ORG010005 SiteMaster Introduction ISSUE 1.0.pptChannel Concepts In
GSM System
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.
BSS Feature Description
Chapter 1 Channel Concepts
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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
2
3
0
1
6
7
4
5
2
3
0
1
slot is called a “burst”
Physical and Logical Channel
the information is carried: 200KHz and 0.577ms
The logical channel consists of the information
carried over the physical channels
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
Two types of Logical Channel
Traffic Channel (TCH) :
Control Channel (CCH) :
Traffic Channel (TCH)
TCH/9.6 Data Channel 9.6kb/s
TCH/4.8 Data Channel 4.8kb/s
TCH/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.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
FCCH
SCH
BCCH( Broadcast Control Channel )
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 )
Carries information for frame synchronization. Contains TDMA frame
number and BSIC.
FCCH( Frequency Correction Channel )
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 )
AGCH( Access Grant Channel )
Downlink only
Used by the network to assign a signaling channel upon successful
decoding of access bursts.
PCH( Paging Channel )
CCCH Channels
SDCCH( Standalone Dedicated Control Channel )
Uplink and Downlink
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.
Works by stealing traffic bursts.
DCCH Channels
SDCCH
SACCH
FACCH
TCH/F
TCH/H
DCCH
TCH
DCH
SDCCH
SACCH
FACCH
TCH/F
TCH/H
DCCH
TCH
DCH
How to use these channels?
Allocate signaling channel
Search for synchronous burst
Chapter 1 Channel Concepts
Bursts
Burst:
Have finite duration
Sent in time and frequency windows ( Slots )
Burst Types:
Normal Burst
Frequency Correction
Synchronization Burst
Access Burst
Dummy Burst
Normal Burst
0.546ms
0.577ms
Carries traffic channel and control channels BCCH, PCH, AGCH,
SDCCH, SACCH and FACCH.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
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
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
Contains BSIC and TDMA Frame number.
Synchronisation Burst
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
Used for Random and Handover access
Access Burst
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
3
3
FRAME1(4.615ms)
FRAME2
Training
sequence
Data
Data
Tail
Bits
Tail
Bits
Flag
Bit
Flag
Bit
Guard
Period
Guard
Period
0.546ms
0.577ms
Transmitted on the unused timeslots of the BCCH carrier in the
downlink.
Dummy Burst
Chapter 1 Channel Concepts
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.
Copyright © 2006 Huawei Technologies Co., Ltd. All rights
reserved.
GSM Multi-frames
TDMA Frames
GSM Multi-frames
TDMA Frames
0
1
0
1
2
2045
2046
2047
3h 28min 53s 760ms
1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr)
multiframes
1
2
3
49
48
47
50
0
1
24
25
0
1
2
23
24
25
0
48
1
2
49
50
2
3
4
5
6
7
6.12s
0
235.38ms
120ms
Basic Idea about various channels used in GSM.
Different types of bursts and frames.
Structure of bursts and frames.
Thank you