GSM Overview GSM Overview
Kameswari Chebrolu
Dept. of Electrical Engineering, IIT Kanpur
GSM Background GSM Background
Global System for Mobile communications Second Generation digital cellular standard First introduction: 1991
Band: 890-960MHz, 1850-1990MHz Developed to serve pan-European cellular service
End of 2004, 1.26 billion GSM subscribers Multi-access: TDMA/FDD
Modulation: GMSK Channel Bandwidth: 200kHz
GSM Services GSM Services
Telephone Services:
Includes emergency calling and facsimile
Bearer Services (data services)
Date rates from 300bps to 9.6kbps
Supplementary ISDN services
Digital in nature
Caller identification, call diversion etc Short Message Services (SMS)
Broadcast of safety and advisory data
GSM Network Architecture GSM Network Architecture
MS
MSC VLR HLRAuC
BSC BTS Base Station Subsystem (BSS)
Network and Switching Subsystem (NSS)
Mobile Station
ISDN PSTN
Data Networks
BTS BSC
BTS BTS
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Network Elements Network Elements
Mobile Station (MS)
Subscriber Identity Module(SIM) Contains subscriber related information.
Mobile Equipment (ME)
Hardware and software specific to the radio interface.
Base Station System (BSS) Connects MS and the NSS.
Base Transceiver station (BTS)
Contains transmitter, receiver and signaling equipment.
Base Station Controller (BSC)
supports radio channel allocation/release and handoff management.
Cont…
Cont…
Network and Switching Subsystem (NSS)
Supports switching functions, subscriber profiles, and mobility management.Mobile Switching center(MSC) Performs basic switching function Manages mobility.
HLR/VLR manage the current location of the mobile.
Authentication center (AuC) Authenticates the subscribers.
Call Origination Operation Call Origination Operation
VLR
MSC PSTN
1) The MS sends a call 1 originating request to MSC.
Call Origination Operation Call Origination Operation
VLR
MSC PSTN
1 2
2) The MSC forwards the request to VLR.
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Call Origination Operation Call Origination Operation
VLR
MSC PSTN
1 2 3
3) The VLR checks the user’s profile and asks the MSC to grant call request.
Call Origination Operation Call Origination Operation
VLR
MSC PSTN
1 2 4 3
4) The MSC sets up the trunk according to standard PSTN call setup.
Call Delivery Operation Call Delivery Operation
PSTN GMSC HLR VLR
MSC
1 2
3 4
6 5
7 The GMSC gets the routing
information from HLR which in turn obtains it from VLR.
The GMSC then contacts MSC to setup the trunk.
GSM Radio Subsystem GSM Radio Subsystem
Uses two 25 Mhz bands for forward and reverse channels
890-915 Mhz for reverse band 935-960 Mhz for forward band Available bands are divided into 200kHz channels (ARFCNs) – 125 channels Each channel is shared by 8 subscribers using TDMA
An ARFCN and a time slot constitutes a physical channel
Each time slot is 576.92 us TDMA frame spans 4.615 ms
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GSM Channel Types GSM Channel Types
Each physical channel is mapped to many logical channels
Logical channels:
Traffic Channels Carry user data and speech Control Channels
Carry signaling and synchronization data between the base-station and the mobile terminal
Broadcast channels (BCHs), Common Control Channels (CCCHs), Dedicated Control Channels (DCCHs)