Gsm channel types
There are two types of GSM logical channels, called traffic channels (TCH) and control channels (CCH). Traffic channels carry digitally encoded user speech or user data and have identical functions and formats on both the forward and reverse link. Control channels carry signaling and synchronizing commands between the base station and the mobile station.
GSM Traffic Channels (TCH):
- GSM traffic channels may be either full-rate or half-rate and may carry either digitized speech or user data.
- When transmitted as full-rate, user data is contained within one TS per frame, when transmitted as half-rate, user data is mapped onto the same time slot, but is sent in alternate frames.
- TCH data is transmitted in consecutive frames. Each group of twenty-six consecutive TDMA frames is called a multiframe.
- For every twenty-six frames, the thirteenth and twenty-sixth frames consist of Slow Associated Control Channel (SACCH) data, or the idle frame, respectively.
- The twenty-sixth frame contains idle bits for the case when full-rate TCHs are used, and contains SACCH data when half-rate TCHs are used.
- Some of the full rate and half rate channels are :
- Full-Rate Speech Channel (TCHIFS): Carries the full-rate speech at 22.8 kbps and user speech which is digitized at a raw data rate of 13 kbps.
- Full-Rate Data Channel for 9600 bps (TCH/F9.6): carries raw user data which is sent at 9600 bps.With additional FEC data is sent at 22.8 kbps.
- Full-Rate Data Channel for 4800 bps (TCH/F4.8): carries raw user data which is sent at 4800 bps. With additional FEC data is sent at 22.8 kbps.
- Full-Rate Data Channel for 2400 bps (TCH/F2.4): carries raw user data which is sent at 2400 bps With additional FEC data is sent at 22.8 kbps.
- Half-Rate Speech Channel (TCH/HS): With GSM channel coding added to the digitized speech, the half-rate speech channel will carry 11.4 kbps.
- Half-Rate Data Channel for 4800 bps (TCH/H4.8): carries raw user data which is sent at 4800 bps. With additional FEC data is sent at 11.4 kbps
- Half-Rate Data Channel for 2400 bps (TCHIH2.4): carries raw user data which is sent at 2400 bps. With additional FEC data is sent at 11.4 kbps
Control channels (CCH):
- There are three main control channels in the GSM system. These are :
- Broadcast control channel (BCCH): A BTS uses this channel to signal information to all MSs within a cell.
- The BTS sends information for frequency correction via the frequency correction channel (FCCH) and information about time synchronization via the synchronization channel (SCH), where both channels are subchannels of the BCCH.
- Common control channel (CCCH): All information regarding connection setup between MS and BS is exchanged via the CCCH. For calls toward an MS, the BTS uses the paging channel (PCH) for paging the appropriate MS.
- If an MS wants to set up a call, it uses the random access channel (RACH) to send data to the BTS.
- The BTS uses the access grant channel (AGCH) to signal an MS that it can use a TCH or SDCCH for further connection setup.
- Dedicated control channel (DCCH): There are three types of dedicated control channels in GSM, and, like traffic channels.
- They are bidirectional and have the same format and function on both the forward and reverse links.
- The three channels are: SDCCH - For signalling exchanges, e.g. during call setup, registration / location updates. FACCHs - FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst. Function not clear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e. using a "SDCCH-like" channel for other purposes than signalling). SACCHs - SDCCH in-band signalling, e.g. for link monitoring.