Gsm Overview: ~varun Ranjit Singh

GSM Overview

~Varun Ranjit Singh

07/23/04

GSM Architecture

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Components of GSM  MS --> ME + SIM  BSS --> BTS + BSC  MSS --> MSC + GMSC + Databases  Databases -->HLR,VLR,EIR,AuC 07/23/04

Addresses in GSM  IMEI  IMSI  MSISDN  MSRN  LAI(CI)  TMSI 07/23/04

GSM Stack

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Physical layer

CM MM RR

RR

LAPDm

LAPDm

RF

RF

MS 07/23/04

BTS

Accessing Technique  FDMA

 TDMA  Frequency hopping

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FDMA aGSM 900 a

Uplink (890-915MHz)

a Downlink(935-960MHz) a124 carriers spaced at 200kHz for each link 2 07/23/04

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TDMA

8 Timeslots per Carrier. Each burst is of 577 microseconds or 156.25 bits. The uplink channel follows the downlink channel after 3 bursts.

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Frequency hopping  1) Frequency diversity  2) Interferer diversity

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Multiframes  Type A multiframe (26 TDMA frames) used for the Traffic channels.

 Type B multiframe(51 TDMA frames) used for the signalling channel.

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Superframes  Super frame of 51 type A multiframe -1326 TDMA frames

 Super frame of 26 type B multiframe -1326 TDMA frames

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Hyper frames  A hyper frame of 2048 super frames -2,715,648 TDMA frames

 nearly 3 1/2 hours  GSM follows hyperframe cycles 07/23/04

Channels

Logical channels Control channels

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Traffic channels

Control Channels

Control channels BCH

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CCCH

DCCH

Broadcast channels

BCH FCCH

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SCH

BCCH

Common Control Channels

CCCH PCH

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AGCH

RACH

Dedicated Control channels

DCCH SDCCH

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SACCH

FACCH

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An example C0,C1,C2 --Carriers used C

C

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

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An Example C0,C1,C2 -- carriers used C -- control channels T -- traffic channels

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Downlink and Uplink,C0,timeslot 0Multiframe mapping

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Uplink and Downlink,C0,timelsot -1 Multiframe mapping

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C0,timeslot 2 - Multiframe mapping

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Transmission of Speech in GSM

Block of 160 samples over 20 ms (1280 bits)

Speech coder

Block of 260 samples (260 bits)

RPE-LTP-LPC Coding of Speech to reduce bit rate 07/23/04

50 VI bits

Block coder

132 important + 4 tail bits

Convolution coder

78 not so important bits Channel coding in GSM 07/23/04

456 bits

First level of Interleaving in GSM 1

2

3

4

10

11

12

13

17

18

19

20

-

-

-

-

-

-

9

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6

7

8

14

15

16

21

22

23

24

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

442 450

-

-

-

-

-

-

-

-

-

-

455

456

5

Second level of Interleaving in GSM

Achieved by transmitting each segment in a burst.

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Layer 2 protocols

CM MM RR

RR

LAPDm

LAPDm

RF

RF

MS 07/23/04

BTS

Link Layer Protocols

1) LAPDm (MS --BTS) 2) LAPD (BTS --BSC) 3) MTP 2 (BSC --MSC) 07/23/04

Functions of link layer 1) Structuring in frames

F

Frame contents

F

LAPDm ------23 octets long frame

LAPD,MTP 2 -----------HDLC format.

2) Segmentation and Re-assembly Maximum length of frames on A interface -

frame control + flags

octets for

Maximum length of frames on Abis interface

3) Error detection and correction LAPD and MTP2 -----FCS LAPDm

---- None

Purposes A) link quality monitoring.

B) frame acknowledgement and repetion

4) Multiplexing 5) Flow control

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LAPDm commands/responses  SABM  UA  DM  DISC  UI  I  RR  RNR  REJ 07/23/04

Radio Link Protocols (RLP) RLP frames --240 bit frame

--576 bit frame Used for user data transmission

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RR sublayer

CM MM RR

RR

LAPDm

LAPDm

RF

RF

MS 07/23/04

BTS

RR Layer Functions  Transmission Management  Handover  Power Control and Timing Advance  Radio Channel Management

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Transmission Management  Transmission Mode Management The set of possible transmission modes differ depending on the type of channel used on the Radio Interface. TCH/8 Signaling Only

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TCH/F

TCH/H

Signaling Only

Signaling Only

Speech

Speech

Data 3.6,6 kbps tr Data Data 12 12 kbps kbps trans non-

Data 3.6 kbps Data 6 kbps trans Data 6 kbps non-tr

Transmission Management  Cipher Mode Management

 RR –session is always started in “ Signaling only” mode

and always in clear text ( not enciphered) .

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Handover  Handover Purposes  Rescue handover

This handover is triggered to avoid losing a call in

Measurement Reporting  Minimum rate of reporting is once per second.  MS can report up to 6 neighbor cells in addition to the measurements relative to the serving cell.

 Reporting is carried by messages on the small signaling channel , SACCH.

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Power Control  Uplink and downlink power control may be applied independently. The initial transmission power of the mobile station and the BTS is chosen by the BSC.

 The BSS computes the required MS transmission power through the reception level measurements performed by the BTS.

 For the downlink , the BTS transmission power is also computed by the BSS for each connection, based on the measurements performed by the MS and reported to the

Power Control Change of values BSC

Initial values BTS

MS

Measurements

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Timing Advance  Once a dedicated channel has been established , the BTS continuously measures the time offset between its own burst schedule and the reception schedule of MS bursts. Based on these measurements , it is able to provide the MS with the required timing advance, on the SACCH. The MS advances its transmission time relative to its basic schedule , by the advance time ( indicated by the infrastructure ).

 The timing advance can take values from 0 to 233µs which is enough to cope with the cells having radius of up to 35km.

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Mobility Management

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MM Procedures  -TMSI reallocation  -Authentication procedure  -Identification procedure  -Abort procedure  -IMSI detach procedure

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 - Location Update Procedure  - Connection management procedures

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Connection Management -Call Control (CC) for GSM services - Short Message Service Support (SMS) for GSM services

- Supplementary Services Support (SS) for GSM services

-Group Call Control for GSM services

Thank you

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