:

1. By the caller to set up a connection to this mobile subscriber2. 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.

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.

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.

Logical channels

Radio Technology

1. GMSK modulation

2. Frequency Hopping

3. Diversity

4. Power Control

5. VAD,DTX,DRX