GSM System Architecture

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GSM System Architecture by BSNL(Bharat Sanchar Nigam Limited)

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Cellular Mobile Communication System

DGM(BSS), BSNL

15-Feb-06

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GSM
Global System for Mobile Communication

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PART-I : Brief History of GSM

PART-II : GSM Architecture

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History of GSM
Date Activities of development of Mobile Communication 1982 Conference Europeenne des postes et Telecommunication – (CEPT)began specifying a European digital telecommunications 1985 standard in the 900 MHz frequency band. This standard later became known as Global System for Mobile Communication (GSM) 1986 Field tests were held in Paris to select which digital transmission technology to use. The choice was TDMA or FDMA 1987 A combination of TDMA and FDMA was selected as the transmission technology for GSM Operation from 12 countries signed a Memorandum of understanding committing themselves to introducing GSM by 1991. 1988 CEPT began producing GSM specifications for a phased implementation.Another five countries signed the MOU

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History
Date 1989 1990 1991 1992 Activity European Telecommunication Standards Institute (ETSI) took over responsibility for GSM Phase 1 specifications were frozen to allow manufacturers to develop network equipment. The GSM 1800 standard was released. An addendum was added to the MoU allowing Countries outside CEPT to sign. Phase 1 specifications were completed. First commercial Ph-I GSM networks were launched. The first international roaming agreement was established between Finland & Vodafone in UK. Australia becomes the first non-European country to sign the MoU. The first commercial DCS 1800 system was launched in UK

1993

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History
Date 1995 Activity The specification for Personal Communication Services (PCS) was developed in USA. This version of GSM operates at 1900MHz The first GSM 1900 system became available. These comply with the PCS 1900 standerd. At the beginning of 1998 the MoU has a total of 253 members in over 100 countries and there are over 70 million GSM subscribers worldwide. GSM subscribers account for 31% of the world’s mobile market

1996 1998

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GSM Specifications
• Designed to be Platform- Independent • Specify Network functions & Interfaces in details, not Actual Hardware requirements • Allows hardware manufacturers to be creative in providing actual functionality. • Allows operators to buy equipment from different suppliers.

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GSM SPECIFICATIONS
Recommended by Groups organized by ETSI
System Frequecies: Wave Length Bandwidth Duplex Distance Carrier Separation Radio Channels Transmission Rate P-GSM 900 UP Link : 890-915 MHz, Down Link : 935-960 MHz ~ 33 cm 25 MHz 45 MHz 200 KHz 125 270 Kbits/s (Over Air Interface)

Modulation Method GMSK (Gaussian Minimum Shift Keying)

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GSM Architecture
OMC
OSS EIR
B T S B T S

SS
AUC

HLR PSTN ISDN

BSC BSC

GMSC/VLR

Data Networks

B T S

BSS
15-Feb-06

MSC

Air interface
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PART - II :GSM Architecture
It Includes following Subsystems: 1. Switching Subsystem(SS) is responsible for
performing switching functions related to call processing & Subscriber related functions.

2. Radio Subsystem includes the equipments and
functions related to the management of the connections on the radio path. It performs all radio related functions

3. Operations and Maintenance subsystem
includes the operation and maintenance of GSM equipment for the radio and network interface.
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Network Architecture
OMC
OSS EIR
B T S B T S

SS
AUC

HLR PSTN ISDN

MS

BSC BSC

GMSC/VLR

Data Networks

B T S

BSS
15-Feb-06

MSC

Air interface
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Architecture of the GSM system
• Several providers setup mobile networks following the GSM standard within each country • Components
– – – – MS (mobile station) BS (base station) MSC (mobile switching center) LR (location register)

• Subsystems
– RSS (radio subsystem): covers all radio aspects
• Base station subsystem

– NSS (network and switching subsystem): call forwarding, handover, switching DGM(BSS), BSNL 15-Feb-06 – OSS (operation subsystem): management of the

12

GSM: system architecture
radio subsystem MS MS Um BTS BTS Abis BSC SS7 MSC ISDN PSTN network and switching subsystem Fixed partner networks

EIR HLR

BTS BTS BSC A BSS MSC IWF

VLR ISDN PSTN PDN

Radio subsystem
radio subsystem MS MS Um BTS BTS Abis BSC MSC

• network and switching subsystem

Components
– MS (Mobile Station) – BSS (Base Station Subsystem): consisting of
• BTS (Base Transceiver Station): sender and receiver • BSC (Base Station Controller): controlling several transceivers


