AIR interface info of GSM

7/29/2019 AIR interface info of GSM

1/45

An Introduction to

Air Interface of GSM


2/45

Evolution Of GSM


3/45

Milestones in GSM


4/45

GSM REFERENCE MODEL


5/45

CM

MM

RR

LAPDm

Layer 1 Layer1

LAPDm

RRBTSM

LAPD

Layer 1 Layer 1

LAPD

BTSM

RR BSSAP

SCCP

MTP MTP MTP

SCCP SCCP

BSSAP TCAP

CM

MMMAP

Um Abis AMSBTS BSC

MSC

V/HLR

Protocol Layer of GSM


6/45

Air Interface Salient Points:

Multiple Access Scheme: TDMA + FDMA

Frequency Bands 900 MHz

Uplink: 890-915 MHz

Downlink: 935-960 MHz

Total No. of Carriers : 124 1800 MHz

Uplink: 1710-1785 MHz


Total No. of Carriers :374

Channel Bandwidth : 200 KHz

Duplex Spacing: 45 MHz

Channels/Carrier: 8


7/45

Traffic Channel Multiframe


8/45

Control Channel Multiframe


9/45

Layers of Protocol Stack

Layer 1 Physical Layer

TDMA/FDMA radio interface on MS to BSS

E1 link on BTS to BSC

MTP Layer1 on BSC to MSC

Layer 2 Data Link Layer

LAPDm used on Air interface

LAPD on BTS to BSC

MTP Layer2 on BSC to MSC


10/45

Layer 3

RR Management Sublayer

Manages the Radio interface.

Terminates at BSS from MS.

MM Sub layer

Terminated at MSC.

Messages from or to MSC are relayed transparently fromBSS.


11/45

CM Sublayer

Terminates at MSC

Contains entities that consist of CC including call-related

supplementary services, SMS, and call independentsupplementary services support (SS).

BTSM

This layer looks after the management of all aspects of radiochannels including radio link layer, control channels andtransceiver management.

SCCP & MTP

SCCP & MTP protocols are used to implement the data linkand Layer 3 transport functions for carrying the call controland mobility management signaling messages on the BSS-MSC Link.


12/45

BSSAP

Used to implement all procedures between the MSC and the BSSthat require interpretation and processing of information related tosingle calls and resource management

TCAP & MAP

- These are the interfaces between the MSC and the HLR/VLR.


13/45

Air Interface

Multiple Access Scheme: TDMA + FDMA

Frequency Bands

Uplink: 890-915 MHz


Channel Bandwidth : 200 KHz

Duplex Spacing: 45 MHz

Total No. of Carriers : 124 Channels/Carrier: 8

Coding on the Air Interface


14/45

MSCA/D

33.85 kbps/user

270.8 kbps/frame

Channel

EncoderInterleaver

Deinter-

leaver

DemodulatorModulator

Error

Correction

Desegmen-

tation

RPE-LTP

decoder

RPE-LTP

encoder

Segmen-

tation

D D

13 kbps

(260 bits/frame)

Duplexers

22.8kbps

456 bits/frame

Coding on the Air Interface


15/45

Coding on the Radio Path

The analog signal is segmented in 20 ms. Segments

Speech coding (RPE-LTP) is used to model the Speechsegments at a data rate of 13 Kbps.

This data is error protected by channel encoding, thusincreasing bit rate to 22.8 Kbps

Interleaving performed to reduce burst errors. Depth of8 is used for interleaving.

This data is used for modulating the carrier. GMSK isused as modulation technique.

TDMA F d B t St t


16/45

TDMA Frame and Burst Structure


17/45

Frame Structure in Air Interface


18/45

Organization of Bursts, TDMA Frames, and

Multiframes for Speech and Data


19/45

Logical Channel Structure in GSMLogical Channels

Full Rate 22.8kbps

Half Rate 11.4kbps

Speech

9.6 kbps

4.8 kbps

2.4 kbps

Data

TCHtraffic

Duplex

FCCHfreq correction

Downlink

SCH

syncronizationDownlink

BCCHBroadcast Control

Downlink

BCHBroadcast

PCHpaging

Downlink

AGCH

access grantDownlink

RACHrandom access

Uplink

CCCHcommon

FACCHfast associated

Duplex

SACCH

slow associatedDuplex

SDCCHstand - alone

Duplex

DCCHdedicated

CCHcontrol


20/45

Logical Control Channels


21/45

Burst Types in GSM


22/45

Traffic Channels

Gross bit rate 22.8 kbps

Net bit rate at 13 kbps for Speech

Gross bit rate at 12, 6, or 3.6 kbps for Data

Transmission rates of the Data services

compatible to the existing 9.6, 4.8, and 2.4 kbpsPSTN & ISDN services

TCH/F


23/45

TCH/H

Gross bit rate of 11.4 kbps

Net bit rate at 5.6 kbps for Speech

Net bit rate at 6 or 3.6 kbps for Data

Generally TCH/H is not used.


