Gsm Concepts

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GSM Concepts



2

GSM

Various subsystems1. Network Subsystem includes the equipments

and functions related to end-to-end call.

2. Radio Subsystem includes the equipments andfunctions related to the management of theconnections on the radio path.

3. Operations and Maintenance subsystemincludes the operation and maintenance of GSMequipment for the radio and network interface.



3

Network Architecture

BT

S

MSC VLR

HLR

PSTNISDN

Data

Networks

Air interface

OSS

B

T

S

B

T

S

MSC VLR

BSC

BSC



4

GSM

Interfaces AUC

MSC

BSC

BTS

MS

OMC

ISDN

PSPDN

CSPDN

PSTN

PLMN A - Interface

BSS

A-bit interface Access to different

services

Base station system

Information transmission

Call connections and information transmission

SS

X.25

VLR EIRHLR

X.25



5

GSM

Network Structure

GSM Service Area

PLMN Service Area MSC Service Area

Location Area Cells



6

GSMPLMN Service Area

V

MSC

MSC

MSC

MSC

VLR

VLR

VLR

I II

IVIII

I

VLR



7

GSMMSC Service Area

MSC

VLR LA1

LA2

LA3

LA6LA4 LA5



8

GSMCells

MSC

VLR LA1

LA2

LA3

LA6LA4 LA5

C1C2 C3

C6 C5C4

C=CELL



9

GSMRelation between areas in GSM

Cell

( Area served by one BTS )

Location Area

PLMN Service Area

( one per operator )

MSC Service Area

GSM Service Area



10

GSM

LA Coding

MCC

LAI

LACMNC

3 digit 3 digit 2 Octets



11

GSMMobile Station

Portable, vehicle mounted, hand held MS identified by unique IMEI

Shall display at least last ten received, dialledand missed calls

Minimum talk time of 1hr 30 min. andstandby time of 80 hrs



12

GSM

Functions of Mobile Station Voice and data transmission

Frequency and time synchronization

Monitoring of power and signal quality of thesurrounding cells for optimum handover

Provision of location updates even duringinactive state

Equalization of multi path distortions

160 characters long SMS



13

GSM

Mobile Station - Power LevelsPower

Class

Max. Peak

Power

Tolerance (dB)

Normal Extreme

1 20W

(43 dBm)

+/- 2 +/- 2.5

2 8W

(39 dBm)

+/- 2 +/- 2.5

3 5W

(37 dBm)

+/- 2 +/- 2.5

4 2W

(33 dBm)

+/- 2 +/- 2.5

5 0.8W

(29 dBm)

+/- 2 +/- 2.5



14

GSM

SIM Card

SIM Module

Unique Subscribers ID IMSI and ISDN PIN

Key Ki, Kc and A3,A5 and A8 algorithms

SIM has CPU, ROM, RAM and EPROM



15

GSM

Mobile Identification Numbers IMEI

MSISDN

IMSI

TMSI

MSRN



16

GSM

MSISDN Mobile Subscribers ISDN Number

The MSISDN is registered in the

telephone directory and used by the

calling party for dialing.

CC NDC SN

1 to 3 digits Variable Variable

MSISDN : not more than 15 digits

N(S)N



17

GSM

IMSI International mobile

subscribers Identity The IMSI is an unique identity which is

used internationally and used within thenetwork to identify the mobile

subscribers. The IMSI is stored on thesubscriber identity module (SIM), theHLR, VLR and AC database.



18

GSM

IMSI

MCC MNC MSIN

3 digits 3 digits Not more than 9 digits

NMSI

IMSI : Not more than 15 digits



19

GSM

TMSI Temporary Mobile subscribers

Identity The TMSI is an identity whichguarantees the integrity of the mobilesubscribers on the radio interface. The

VLR assigns a TMSI to each mobilesubscribers entering the VLR area.



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MSRN ± Mobile Subscriber Roaming Number

It is a temporary number used for routing the call to MS.

Format : MSRN = CC + NDC + SN

CC = Country Code

NDC = Network Destination Code

For example for Patna MSC in IMPCS network MSRN looks

like 91 98750 00100.