BTS BSC BTS BSS A MSC

Interfaces
– Um : radio interface – Abis : standardized, open interface with 16 kbit/s user channels – A: standardized, open interface with 64 kbit/s user channels

Network Subsystems
1. Network Subsystem(NSS) includes the equipments and functions related to end-to-end call. 2. Base Subsystem(BSS) includes the equipments and functions related to the management of the connections on the radio path. 3. Operations and Maintenance subsystem(OMC) includes the operation and maintenance of GSM equipment for the radio and network interface.
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SS Components:
• • • • • Mobile Services switching Center (MSC) Home Location Register (HLR) Visitor Location Register (VLR) Authentication Center(AUC) Equipment Identity Register(EIR)

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BSS Components:
• Base Station Controller (BSC) • Base Transceiver (BTS)

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GSM Network Structure
• • • • • GSM Service Area PLMN Service Area MSC Service Area Location Area Cells
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GSM
PLMN Service Area
I MSC VLR VLR MSC III IV VLR II MSC VLR

MSC

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GSM
MSC Service Area

LA2 LA1 VLR MSC LA3

LA4

LA6 LA5

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GSM
Cells

LA2 LA1
MSC/VLR

LA3 C2 C7 C3 C1 LA5 C4 C5 C=CELL C6

LA6

LA4

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GSM
Relation between areas in GSM

Cell Location Area Area served by a BTS Location Area MSC Service Area PLMN Service Area GSM Service Area
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Typical Engineering Groups and Activities
Marketing
RF Design & Planning OMC/NMC O&M Engineering

Customer Care
Optimization Quality Assurance Switching Operations

Site Planning

Faults Management

BTS Maintenance

Statistics Analysis

Performance Verification

Maintenance

Core Design

Statitics Generation

Air Interface Troubleshooting

RF Tests

Benchmarking

Planning

RF Planning

Databse Management

RF and Parametric Tuning

Transmission

Implementing Changes
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Value Added Services
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RF Planning and Design – Site Planning

Reject Acquire
Action Steps

Search Area Selection Site Physical Qualification

Site Planning

Test Transmission
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Band Clearance
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Coverage Prediction
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RF Planning and Design – Frequency Planning
GSM uses concept of cells One cell covers small part of network Network has many cells Frequency used in one cell can be used in another cells This is known as Frequency Re-use
F4 F3 F1 F4 F3 F1 F2 F2

F= 1,2,3,4,5,6,7,8,9,10

Clusters
F=2 F=7 F=1 F=6,10 F=2 F=7 F=1 F=3 F=7 F=4,8 F=5,9 F=6,10 F=5,9 F=1 F=4,8 F=5,9 F=2 F=3 F=4,8 F=3

F2 F1 F2

F=6,10

Co-Channel ( Re-use ) Cells

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Interference
Interference in GSM systems are classified into three major categories . Co-Channel Interference Adjacent Channel Interference External Interference

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Co - Channel interference
Interference on a channel caused by another cell/mobile using the same frequency. C/Ic is the measure of co-channel interference GSM specifies the C/Ic threshold of 9dB for a service quality of 0.4% BER. 9db also includes 2db implementation margin 9db is decided considering the implementation of Synthesised Frequency Hopping Without SFH, the preferred threshold is 12 dB.

C / Ic 9 db

Ic

C

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Co-Channel Interference
Noise

C

Ic

ARFCN "N"

ARFCN "N"

Causes
Distant Cells due to tight frequency re-use patterns. Distant Cells due to errors in frequency planning. Mulitpath from Distant cells( strong reflector, Water). C/Ic will degrade the Ec/No, so if Noise floor itself is high, then even a high value of C/Ic can deteriorate quality.
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Adjacent Channel Interference
Interference caused when wanted and unwanted GSM RF channels co-exist.

Ia C

GSM receivers are designed for an Adjacent Channel Suppression of minimum 18db at an offset of 200 Khz, 50db at 400 Khz and 58db at 600 Khz.

