GSM Channel are divided into two types:
Traffic channels (TCHs):
The traffic channes are intended to carry encoded speech or user data.
Traffic channels are intended to carry encoded speech and user data.
Full rate traffic channels at a net bit rate of 22.8 Kb/s (TCH/F)
Half rate traffic channels at a net bit rate of 11.4 Kb/s (TCH/H)
Speech channels are defined for both full rate and half rate traffic channels.
Data channels support a variety of data rates (2.4, 4.8 and 9.6 Kb/s) on both half and full rate traffic channels. The 9.6 Kb/s data rate is only for full rate application.
The traffic channels(TCH) support two types of information rates Full rate (TCH/F) and Half rate (TCH/H)
When transmitted as full rate, the user data is occupied within TS per frame. When transmitted as half rate, the user data is occupied into the same time slot but sent in alternate frames.
The 26th frame contains idle bits if full rate TCHs are used and contains SACCH data if half rate TCHS are used
- Full Rate TCH for data and speech channels:-
A. Full - rate Speech Channel )TCH/Fs): At 16 kbps the full rate speech channel is digitized. The full rate speech channel caries 55.8kbps after adding the GSM channel coding to the digitized speech.
B. Full-rate Data Channel for 9600 bps (TCH/F9.6): The full rate traffic data channel contains raw data that is transmitted at 9.6 kbps. After the application of additional forward error correction coding with the GSM standards, 9600 kbps is transferred at 22.8 kbps.
C. Full-rate Data Channel for 4500 bps(TCH/F4.8): The full rate traffic date channel contains data that is transmitted at 4.8 Kbps. After the application of additional forward error correction coding with GSM standards, the 4.8 kbps is transferred at 22.8 kbps.
D. Full Rate Data Channel for 2400 bps (TCH/F2.4): The full rate traffic data channel contains raw data that is transmitted at 2.4 kbps. After the application of additional forward error correction coding with GSM standards, the 2.4 kbps data is transferred at 22.8 kbps.
. Half Rate TCH for data and speech channels:
A. Half Rate Speech Channels (TCH/HS): The half tate speech channel can carry digitized speech that is sampled at a rate half that of full rate channel. GSM anticipates the availibility of speech coders. It can digitize speech at about 6.5 kbps. After adding GSM channel coding to the digitized speech, the half rate Speech channel will carry 11.4 kbps.
B. Half Rate Data Channel for 4800 bps (TCH/H4.8): The half rate traffic data channel carries raw data that is sentat 4800 bps. After the application of forward error correction using GSM standards, 4800 bps data is sent at 11.4 kbps.
C Half Rate Data Channe for 2400 kbps (TCH/H 2.4): The half rate traffic data channel carries raw user data that is sent at 2400 bps. After application of additional forward error corretion using GSM standards, 2400 bps data is sent to 11.4 bps.
Control Channel (CCH):
. Control channels carry signaling information between an MS and a BTS.
a) Broadcast control channel:
. Broadcast control channels are transmitted in downlink direction only i.e. only transmitted by BTS.
. The broadcast channels are used to broadcast synchronization and general network information to all the MSs within a cell.
. It has three types:
a. FREQUENCY CORRECTION CHANNEL (FCCH):
. Used for the frequency correction / synchronization of a mobile station.
. The repeated (every 10 sec) transmission of Frequency Bursts is called FCCH.
b. SYNCHRONISATION CHANNEL (SCH):
. Allows the mobile station to synchronize time wise with the BTS.
. Repeated broadcast (every 10 frames) of Synchronization Bursts is called (SCH).
c. BROADCAST CONTROL CHANNEL (BCH):
. The BROADCAST CONTROL CHANNEL (BCCH) is used to broadcast control information to every MS within a cell.
. This information includes details of the control channel configuration used at the BTS, a list of the BCCH carrier frequencies used at the neighboring BTSs and a number of parameters that are used by the MS when accessing the BTS.
Common Control Channel:
The common control channels are used by an MS during the paging and access procedures. Common control channels are of three types.
(PCH) PAGING CHANNEL:
. Within certain time intervals the MS will listen to the Paging channel, PCH, to see if the network wants to get in contact with the MS.
. The reason could be an incoming call or an incoming Short Message.
2.(RACH) RANDOM ACCESS CHANNEL:
- If listening to the PCH, the MS will realize it is being paged.
- The MS answers, requesting a signalling channel, on the Random Access channel, RACH.
- RACH can also be used if the MS wants to get in contact with the network, e/g. when setting up a mobile originated call.
3.(AGCH) ACCESS GRANTED CHANNEL:
- The access grant channel (AGCH) is carried data which instructs the mobile to operate in a particular physical channel (Time slot or ARFCN).
- It uses normal burst.
C) Dedicated Control Channels (DCCHs):
- Signaling information is carried between an MS and a BTS using associated and dedicated control channels during or not during a call, They are of three types:
A. (SDCCH STAND ALONE DEDICATED CONTROL CHANNEL:
Non-urgent information, e.g. transmitter power control, is transmitted using the slow associated control channel (SACCH).
On the uplink MS sends averaged measurements on own base station (signal strength and quality) and neighboring base stations (signal strength).
On the downlink the MS receives system information, which transmitting power and what timing advance to use. It is transmitted at 13thFrame of TCH. As seen, SACCH is transmitted on both up-and downlink, point-to-point.
It uses normal burst.
B. (SAACH)SLOW ASSOCIATED CONTROL CHANNEL:
In some situations, signaling information must flow between a network and an MS when a call is not in progress, e.g. during a location update.
This could be accommodated by allocating either a full-rate or half-rate TCH and by using either the SACCH or FACCH to carry the information.
C. (FACCH) FAST ASSOCIATED CONTROL CHANNEL:
More urgent information, e.g. a handover command, is sent using time slots that are 'stolen' from the traffic channel.
If, suddenly, during the conversation a handover must be performed the Fast Associated Control channel, FACCH, is used.
FACCH works in stealing mode, meaning that one 2. ms segment of speech is exchanged for signaling information necessary for the handover.