Hyoung Joo. Nam. 1CommunicationProtocolEngineering Lab.

GSM System Overview

Wireless and Mobile Network Architecture2003. 3. 18.

Nam Hyoung-Joo (jsnake@kw.ac.kr)

Communication Protocol Engineering Lab.

Hyoung Joo. Nam. 2CommunicationProtocolEngineering Lab.

Contents Introduction GSM Architecture Location Tracking and Call Setup Security Data services Unstructured Supplementary Service Data

Hyoung Joo. Nam. 3CommunicationProtocolEngineering Lab.

Introduction GSM(Global System for Mobile Communications) is

digital wireless network standard Major European telecommunications operators and

manufacturers The basic requirements of GSM

– Service – Quality of services and security– Radio frequency utilization– Network– Cost

Hyoung Joo. Nam. 4CommunicationProtocolEngineering Lab.

GSM architecture

MS : Mobile Station BSS : Base Station System NSS : Network and Switching System MSC : Mobile Switching Center

BSCBSS

MS MS

MSC

VLRHLR AuC

BTS BTS

ME

SIM

Radio Interface

MS

BSS

The A Interface

NSS

A-bis

Hyoung Joo. Nam. 5CommunicationProtocolEngineering Lab.

Mobile station Composition

– SIM (Subscriber Identity Module) A smart card, usually the size of a credit card A smaller-sized “plug-in SIM” Protected by a PIN (Personal Identity Number) Modified by subscriber Updated over the air though SIM Toolkit

– ME (Mobile equipment) Contains the noncustomer-related hardware and software specific to the r

adio interface Property of the subscriber

– TE (Terminal Equipment)– ME and SIM are called the MT (Mobile Terminal)

Hyoung Joo. Nam. 6CommunicationProtocolEngineering Lab.

BSS Composition

– BTS (Base Transceiver Station) Contains transmitter, receiver, signaling equipment TRAU (Transcoder/Rate Adapter Unit)

Speech encoding/decoding and rate adaption in data trasmission

– BSC (Base Station Controller) Responsible for the switching function in the BSS Supports radio channel allocation/release and handoff management Communicates with the BTSs using ISDN protocol via the A-bis interface

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NSS Supports the switching functions, subscriber profiles, and mo

bile management Interface

– Signaling protocol used in the telephone network Current location of a MS

– Maintained by HLR and VLR AuC (Authentication Center)

– security data management for the authentication of subscriber MSC

– GMSC (Gateway MSC)

Hyoung Joo. Nam. 8CommunicationProtocolEngineering Lab.

Radio interface Radio link

– FDMA and TDMA technologies Frequency band

– Downlink signal : 935 ~ 960 MHz– Uplink signal : 890 ~ 915 MHz– 200 KHz carrier frequency : 124 pairs of frequency duplex

channel The frame is divided into eight bursts (time slots)

– GSM burst structureTailing Data Flag Training Guard

3 57 bits 1 26 bits 1 57 bits 3 8.25 bits

Burst (148 bits or 0.546 msec)

Time Slot (156.25 bits or 0.577 msec)

Flag Data Tailing

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Logical channel(1/2) Two types

– TCHs (Traffic Channels)– CCHs (Control Channels)

TCH– TCH/F (Full-rate TCH)

For speech : 13 Kbps For data : 9.6, 4.8, or 2.4 Kbps

– TCH/H (Half-rate TCH) For speech : 6.5 Kbps For data : 4.8 or 2.4 Kbps

CCH– Carry signaling information– Three type

CCCHs (Common control channels) DCCHs (Dedicated control channels) BCHs (Broadcast channels)

Hyoung Joo. Nam. 10CommunicationProtocolEngineering Lab.