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GSM LA Coding

MCC 3 digit

MNC 3 digit LAI

LAC 2 Octets

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GSM Functions of Mobile Station
• Voice and data transmission • Frequency and time synchronization • Monitoring of power and signal quality of the surrounding cells • Provision of location updates even during inactive state • Equalization of multi path distortions
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GSM Mobile Station
• Portable, vehicle mounted, hand held • MS identified by unique IMEI (*#06#) • Shall display at least last ten received, dialled and missed calls • Minimum talk time of 1hr 30 min. and standby time of 80 hrs

• 160 characters long SMS

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GSM Mobile Station - Power Levels
Power Class 1 2 3 4 5
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Max. Peak Power 20W (43 dBm) 8W (39 dBm) 5W (37 dBm) 2W (33 dBm) 0.8W (29 dBm)

Tolerance (dB) Normal Extreme +/- 2 +/- 2.5 +/- 2 +/- 2 +/- 2 +/- 2
15-Feb-06

+/- 2.5 +/- 2.5 +/- 2.5 +/- 2.5
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GSM SIM Card
• • • • • SIM Module Unique Subscriber’s ID IMSI and ISDN PIN Key Ki, Kc and A3,A5 and A8 algorithms SIM has CPU, ROM, RAM and EPROM

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GSM Mobile Identification Numbers
• • • • • IMEI MSISDN IMSI TMSI MSRN

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GSM MSISDN
• Mobile Subscriber’s ISDN Number • The MSISDN is registered in the telephone directory and used by the calling party for dialing.
1 to 3 digits CC=+91 Variable NDC=94170 Variable SN=12345

N(S)N MSISDN : not more than 15 digits
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GSM IMSI
• International mobile subscriber’s Identity
• The IMSI is an unique identity which is used internationally and used within the network to identify the mobile subscribers. • The IMSI is stored on the subscriber identity module (SIM), the HLR, VLR and AC database.

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GSM IMSI
3 digits MCC 2 digits MNC NMSI
MSC in HLR Area (1); Pre/Post (1); Vendor ID (1); Running Counter=6

Not more than 9 digits MSIN

IMSI : Not more than 15 digits

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GSM TMSI
• Temporary Mobile subscriber’s Identity
• The TMSI is an identity which guarantees the integrity of the mobile subscribers on the radio interface. • The VLR assigns a TMSI to each mobile subscribers entering the VLR area.

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GSM MSRN
Mobile Station Roaming Number
• The MSRN is used in the GMSC to set up a connection to the visited MSC/VLR.

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GSM IMEI
• International Mobile Equipment Identity
• The IMEI is an unique code allocated to each mobile equipment. It is checked in the EIR. • IMEI check • White List • Grey List • Black List

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RADIO SUB SYSTEM (RSS)

MSC/VLR

BSC BSC

BSC

RSS RSS
n BTS
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n BTS
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GSM
FUNCTION OF BTS -I
• Encodes, encrypts, multiplexes, modulates and feeds the RF signals to the antenna Transcoding and rate adaption Functionality Time and frequency synchronisation signals transmission. 11 power classes from .01 watts (Micro cell) to 320 watts (Umbrella cell)
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• •



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GSM
FUNCTIONS OF BSC-I
• It is connected to BTS and offloads MSC • Radio resource management • Inter-cell handover • Reallocation of frequencies • Power control

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GSM
FUNCTION OF BTS -II
• Frequency hopping • Random access detection • Uplink radio channel measurements • BTS mainly consists of a set of transceivers (TRX). Can accommodate 1 to 7 TRX per Sector

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GSM
FUNCTIONS OF BSC-II
• Time delay measurement of the received signals from MS with respect to BTS clock. • Performs traffic concentration to reduce the number of lines from BSC to MSC. • Provide interface TCP/IP X.25 to the OMS

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GSM MSC-BSS Configurations
Multi - cell site (sector Cells Configuration -5 A BTS A BSS Single - cell site Configuration -1 BTS BTS BTS Many single cell sites

MSC

A

BSS

A-bis BSC A-bis

BTS

BTS

MCC: Mobile Switching Centre BSS: Base Station System BSC: Base Station Controller BTS: Base Transceiver Station

Configuration -6

BTS

Multi - cell site = multi--BTS site

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Network and Switching Subsystem (NSS)
SS7 Signalling Traffic Path
VLR D C HLR E EIR A AUC

F

MSC

Other MSC

(PSTN) (BSS)
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GSM
MSC ( 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 .
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GSM MSC Functions - I
• Paging, specifically call handling • Location updation. • Handover management. • Billing for all subscribers based in its area. • Reallocation of frequencies to BTSs in its area to meet heavy demands.
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GSM MSC Functions - II
• Echo canceller operation control • Signaling interface to databases like HLR, VLR. • Gateway to SMS between SMS centers and subscribers • Handle interworking function while working as GMSC

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GSM
VISITOR LOCATION REGISTER (VLR)-I
• It controls those mobiles roaming in its area. • VLR reduces the number of queries to HLR • One VLR may be incharge of one or more LA. • VLR is updated by HLR on entry of MS its area. • VLR assigns TMSI which keeps on changing.

