Channel Concepts

Channel Concepts In GSM System www.huawei.com

GBSS Training Team Copyright © 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

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

Page2



BSS Feature Description



BSS Signaling Analysis Manual



BSC Technical Manual

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Page3

Chapter 1 Channel Concepts Chapter 2 Bursts Chapter 3 Frames

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Channel Concepts Downlink

Uplink

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

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Physical and Logical Channel 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

0

1 2

Timeslot

3 4

5 6

7 0

1 2

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

TDMA FRAME Copyright © 2006 Huawei Technologies Co., Ltd. All rights reserved.

TDMA FRAME Page6

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

TCH/HS

TCH Traffic Channel TCH/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

Data

TCH/9.6

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TCH/2.4

TCH/4.8 Page8

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.

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Page9

Control Channel (CCH) CCH (Control Channels) DCCH SDCCH

FACCH

BCH

ACCH

SACCH

Broadcast Control Channel – BCCH Common Control Channel – CCCH Dedicated Control Channel – DCCH Associated Control Channel – ACCH

BCCH CCCH RACH

SCH

CBCH

PCH/AGCH

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Page10

Synch. CH.

FCCH

BCH Channels 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.

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CCCH 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.

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Page12

DCCH 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.

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Page13

Uplink Logical channel

RACH

CCCH

CCH

SDCCH SACCH

DCCH

FACCH TCH/F TCH/H

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TCH

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DCH

Downlink Logical channel FCCH

CCH

BCCH

SCH BCCH

CCCH

PCH AGCH

SDCCH

DCH

DCCH

SACCH FACCH

TCH

TCH/F TCH/H

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Page15

How to use these channels? Power-off

Search for frequency correction burst

FCCH

Search for synchronous burst

SCH

Extract system information

BCCH

Monitor paging message

PCH

Send access burst

RACH

Allocate signaling channel

AGCH

Set up the call

SDCCH

Allocate voice channel

SDCCH

Conversation

TCH

Release the call

FACCH

Idle mode

Dedicated mode

Idle mode

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Chapter 1 Channel Concepts Chapter 2 Bursts Chapter 3 Frames

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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 FRAME1(4.615ms)

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FRAME2

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0.577ms 0.546ms 3 Guard Tail Period Bits

57 bits Data

26 bits Flag Training Bit sequence

57 bits Flag Bit

Data

3 Tail Guard Bits Period (8.25 bits)

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

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

Page19

Normal Burst 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.

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Page20

Frequency Correction Burst FRAME1(4.615ms)

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FRAME2

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0.577ms 0.546ms 3 Guard Tail Period Bits

• • •

142 bits

3

Fixed Data

Carries FCCH channel. Made up of 142 consecutive zeros. Enables MS to correct its local oscillator locking it to that of the BTS.

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Page21

Tail Guard Bits Period (8.25 bits)

Synchronisation Burst FRAME1(4.615ms)

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FRAME2

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0.577ms 0.546ms 3

39 bits

Guard Tail Encrypted Bits Period Bits

• • •

64 bits Synchronisation Sequence

39 bits Encrypted Bits

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

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Page22

3 Tail Guard Bits Period (8.25 bits)

Access Burst FRAME1(4.615ms)

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FRAME2

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0.577ms 8

41 bits

Guard Tail Synchronization Bits Period Bits

• •

36 bits

3

68.25 bits

Encrypted Sequence

Tail Bits

Guard Period

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

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Page23

6

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Dummy Burst FRAME1(4.615ms)

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FRAME2

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0.577ms 0.546ms 3 Guard Tail Period Bits

•

57 bits Data

26 bits Flag Training Bit sequence

57 bits Flag Bit

Data

3 Tail Guard Bits Period

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

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Page24

Chapter 1 Channel Concepts Chapter 2 Bursts Chapter 3 Frames

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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.

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Page26

GSM Multi-frames TDMA Frames 0

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

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46 47 48 49 50

51 – Frame Multi-frames CONTROL CHANNELS

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GSM Multi-frames TDMA Frames 0

1

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

0 1 2 3 4

21 22 23 24 25

26 – Frame Multi-frames TRAFFIC CHANNELS

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Hyperframe and Superframe Structure 3h 28min 53s 760ms 0

1 6.12s 1

0

1 Hyperframe = 2048 superframes = 2,715,648 TDMA frames 2

2045

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120ms 1

2047

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

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2046

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235.38ms 23

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Traffic 26 - Frame Multiframe

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Control 51 - Frame Multiframe 4.615ms 0

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TDMA Frame Page29



Basic Idea about various channels used in GSM.



Different types of bursts and frames.



Structure of bursts and frames.

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