17706026 Channel Concepts

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Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved.

Channel Concepts

In GSM System

GBSS Training Team


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Page2Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved.

After the completion of this

course, the trainees should

understand about:

Types of Channels in GSM

Different Burst and frame

definition


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BSS Feature Description

BSS Signaling Analysis

Manual

BSC Technical Manual


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Chapter 1 Channel Concepts

Chapter 1 Channel Concepts

Chapter 2 BurstsChapter 2 Bursts

Chapter 3 FramesChapter 3 Frames


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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


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2 30 16 74 52 30 1

TDMA FRAME

Timeslot The information carried in one timeslot 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


7/31Page7Copyright 2006 Huawei Technologies Co., Ltd. All rights reserved.

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.


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Traffic Channel (TCH)

TCH

Traffic 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/s

TCH/4.8 Data Channel 4.8kb/s

TCH/2.4 Data Channel 2.4Kb/s


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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 TCHs can share one

physical channel, thus doubling the capacity of a cell.


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FCCHSCH

CCH (Control Channels)

DCCH

SDCCH

BCH

BCCH Synch. CH.ACCH

SACCHFACCH CCCH

RACH CBCH

PCH/AGCH

Broadcast Control Channel BCCH

Common Control Channel CCCH

Dedicated Control Channel DCCH

Associated Control Channel ACCH

Control Channel (CCH)


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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


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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


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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


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RACH CCCHCCH

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

DCH

Uplink Logical channel


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FCCH

SCH

BCCH

PCH

AGCH

BCCH

CCCH

CCH

SDCCH

SACCH

FACCH

TCH/F

TCH/H

DCCH

TCH

DCH

Downlink Logical channel


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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


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Chapter 1 Channel ConceptsChapter 1 Channel Concepts




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Bursts

Burst:

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


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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

sequenceData Data

Tail

Bits

Tail

Bits

Flag

Bit

Flag

Bit

Guard

Period

Guard

Period(8.25 bits)

0.546ms0.577ms

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

FACCH.


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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 theburst 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 thetransmitter 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


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

142 bits 33


Fixed Data

Tail

Bits

Tail

Bits

Guard

Period

Guard

Period(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


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

39 bits 33


Synchronisation

Sequence

Tail

Bits

Tail

Bits

Guard

Period

Guard

Period

(8.25 bits)

0.546ms0.577ms

64 bits 39 bits

Encrypted

Bits

Encrypted

Bits

Carries SCH channel. Enables MS to synchronies its timings with the BTS. Contains BSIC and TDMA Frame number.

Synchronisation Burst


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

41 bits8


Encrypted

Sequence

Tail

Bits

Guard

Period GuardPeriod

0.577ms

36 bits 68.25 bits

Synchronization

Bits

Carries RACH and FACCH channels. Used for Random and Handover access

Access Burst

3

Tail

Bits


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

57 bits 57 bits26 bits 33


Training

sequenceData Data

Tail

Bits

Tail

Bits

Flag

Bit

Flag

Bit

Guard

Period

Guard

Period

0.546ms0.577ms

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

Dummy Burst


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Chapter 1 Channel ConceptsChapter 1 Channel Concepts




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Frames

26 TDMA frame multiframe: Used to carry

traffic channels. TCH,SACCH and FACCH.

51 TDMA frame multiframe: Used to carrycontrol channels BCCH,CCCH,SDCCH and

SACCH.


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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


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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


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0 1

0 1 2 2045 2046 2047

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

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

1 2 3 494847 50

0 1 24 25

0 1 2 23 24 25 0 481 2 49 50

2 3 4 5 6 7

6.12s

0

235.38ms120ms

Control 51 - Frame MultiframeTraffic 26 - Frame Multiframe

4.615ms

TDMA Frame

Hyperframe and Superframe Structure


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Basic Idea about various channels used

in GSM.

Different types of bursts and frames.

Structure of bursts and frames.


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17706026 Channel Concepts