SN = Subscriber Number



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PSTN GMSC

HLR

MSC VLR

MSISDN

1

MSISDN2

IMSI

3

IMSI MSRN4

MSRN

5

MSRN 6

MSRN

7



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GSM

MSRNMobile Station Roaming

Number

The MSRN is used in the GMSC to set up aconnection to the visited MSC/VLR.



23

GSM

IMEI International Mobile Equipment

Identity

The IMEI is an unique code allocated to eachmobile equipment. It is checked in the EIR.

IMEI check

y

White List y Grey List

y Black List



24

RADIO SUB SYSTEM (RSS)RADIO SUB SYSTEM (RSS)

n BTS n BTS

BSC

BSC

BSC

MSC/VLR

RSS



25

GSMFUNCTION OF BTS -I

Encodes, encrypts, multiplexes, modulatesand feeds the RF signals to the antenna

Transcoding and rate adaption Functionality

Time and frequency synchronisation signalstransmission.

Communication on full or half rate channel

11 power classes from .01 watts (Micro cell)

to 320 watts (Umbrella cell)



26

GSMFUNCTION OF BTS -II

Frequency hopping

Random access detection

Uplink radio channel measurements

BTS mainly consists of a set of transceivers(TRX)



27

GSMFUNCTIONS OF BSC-I

Radio resource management

Assigns the frequency and timeslots for allMSs in its area.

Inter-cell handover

Power control



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GSMFUNCTIONS OF BSC-II

Time delay measurement of the receivedsignals from MS with respect to BTS clock.

Performs traffic concentration to reduce thenumber of lines from BSC to MSC.

Provide interface TCP/IP X.25 to the OMS



29

GSMFUNCTIONS OF BSC-III

BSC performs call processing

TRAU are generally located at the site of MSC.

BSC- BTS configurations as per requirement.

Data from OMC and can be down loaded to

BSC



30

GSM

MSC-BSS Configurations

BTSBTS

BTS

BTS BTS

BTS

A-bis

BSC

BSS

Configuration -6 Multi - cell site =multi--BTS site

Many single

cell sites

BSS

MSC

BTS

A A

A

Single - cell site

Configuration -1

Multi - cell site (sector CellsConfiguration -5

MCC: Mobile Switching Centre

BSS: Base Station System

BSC: Base Station Controller

BTS: Base Transceiver Station

A-bis



31

GSM

Transcoder ConfigurationsBTS TRAU BSC MSCTo MS To fixed

networks

BSC TRAUBTS MSCTo MS To fixednetworks

TRAU MSCBTS BSCTo MS To fixednetworks

RF Air Interface

A-bis

Interface

AInterface

13 kbps encoded voice/ 12 kbps data16 kbps transmission

64 kbps transmission

Transcoder (XC) configurations



32

Network and Switching

Subsystem (NSS)

MSC

(PSTN)

VLR

HLR AUC

EIR

D

C

SS7 Signalling

Traffic Path

F

( BSS)

A

E

Other

MSC



33

GSMMSC ( MOBILE SWITCHING CENTRE)

Manages communication between GSM &

other network

Call setup functions, basic switching are done

MSC takes into account the RR allocation in

addition to normal exchange functions

MSC does gateway function while its customers

roams to other network by using HLR /VLR



34

GSM

MSC Functions - I Paging, specifically call handling

Dynamic allocation of resources in coordination withBSS

± MSC decides what type of channel and when to beused

± BSS decides channel identity and radio parameters

Location registration

Handover management Billing for all subscribers based in its area

Reallocation of frequencies to BTSs in its area to meetheavy demands



35

GSM

MSC Functions - II Echo canceller operation control

Signaling interface to databases like HLR, VLR.

Gateway to SMS between SMS centers andsubscribers

Handle interworking function while working asGMSC



36

GSM VISITOR LOCATION REGISTER (VLR)-I

It controls those mobiles roaming in its area.

Main task of VLR is to reduce the number ofqueries to HLR

One VLR may be incharge of one or more LA.