24/45

FCCHThis downlink channel continuously transmits

unmodulated carrier frequency for frequency

correction of the MS.

SCHUsed for frame synchronization of MS

Contains information about TDMA frame

number and BSIC.

Broadcast Control Channels


25/45

BCCH

Broadcasts general information about the cell. viz

Neighbour Cell information

MS transmit power maximum on CCHs

Local Area Identification


26/45

Common Control Channels

PCH

Used to page the MS during mobile terminated

call setup.

Paging messages are sent to the area where the

recipient MS is located.

AGCH


27/45

AGCH

Used for sending access grant messages which

assign the MS to a SDCCH or directly to TCH in

response to a request placed on RACH by MS.

RACHUsed to transfer uplink messages in response to the

call initiation request placed by the MS or in

response to a paging request or automatically aspart of location update


28/45

Dedicated Control Channels

FACCHUsed as a main signaling link for the transmission of

data e.g. handover commands.Required for every call setup and release

FACCH is transmitted in the burst stealing mode bypre-empting a portion of the speech/user data

information bits during active call.


29/45

SACCH

Uplink SACCH is used by the MS to report the radio

strengths of the surrounding cells and receive quality of

the serving cell TCH.

Downlink SACCH is used to transmit some common

information also present on BCCH. Thus the MS,

while on TCH, can receive the relevant broadcast

information inspite of being unable to monitor theBCCH.


30/45

SDCCH

A duplex pt-to-pt channel which is used forsignaling.

Carries all signaling between the BTS and the

MS when no TCH is allocated.

Used for service requests (e.g SMS), locationupdates, subscriber authentication, ciphering

initiation, equipment validation and assignment toa TCH


31/45

Logical Channel to Physical

Channel Mapping

Logical channels FCCH,SCH,BCCH,CCCH are time

division multiplexed on carrier 0 (C0) in time slot 0

(T0).

C0 T0 is always a broadcast control channel and is

always transmitting.

Remaining 7 time slots of C0 can be used for traffic.

T ffi Ch l M ltif St t


32/45

Traffic Channel Multiframe Structure

24 slots used for traffic out of 26

Slot 12 used for SACCH

Slot 25 is empty


33/45

FCCH and SCH

One SCH slot follows each FCCH slot 8 BPs later

Each channel uses 5 slots in each 51 multiframe.

Common Channels Multiframe Structure


34/45

BCCH and PAGCH

BCCH is repeated 4 times every 51 multiframe

PAGCH ( PCH + AGCH) is repeated 36 times every 51 multiframe

Common Channels Combination


35/45

Common Channels Combination


36/45

Types of Bursts in GSM

Access Burst

Used by the RACH when no channel parameters are known.

41 synch bits, 36 data bits and 68.25 guard bits

8 TB 8 TB41 SYNCHRONIZATION BITS 36 DATA BITS 68.25 GP

Synchronization Burst


37/45

Synchronization Burst

Used by BTS for synchronization with MS.

Used by SCH.

Frequency Correction Burst

Used by the FCCH.

Contains all data bits as 0.

Generates a sine wave which is used for frequency correction.

8 TB

8 TB8 TB

8 TB64 SYNCHRONIZATION BITS39 DATA BITS 39 DATA BITS 8.25 GP

8.25 GP142 FIXED BITS (ALL ZEROS)

Normal Burst


38/45

Normal Burst

Has 114 bits of data.

Used by all other channels except those mentioned above.

Has training sequence which is used for identifying the correct signal

from all the multi path signals

Stealing bits are used by FACCH for using the TCH.

3 TB 57 DATA BITS 57 DATA BITS26 TRAINING

SEQUENCE

STEALING BITS

3 TB 8.25 GP


39/45

Dummy Burst

Transmitted when no other bursts are to be transmitted

Used for signal quality monitoring.

8.25 GP3 TB 3 TB26 TRAINING

SEQUENCE


40/45

Call Setup Scenarios

Mobile Originated Call


41/45


42/45


43/45

Mobile Terminated Calls


44/45


45/45

Date post:	04-Apr-2018
Category:	Documents
Upload:	raja
View:	220 times
Download:	0 times

AIR interface info of GSM

Documents