GSM System

Global System for Mobile Communications

Originally GSM stood for Groupe Speciale Mobile

GSM to meet the following business objectives

1. Support for international roaming

2. Good speech quality

3. Ability to support handheld terminals

4. Low terminal and service cost

5. Spectral efficiency

6. Support for a range of new services and facilities

7. ISDN compatibility

GSM System Hierarchy

GSM System Hierarchy

Consists at the minimum one administrative region assigned to one MSC (Mobile Switching Centre)

Administrative region is commonly known as PLMN (Public Land Mobile Network)

Each administrative region is subdivided into one or many Location Area (LA)

One LA consists of many cell groups and each cell group is assigned to one BSC (Base Station Controller)

For each LA, there will be at least one BSC while cells in one BSC can belong to different LAs

GSM Architecture

Operational Architecture of GSM

Mobile Station Mobile Station (MS) consists of two main elements: mobile equipment or mobile device (that is the phone without the SIM card) and Subscriber Identity Module (SIM)

Terminals distinguished principally by their power and application

SIM is installed in every GSM phone and identifies the terminal

SIM cards used in GSM phones are smart processor cards with a processor and a small memory

SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other security information

Radio subsystem• The Radio Subsystem (RSS) comprises the cellular mobile network up

to the switching centers• Components

– Base Station Subsystem (BSS):• Base Transceiver Station (BTS): radio components including sender, receiver,

antenna - if directed antennas are used one BTS can cover several cells• Base Station Controller (BSC): switching between BTSs, controlling BTSs,

managing of network resources, mapping of radio channels (Um) onto terrestrial channels (A interface)

• BSS = BSC + sum(BTS) + interconnection

– Mobile Stations (MS)

System architecture: radio subsystem

•Interfaces– Um : radio interface– Abis : standardized, open interface

with 16 kbit/s user channels

– A: standardized, open interface with 64 kbit/s user channels

Um

Abis

A

BSS

radiosubsystem

network and switchingsubsystem

MS MS

BTSBSC MSC

BTS

BTSBSC

BTSMSC

Network and Switching Subsystem

Central component of the Network Subsystem is the Mobile Switching Center (MSC)

Signaling between functional entities in the Network Subsystem uses Signaling System Number 7 (SS7)

MSC together with Home Location Register (HLR) and Visitor Location Register (VLR) databases, provide the call-routing and roaming capabilities of GSM

MSC does the following functions:

1. It acts like a normal switching node for mobile subscribers of the same network (connection between mobile phone to mobile phone within the same network)

Network and switching subsystem

• NSS is the main component of the public mobile network GSM– switching, mobility management, interconnection to other networks,

system control• Components

– Mobile Services Switching Center (MSC)controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSC can belong to a MSC

– Databases (important: scalability, high capacity, low delay)• Home Location Register (HLR)

central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR (one provider can have several HLRs)

• Visitor Location Register (VLR)local database for a subset of user data, including data about all user currently in the domain of the VLR

Mobile Services Switching Center

• The MSC (mobile services switching center) plays a central role in GSM– switching functions– additional functions for mobility support– management of network resources– interworking functions via Gateway MSC (GMSC)– integration of several databases

• Functions of a MSC– specific functions for paging and call forwarding– termination of SS7 (signaling system no. 7)– mobility specific signaling– location registration and forwarding of location information– provision of new services (fax, data calls)– support of short message service (SMS)– generation and forwarding of accounting and billing information

Operation subsystem• The OSS (Operation Subsystem) enables centralized operation,

management, and maintenance of all GSM subsystems• Components

– Authentication Center (AUC)• generates user specific authentication parameters on request of a VLR • authentication parameters used for authentication of mobile terminals and

encryption of user data on the air interface within the GSM system – Equipment Identity Register (EIR)

• registers GSM mobile stations and user rights• stolen or malfunctioning mobile stations can be locked and sometimes even

localized– Operation and Maintenance Center (OMC)

• different control capabilities for the radio subsystem and the network subsystem

AIR INTERFACEFrequency Allocation

Radio ChannelDOWNLINK935 - 960 MHz1805-1880 MHz

UPLINK890-915 MHz1710-1785 MHz

Air Interface

Cell Site Mobile

GSM Frequency Allocation

Each way the bandwidth for the GSM system is 25 MHz which provides 125 carriers uplink/downlink each having a bandwidth of 200 kHz.

ARFCN (Absolute Radio Frequency Channel Numbers) denote a forward and reverse channel pair which is separated in frequency by 45 MHz.

Practically, a guard band of 100 kHz is provided at the upper and lower end of the GSM 900 MHz spectrum and only 124 (duplex) channels are implemented.

GSM Frequency Allocation GSM uses TDMA and FDMA

One or more carrier frequencies are assigned to each base station and each of these carrier frequencies is then divided in time using a TDMA scheme where fundamental unit is called a burst period lasting approximately 0.577 ms.

Eight burst periods are grouped into a TDMA frame of approximately 4.615 ms which forms the basic unit for the definition of logical channels.

One physical channel is one burst period per TDMA frame while, normally, channels are defined by the number and position of their corresponding burst periods.

GSM Frequency Allocation

Carrier frequencies and TDMA frames

tch –traffic control channelSacch-slow associated control channel