GSM Channels

GSM Channels

• Physical Channel– One time slot on one carrier is called physical

channel.

• Logical Channel– Information carried by physical channels is called

logical Channels.– Logical channels are mapped on physical channels.

Logical Channels

• Traffic channels: Used for speech and data– Full Rate(TCH/F) – Half Rate(TCH/H)

• Control channels: Used for signaling .i.e. setting up a radio connection, call or controlling an MS during conversation– BCH(Broadcast channels)– CCCH(common control channels)– DCCH(dedicated control channels)

Traffic Channels(TCH)

TCH/F(full Rate)

TCH/H(half Rate)

Traffic Channels(TCH)

Control Channels(CCH)

CCH(Control Channel)

BCH CCCH DCCH

CCH RACH CBCH SDCCH ACCHSynch.Chanels

SACCHFACCHPCH/

AGCHFCCHSCH

BCH(Broadcast Channels)

• BCCH(Broadcast Control Channels)– Downlink Only.– Broadcast information of the serving cell

(System Information).– Transmitted on timeslot zero of BCCH carrier.– Reads only by idle mobile at least once every

30 secs.

BCH(Broadcast Channels) cont’d

• SCH(Synchronisation Channels)– Downlink Only– Carries information for frame synchronisation.– Contains frame number and BSIC(Base Station

Identity Code).

BCH(Broadcast Channels) cont’d

• FCCH(Frequency Correction Channels)– Downlink Only.– Enable MS to synchronies to the frequency.

CCCH(Common Control Channel)

• RACH(Random Access Channel)– Uplink only.– Used by the MS when making its first access

to the Network.– The reason for access could be initiation of a

call or a page response.

CCCH(Common Control Channel) cont’d

• AGCH(Assess Grant Channel)– Downlink only.– Used for acknowledgement of the access

attempt sent on RACH.– Used by the network to assign a signaling

cannel upon successful decoding of access bursts.

CCCH(Common Control Channel) cont’d

• PCH(Paging Channel)– Downlink only.– The network will page the MS ,if there is a

incoming call or a short Message.– It contains the MS identity number, the IMSI or

TMSI.

DCCH(Dedicated Control Channel)

• SDCCH (Stand-alone Dedicated Control Channel)– Uplink and Downlink.– Used for call setup, authentication, ciphering

location update and SMS.

DCCH(Dedicated Control Channel) cont’d

• SACCH(Slow Associated Control Channel)– Downlink and Uplink.– Used to transfer signal while MS have ongoing

conversation on traffic or while SDCCH is being used.

– On the forward link, the SACCH is used to send slow but regularly changing control information to each mobile on that ARFCN, such as power control instructions and specific timing advance instructions

• SACCH(Slow Associated Control Channel) cont’d– The reverse SACCH carries

information about the received signal strength and quality of the TCH, as well as BCH measurement results from neighboring cells.

DCCH(Dedicated Control Channel) cont’d

• FACCH(Fast Associated Control Channel)– Downlink and uplink.– Associate with TCH only.– It is used to send fast message like hand over

message.– Work by stealing traffic bursts.

Mapping on Physical Channels

• The Logical channels are mapped on the physical channels.

• The TDMA frames are grouped together into multi-frame.– 26 TDMA multi-frame for Traffic.– 51 TDMA multi-frame for control signal.

Channel Combination

• Combined – All the controlling signals are in the time slot 0

of the Multi-frame.

• Non Combined– Dedicated controlling signals are in time slot 1

of the Multi-frame.

Combined

• Cell with single carrier.– Timeslot 0 :BCCH+CCCH+SDCCH.– Timeslot 1-7 :TCH/FACCH+SACCH.

Non Combined

• Cell with Two carrier– Timeslot 0 (of carrier 1) BCCH+CCCH.– Timeslot 1 (of carrier1) SDCCH+SACCH.– Timeslot 2-7 & 0-7(of both carriers)

TCH/FACCH+SACCH.

Combined Multi-frame

• Mapping of All Control Channel on  Timeslot 0 of The multi-frame.– BCCH+CCCH+SDCCH in Timeslot 0

BCCH/CCH Combined Multi-frame Downlink

A1

A0

D3

D2

D1

D0

A2

A3

D3

D2

D1

D0

0

10

20

30

40

50

51

61

71

81

91

101

IDEL

SCH

CCCH

BCCH

SACCH

SDCCH

FCCH

Mapping of logical channel

on  Timeslot 0

BCCH/CCH Combined Multi-frame Uplink

D2

D1

D0

A3

A1

D3

51

101D2

D1

D0

D3

A0

A2

0

10

40

50

30

20

 

SACCH

SDCCH

FACH

Mapping of logical channel on  Timeslot 0

NON Combined Multi-frame

Mapping of All Control Channels on  

Timeslot 0 and 1of The multi-frame.– BCCH+CCCH in Timeslot 0 – SDCCH+SACCH in Timeslot 1

BCCH/CCH Non combined Multi-frame Downlink

0

10

20

30

40

50

FCCH

Idle

CCCH

SCH

BCCH

Mapping of logical channel On Timeslot 0

BCCH/CCH Non combined Multi-frame Uplink

RACH

0

50

10

20

30

40

Mapping of logical Channel on Timeslot 0

DCCH Non combined Multi-frame Downlink

A3

A2

A1

A0

D7

D6

D5

D4

D3

D2

D1

D0

0

4

8

12

16

20

24

28

32

36

40

44

48

50

A7

A6

A5

A4

D7

D6

D5

D4

D3

D2

D1

D0

51

55

59

63

67

71

75

79

83

87

91

95

99101

SACCH

IDLE

SDCCH

Mapping of logical channel on Timeslot 1

DCCH Non combined Multi-frame uplink

A2

A1

A0

D7

D6

D4

D5

A1

D3

D2

D1

D0 D0

A7

A6

A5

A4

D7

D6

D5

D4

D3

D2

D1

IDLE

SDCCH

4

8

12

15

19

23

27

31

35

39

43

47

50

84

55

59

63

66

70

74

78

82

88

92

96

101

51

3

Mapping of logical channel on Timeslot 1

SACCH