Hyoung Joo. Nam. 1CommunicationProtocolEngineering Lab.
GSM System Overview
Wireless and Mobile Network Architecture2003. 3. 18.
Nam Hyoung-Joo ([email protected])
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
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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)
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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)
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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)
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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
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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
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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
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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
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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
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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
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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
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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