• IMSI detach and attach operation
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GSM
Data in VLR
• • • • • • • IMSI & TMSI MSISDN MSRN. Location Area Supplementary service parameters MS category Authentication Key

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GSM
Home Location Register(HLR)-I
• Reference store for subscriber’s parameters, numbers, authentication & Encryption values. • Current subscriber status and associated VLR. • Both VLR and HLR can be implemented in the same equipment in an MSC. • one PLMN may contain one or several HLR.

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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
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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
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GSM
AUTHENTICATION CENTRE (AUC )-I
• AUC is a separate entity and physically included in HLR • Protect against intruders in air interface • Authentication (Ki) and ciphering (Kc) key are stored in this data base. • Keys change randomly with each call • Keys are never transmitted to MS on air Only calculated response are sent.
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GSM
EQUIPMENT IDENTITY REGISTER ( EIR )
• This data base stores IMEI for all registered mobile 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.

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Operations and Maintenance Centre (OMC)
The centralized operation of the various units in the system and functions needed to maintain the subsystems. Dynamic monitoring and controlling of the network Separate OMC-S and OMC-R for NSS and RSS

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Functions of OMC
- 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
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Mobile Terminated Call
• • • • • • • • • • • 1: calling a GSM subscriber 2: forwarding call to GMSC 3: signal call setup to HLR 4, 5: connect with current VLR 6: forward responsible MSC to GMSC calling station 1 7: forward call to current MSC 8, 9: get current status of MS 10, 11: paging of MS 12, 13: MS answers 14, 15: security checks 16, 17: set up connection
HLR

4 5 7

VLR

3 6
PSTN

8 9 14 15
MSC

2

GMSC

10
BSS

10 13 16
BSS

10
BSS

11

11 11 12 17
MS

11

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Mobile Originated Call
• 1, 2: connection request • 3, 4: security check • 5-8: check resources (free circuit) • 9-10: set up call
VLR

3 4 6
PSTN GMSC

5
MSC

7

8 2 9
MS

1 10

BSS

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MTC/MOC
MS

MTC
paging request channel request

BTS

MS

MOC
channel request

BTS

immediate assignment paging response authentication request authentication response ciphering command ciphering complete setup call confirmed assignment command assignment complete alerting connect connect acknowledge data/speech exchange
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immediate assignment service request authentication request authentication response ciphering command ciphering complete setup call confirmed assignment command assignment complete alerting connect connect acknowledge data/speech exchange
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4 types of handover
1 MS 2 MS 3 MS 4 MS

BTS

BTS BSC

BTS BSC MSC

BTS BSC MSC

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Handover decision
receive level BTSold receive level BTSold

HO_MARGIN MS BTSold MS BTSnew

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GSM Security Management
• Four basic security services provided by GSM • • • • Anonymity : TMSI Assignment Authentication Encryption: PIN

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GSM Generic Authentication Process
RAND Ki

Ki IMSI

RAND Radio Path

A3

A3 SRES Compare Yes/No

IMSI

Response

SRES

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GSM Encryption Process
Plain Text

KEY

Encryption Process

Cipher-text
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GSM - authentication
mobile network Ki AC 128 bit A3 SRES* 32 bit SRES RAND 128 bit RAND SIM RAND 128 bit A3 SIM 32 bit Ki 128 bit

MSC

SRES* =? SRES

SRES 32 bit

SRES

Ki: individual subscriber authentication key
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SRES: signed response
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15-Feb-06

GSM - key generation and encryption
mobile network (BTS) Ki AC 128 bit A8 cipher key Kc 64 bit data A5
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MS with SIM RAND RAND 128 bit A8 Ki 128 bit SIM

RAND 128 bit

Kc 64 bit encrypted data SRES data MS A5
70

BTS

GSM
AUTHENTICATION & ENCRIPTION • AUC
Database
IMSI1 IMSI2 IMSI3

ki1 ki2 ki3

Kc
Algorithm for Ciphering A8 Algorithm for Authentication A3

SRES RAND

HLR

Kc 64 bits SRES 32 bits RAND
71

Generation of Random Number RAND
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Thank You
DGM(BSS), BSNL 15-Feb-06 72

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