Logical channel(2/2) CCCH

– PCH (paging channel)– AGCH (Access grant channel)– RACH (Random access channel)

DCCH– SDCCH (Standard dedicated control channel)– SACCH (Slow associated control channel)– FACCH (Fast associated control channel)– CBCH (Cell broadcast channel)

BCH– FCCH (Frequency correction channel)– BCCH (Broadcast control channel)

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Origination and termination

PCH (page MS)

RACH (request signaling channel)

AGCH (assign signaling channel)

SDCCH (respond to paging)

MS BSS

Call setup procedure

GSM call termination

SDCCH message exchanges for call setup

RACH (request signaling channel)

AGCH (assign signaling channel)

SDCCH (request call setup)

SDCCH (assign TCH)

FACCH (complete assignment)

GSM call origination

MS BSS

Hyoung Joo. Nam. 12CommunicationProtocolEngineering Lab.

Location tracking and call setup(1/2) Registration

OLDVLR

NEWVLR

HLR

35

2

1 4

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Location tracking and call setup(2/2) Termination

Otherswitches

Otherswitches

Telephone

VLRHLRGMSC

MSC

1 1

1 1

22

3

3

3

Hyoung Joo. Nam. 14CommunicationProtocolEngineering Lab.

Security Two aspects

– Authentication– Encryption Ki

A8 A3 A8A3

Ki

SRES

RAND

SRESEqual?

Kc

Kc

Kc

ciphering

deciphering

deciphering

ciphering

Frame number

MS Home system

AuthenticationEncryption

Visited system

Chipheredinformation

A5 A5

data

data data

data

reject

no

yesaccept

Hyoung Joo. Nam. 15CommunicationProtocolEngineering Lab.

Data services GSM phase 2 supports two data service groups

– Short message services– Bearer services

Short message services– Part of the basic subscription package

Bearer services– Similar to the ISDN services– Data rate : 9.6 Kbps– Circuit-switched connection– Channel resources are reserved : call setup time is too long

GSM phase 2+ standard– HSCSD (High speed Circuit-Switched Data)– GPRS (General Packet Radio Service)

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HSCSD(1/2) HSCSD architecture

– TE (Terminal Equipment)– IWF (Interworking Function)– TAF (Terminal Adaption Function)– PSDN (Public Switched Data Network) – PSTN (Public Switched Telephone Network)

PSTNPSDNISDN

TETAF

BSS

MSC

IWF

MS Radiointerface

Hyoung Joo. Nam. 17CommunicationProtocolEngineering Lab.

HSCSD(2/2) Circuit-switched protocol for large file transfer and

multimedia applications Using multiple TDMA time slot and data compression

techniques Radio interface

– Multiple, independent time slot– RLP (Radio Link Protocol)

Disadvantage of multiple time-slot– Blocking rate of the system will be increased– Solution

Flexible resource assignment

Hyoung Joo. Nam. 18CommunicationProtocolEngineering Lab.

GPRS(1/2) GPRS architecture

– SGSN (Serving GPRS Support Node)– GGSN (Gateway GPRS Support Node)

TE

MS

TAF

BSS

SGSNIWF

GGSN

HLRVLR

MSC

PSTNPSDNISDN

Radiointerface

Signaling link

Hyoung Joo. Nam. 19CommunicationProtocolEngineering Lab.

GPRS(2/2) Packet-switched protocol Requires its own transport network Two new entities

– SGSN : receives and transmits packets between the MS and PSDN– GGSN : interworks with the PSDN

Cache– Containing location information to speed up the routing procedure

Radio link– MAC (Media Access Control) for packet switching

GPRS supports– Up to 100 users with one to eight channels– Broadcast and multisessions

Hyoung Joo. Nam. 20CommunicationProtocolEngineering Lab.

USSD (Unstructured Supplementary Service Data)– Supports new services in old MS– GSM 02.90, 03.90, 04.90– Used as a GSM transparent bearer for old MS

Flexible message length– Plus the asterisk (*) and pound (#) keys– For example

Command code 159 for call forwarding *159*5288128#

USSD architectureGSM Network

MSC VLR HLR

App.Server

USSDgateway

1

4

5

2 3