VLR is a database and gets updated whenevera new MS enters its area, by HLR database.



37

GSM VISITOR LOCATION REGISTER (VLR)-II

Authentication along with HLR and AUC.

Relays ciphering key from HLR to BSS.

VLR is responsible for assigning TMSIand can be periodically changed to

secure the subscribers identity.

IMSI detach and attach operation



38

GSMData in VLR IMSI & TMSI

MSISDN

MSRN.

Location Area

Supplementary service parameters

MS category Authentication Key



39

GSM

Home Location Register(HLR)-I

Reference store for subscribers parameters,numbers, authentication & Encryption values.

Current subscriber status and associated VLR.

one PLMN may contain one or several HLR.

Logically there is only one HLR per PLMN.



40

GSM

Home Location Register(HLR)-II Permanent data in HLR Data stored is changed only by man-machine.

IMSI, MS-ISDN number.

Category of MS ( whether pay phone or not )

Roaming restriction ( allowed or not ).

Supplementary services like call forwarding



41

GSM

Home Location Register(HLR)-III Temporary data in HLR The data changes from call to call & is dynamic

MSRN

RAND /SRES and Kc

VLR address , MSC address.

Messages waiting data used for SMS



42

GSM AUTHENTICATION CENTRE (AUC )-I

Protect against intruders in air interface

Authentication and ciphering key arestored securely in this data base.

Ki Kc keys are never transmitted to MSon air



43

GSM AUTHENTICATION CENTRE (AUC )-II

Only calculated response are sent overthe air interface

Cipher key is transmitted over SS7 linkbetween home HLR and AUC

Random key and cipher key changesrandomly with each phone call

AUC is a separate entity and physicallyincluded in HLR



44

GSMEQUIPMENT IDENTITY REGISTER ( EIR )

This data base stores IMEI for all registeredmobile equipments and is unique to every ME.

Only one EIR per PLMN.

White list : IMEI, assigned to valid ME.

Black list : IMEI reported stolen Gray list : IMEI having problems like faulty

software, wrong make of equipment etc.



45

Architecture For Mobile Originated Call

BSS-AMS

VLR-A

MSC-A

PLMN

A- I/F

Signaling

IAM ( ISUP )Radio I/F

Signaling

Send Info

for o/g call

/ ACK

A hit t F M bil T i t d C ll



46

Architecture For Mobile Terminated Call

GMSC- B BSS-B

MS

VLR-B

MSC-B

VISITING PLMN

A- Interface

Air Interface

HLR-B

IAM ( ISUP )



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The centralized operation of the various units inthe system and functions needed to maintain thesubsystems.

Dynamic monitoring and controlling of thenetwork

Separate OMC-S and OMC-R for NSS and RSS

Operations and Maintenance Centre

OMC

Operations and Maintenance Centre

OMC



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functions

-O&M data function

-Configuration management

--Fault report and alarm handling

-Performance supervision/management

-Storage of system software and data

-Support GUI for operation and Maintenance

-Collect and store data for a minimum 1 year.

Functions Of OMCFunctions Of OMC



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GSM

Encryption Process

Encryption

ProcessKEY

Plain Text

Cipher-text

GSM



51

GSMGeneric Authentication

Process

A3 A3

Ki KiRAND

RAND

CompareSRES

SRES

Response

IMSIIMSI

Yes/No

Radio Path



52

AUTHENTICATION & ENCRIPTION AUC

Database

Generation

of Random Number

RANDRAND

IMSI1

IMSI3

IMSI2

ki1

ki2

ki3

RANDSRESKc

Algorithm for Ciphering

A8

Algorithm for

Authentication

A3

Kc

64 bits

SRES32 bits

HLR



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AUTHENTICATION & ENCRIPTION

HLR SRES=

SRESc

A3Sim

Card

Key Pad

A8

Store

Kc

AccessGranted

RAND

Yes

MSC/VLR RAND Ki

SRESc

(128)

SRES

(32)

Kc

Cipher

Key Transfer Kc to

BTS

no



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THANK THANKS !

ALT CENTRE

ALTTC

Gsm Concepts

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