Channel Concepts

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Channel Concepts In GSM System

GBSS Training Team


After the completion of this

course, the trainees should

understand about:

Types of Channels in GSM

Different Burst and frame

definition


BSS Feature Description

BSS Signaling Analysis

Manual

BSC Technical Manual


Chapter 1 Channel ConceptsChapter 1 Channel Concepts

Chapter 2 BurstsChapter 2 Bursts

Chapter 3 FramesChapter 3 Frames


Physical channel - Each timeslot on a carrier is referred to as a physical channel. Per carrier there are 8 physical channels.

Logical channel - Variety of information is transmitted between the MS and BTS. There are different logical channels depending on the information sent. The logical channels are of two types

• Traffic channel• Control channel

Downlink

Uplink

Channel Concepts


2 30 16 74 52 30 1

TDMA FRAME

TimeslotThe information carried in one time

slot is called a “burst”

The information carried in one time slot is called a “burst”

Physical and Logical Channel

TDMA FRAME

The physical channel is the medium over which the information is carried: 200KHz and 0.577ms

The logical channel consists of the information carried over the physical channels


Two types of Logical Channel

Traffic Channel (TCH) :

Transmits traffic information, include data and speech.

Control Channel (CCH) :

Or Signaling Channel, transmits all kinds of control information.


Traffic Channel (TCH)

TCHTraffic Channels

Speech

TCH/FS

Data

TCH/HSTCH/9.6 TCH/2.4

TCH/4.8

TCH Traffic ChannelTCH/FS Full rate Speech Channel TCH/HS Half rate Speech Channel TCH/9.6 Data Channel 9.6kb/sTCH/4.8 Data Channel 4.8kb/sTCH/2.4 Data Channel 2.4Kb/s


Traffic Channel (TCH)

User Data Transmission:

Speech and data (not SMS) are transmitted using traffic

channel (TCH).

Full Rate (TCH): Transmits full rate speech

(13Kbits/sec). A full rate TCH occupies one physical

channel.

Half Rate (TCH/2): Transmits half rate speech

(6.5Kbits/sec). Two half rate TCH’s can share one

physical channel, thus doubling the capacity of a cell.


FCCHSCH

CCH (Control Channels)

DCCH

SDCCH

BCH

BCCH Synch. CH.ACCH

SACCHFACCH CCCH

RACH CBCH

PCH/AGCH

Broadcast Control Channel – BCCHCommon Control Channel – CCCHDedicated Control Channel – DCCHAssociated Control Channel – ACCH

Control Channel (CCH)


BCCH( Broadcast Control Channel )

• Downlink only

• Broadcasts general information of the serving cell called System Information

• BCCH is transmitted on timeslot zero of BCCH carrier

• Read only by idle mobile at least once every 30 sec.SCH( Synchronization Channel )

• Downlink only

• Carries information for frame synchronization. Contains TDMA frame number and BSIC.

FCCH( Frequency Correction Channel )

• Downlink only.

• Enables MS to synchronies to the frequency.

• Also helps mobiles of the n cells to locate TS 0 of BCCH carrier.

BCH Channels


RACH( Random Access Channel )

• Uplink only• Used by the MS to access the Network.

AGCH( Access Grant Channel )

• Downlink only• Used by the network to assign a signaling channel upon

successful decoding of access bursts.

PCH( Paging Channel )

• Downlink only.• Used by the Network to contact the MS.

CCCH Channels


SDCCH( Standalone Dedicated Control Channel )

• Uplink and Downlink

• Used for call setup, location update and SMS.

SACCH( Slow Associated Control Channel )

• Used on Uplink and Downlink only in dedicated mode.

• Uplink SACCH messages - Measurement reports.

• Downlink SACCH messages - control info.

FACCH( Fast Associated Control Channel )

• Uplink and Downlink.

• Associated with TCH only.

• Is used to send fast messages like handover messages.

• Works by stealing traffic bursts.

DCCH Channels


RACH CCCHCCH

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

DCH

Uplink Logical channel


FCCH

SCH

BCCH

PCH

AGCH

BCCH

CCCH

CCH

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

DCH

Downlink Logical channel


Allocate signaling channel

Power-off

Idle mode

Dedicated mode

Idle mode

How to use these channels?

