Evolution of GSM Originally described a digital, circuit-switched network optimized for

fully duplex voice telephony. Expanded over time to include data communications.

1. circuit-switched transport 2. packet data transport via GPRS

GPRS (General Packet Radio Service) having data rates upto 171kbits/s

GSM (Global System for Mobile Communications) is a common second generation cell phone standard.

GSM is a digital standard that was adopted over the original analog system. The GSM network operates at 850 MHz or 1900 MHz bands in the US. GSM is a common second generation cell phone standard.

Good subjective voice quality Low service cost Support for international roaming Support for handheld terminals Support for new services Spectrum efficient GSM SYSTEM ARCHITECTURE

Mobile Station (MS) Mobile Equipment (ME) Subscriber Identity Module (SIM) Base Station Subsystem (BSS) Base Transceiver Station (BTS) Base Station Controller (BSC) Network Switching Subsystem(NSS) Mobile Switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR) Authentication Center (AUC)

Equipment Identity Register (EIR)

SYSTEM ARCHITECTURE MOBILE STATION (MS)The Mobile Station is made up of two entities. Mobile Equipment (ME) Subscriber Identity Module (SIM) MOBILE EQUIPMENT:

Portable, vehicle mounted, hand held device Uniquely identified by an IMEI (International Mobile Equipment Identity) Voice and data transmission Monitoring power and signal qualityof surrounding cells for optimum handover Power level : 0.8W – 20 W 160 character long SMS. SUBSCRIBER IDENTITY MODULE(SIM): Smart card contains the International Mobile Subscriber Identity (IMSI) Allows user to send and receive calls and receive other subscribed services Protected by a password or PIN Can be moved from phone to phone – contains key information to activate the phone

SYSTEM ARCHITECTURE BASE STATION SUBSYSTEM (BSS) Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing operation between components made by different suppliers

1. Base Transceiver Station (BTS)2. Base Station Controller (BSC)

BASE TRANSCEIVER STATION (BTS): Encodes, encrypts, multiplexes, modulates and feeds the RF signals to the antenna. Communicates with Mobile station and BSC Consists of Transceivers (TRX) units BASE STATION CONTROLLER (BSC):Manages Radio resources for BTSAssigns Frequency and time slots for all MSs in its area Handles call set up Handover for each MS It communicates with MSC and BTS

SYSTEM ARCHITECTURE NETWORK SWITCHING SUBSYSTEM(NSS)

The system contains the following functional units Mobile Switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR) Authentication Center (AUC) Equipment Identity Register (EIR) Network switching subsystem (NSS) is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations.

There are basically three database in the NSS   Network Switching Subsystem of GSM Network

Home Location Resistor Visitor Location Register Authentication Centre

MOBILE SWITCHING CENTER (MSC):

Manages communication between GSM and other networks• Billing information and collection• Mobility management - Registration - Location Updating - Inter BSS and inter MSC call handoff

The MSC is the most central unit in the NSS MSC is responsible for controlling the traffic among all of the present

BSCs. A MSC is the hardware part of any wireless communication switch which

communicate with the PSTN switch using signalling system-7 protocol.HOME LOCATION REGISTERS (HLR): Stores information about each subscriber that belongs to it MSC in permanent and temporary fashion. As soon as mobile subscriber leaves its current local area, the information in the HLR is updated. database contains IMSI, MSISDN, prepaid/ postpaid, roaming restrictions, supplementary services.

It is responsible for containing the subscriber information and location information for each user who reside in the same city as the city of the MSC.

Each subscriber is assigned a Unique International Identity (IMSI) and this is the number which is used to identify the home user.  

HLR  is  the  person  using the  mobile  in  the  local  area .VISITOR LOCATION REGISTERS (VLR):

Temporary database which updates whenever new MS enters its area, by HLR database Assigns a TMSI to each MS entering the VLR area which keeps on changing. Controls those mobiles roaming in its area Database contains IMSI, MSISDN, Location Area, authentication key temporaraily stores the IMSI an customer information for each roaming subscriber who is visiting the coverage information for each and a particular MSC.

MSC sends the necessary information to the visiting subscriber’s HLR so that calls to the roaming mobile can be appropriately routed over the PSTN by the roaming user’s HLR.

AUTHENTICATION CENTER (AUC):

Contains the algorithms for authentication as well as the keys for encryption. Protects network operators from fraud. Situated in special protected part of the HLR.

The Authentication centre is a strongly protected database. it is responsible for handling the authentication and encryption keys for

every single subscriber in the HLR and VLR. Once the authentication is successful, the HLR is allowed to manage the

SIM and services EQUIPMENT IDENTITY REGISTER (EIR): Stores all devices identifications registered for this network. Database that is used to track handsets using the IMEI (International Mobile Equipment Identity) Prevents calls from stolen, unauthorised or defective mobile devices

This database is used in NSS in order to track the handsets using the IMEI (International Mobile Equipment Identity) number.

.This is designed to allow tracking of stolen mobile phonesModulation

GSM uses GMSK or Gaussian minimum shift keying for modulation GMSK is a kind of continuous frequency shift keying

Advantages

Digital data allows for better more effective use of bandwidth Radio signals use less power, this allows batteries to last longer Digital voice encoding allowed for digital error checking to be used. Digital services such as email and SMS or Short Message Service (text

messages) were added.

Security

GSM uses some simple encoding algorithms but has been found to be less effective then originally believed.

Previous analog systems provided virtually no security. Phones were prone to being “cloned” with analog systems. GSM prevents cloning problem but also stops legitimate uses of this property.

Disadvantages The weaker digital signal may not be able to reach cell tower

Digital signal will drop out faster then an analog signal.

Choice of network

In GSM the following procedure is followed:

The latest used network is stored on the SIM

As long as a cell that fulfils the criteria is available from this network, the mobile will not search for alternatives (the exception is national roaming, in which case the mobile will periodically search for the home network and connect when this becomes available)

If the previously used network is not available, the mobile searches for alternative networks

The mobiles can perform manually or automatic choice of network

A location update is performed when:

The mobile is connecting to a cell and discovers that the LAI read is different than the one stored in the mobile

ME

SIMSIM

BTSBTS

BTSBTS

BSCBSC

BSCBSC

MSC

VLR

EIR

HLR

AuC

AUmAbis

PSTN,ISDN

Mobile station

Base Station Subsystem

NetworkSubsystem

MS BSS NSS

The mobile has been turned on, but not used, for a pre-defined period of time since the last location update (periodic location update)

IMSI detach/attach:

An additional function where the mobile informs that it is turned on or off (in the same LA), saves resources on the radio interface and leads to faster response on incoming calls

Periodic detach

A network functionality where the network assumes that the mobile has been turned off if periodic location update has not been performed and no other activity has been observed for a pre-defined amount of time

TYPE OF HANDOVER

Intra cell (to another channel in the same cell) (1) Inter cell, intra BSC (2) Inter BSC, intra MSC (3) Inter MSC (4) In addition inter system handover can sometimes be performed, e.g. GSM

toUMTS Complicated, special rules apply Type of handover has network implications, but the algorithms to decide

handover are the same