Search for frequency correction burst

Search for synchronous burst

Extract system information

Monitor paging message

Send access burst

Set up the call

Allocate voice channel

Conversation

Release the call

SCH

FCCH

BCCH

PCH

RACH

AGCH

SDCCH

SDCCH

TCH

FACCH


BurstsBurst:

• Unit of transmission ( about hundred modulated bits )• Have finite duration• Occupy a finite number of spectrum• Sent in time and frequency windows ( Slots )

Burst Types:

• Normal Burst• Frequency Correction• Synchronization Burst• Access Burst• Dummy Burst


Normal Burst

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

57 bits 57 bits26 bits 33

FRAME1(4.615ms) FRAME2

Training sequence

Data DataTailBits

TailBits

FlagBit

FlagBit

GuardPeriod

GuardPeriod

(8.25 bits)

0.546ms0.577ms

• Carries traffic channel and control channels BCCH, PCH, AGCH, SDCCH, SACCH and FACCH.


Data - Two blocks of 57 bits each. Carries speech, data or control info.Tail bits - Used to indicate the start and end of each burst. Three bits always 000.

Guard period - 8.25 bits long. The receiver can only receive and decode if the burst is received within the timeslot designated for it.Since the MS are moving. Exact synchronization of burst is not possible practically. Hence 8.25bits corresponding to about 30us is available as guard period for a small margin of error.

Flag bits - This bit is used to indicate if the 57 bits data block is used as FACCH.

Training Sequence - This is a set sequence of bits known by both the transmitter and the receiver( BCC of BSIC). When a burst of information is received the equalizer searches for the training sequence code. The receiver measures and then mimics the distortion which the signal has been subjected to. The receiver then compares the received data with the distorted possible transmitted sequence and chooses the most likely one.

Normal Burst


0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

142 bits 33


Fixed DataTailBits

TailBits

GuardPeriod

GuardPeriod

(8.25 bits)

0.546ms0.577ms

• Carries FCCH channel.

• Made up of 142 consecutive zeros.

• Enables MS to correct its local oscillator locking it to that of the BTS.

Frequency Correction Burst


0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

39 bits 33


SynchronisationSequence

TailBits

TailBits

GuardPeriod

GuardPeriod

(8.25 bits)

0.546ms0.577ms

64 bits 39 bits

EncryptedBits

EncryptedBits

• Carries SCH channel.

• Enables MS to synchronies its timings with the BTS.

• Contains BSIC and TDMA Frame number.

Synchronisation Burst


0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

41 bits8


EncryptedSequence

TailBits

GuardPeriod

GuardPeriod

0.577ms

36 bits 68.25 bits

SynchronizationBits

• Carries RACH and FACCH channels.

• Used for Random and Handover access

Access Burst

3

TailBits


0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

57 bits 57 bits26 bits 33


Training sequence

Data DataTailBits

TailBits

FlagBit

FlagBit

GuardPeriod

GuardPeriod

0.546ms0.577ms

• Transmitted on the unused timeslots of the BCCH carrier in the downlink.

Dummy Burst


Frames

26 TDMA frame multiframe: Used to carry

traffic channels. TCH,SACCH and FACCH.

51 TDMA frame multiframe: Used to carry

control channels BCCH,CCCH,SDCCH and

SACCH.


TDMA Frames

0 1

0 1 2 43 46 47 48 5049

51 – Frame Multi-frames

0 1 10

CONTROL CHANNELS

2 3 4 5 6 7 2 3 5 764

GSM Multi-frames


GSM Multi-frames

TDMA Frames

0 1

0 1 2 43 21 22 23 2524

26 – Frame Multi-frames

0 1 10

TRAFFIC CHANNELS

2 3 4 5 6 7 2 3 5 764


00 11

00 11 22 20452045 20462046 20472047

1 Hyperframe = 2048 superframes = 2,715,648 TDMA frames3h 28min 53s 760ms

1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr) multiframes

11 2 33 494847 50

00 11 2424 2525

00 11 22 2323 2424 2525 00 484811 22 4949 5050

22 33 44 55 66 77

6.12s

0

235.38ms120ms

Control 51 - Frame MultiframeTraffic 26 - Frame Multiframe

4.615ms

TDMA Frame

Hyperframe and Superframe Structure


Basic Idea about various channels used

in GSM.

Different types of bursts and frames.

Structure of bursts and frames.

Thank youwww.huawei.com

Channel Concepts

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