A traffic channel (TCH) is used to carry speech and data traffic. Traffic channels are defined using a 26-frame multi frame, or group of 26 TDMA frames. The length of a 26-frame multi frame is 120ms. Out of the 26 frames, 24 are used for traffic, 1 is used for the slow associated control channel (SACCH) and 1 is currently unused.
Full Rate & Half Rate TCH
They can be defined as full-rate TCHs (TCH/F, 22.8 kbps) and half-rate TCHs (TCH/H, 11.4 kbps). Half-rate TCHs double the capacity of a system effectively by making it possible to transmit two calls in a single channel. If a TCH/F is used for data communications, the usable data rate drops to 9.6 kbps (in TCH/H: max. 4.8 kbps) due to the enhanced security algorithms. Eighth-rate TCHs are also specified, and are used for signaling. In the GSM Recommendations, they are called stand-alone dedicated control channels (SDCCH)
Signaling channels
The signaling channels on the air interface are used for call establishment, paging, call maintenance, synchronization, etc.
There are three type of signaling channels
- Broadcast Channels
- Common Control Channels
- Dedicated Control Channel
Broadcast Channels (BCH)
Carry only downlink information and are responsible mainly for synchronization and frequency correction. This is the only channel type enabling point-to-multipoint communications in which short messages are simultaneously transmitted to several mobiles
BCH Characteristics
• Each cell has a designated BCH carrier
• All BCH timeslots transmit continuously on full power
• TS 0 contains logical control channels
• TS1-7 optionally carries traffic
• BCCH block occur once each 51-frame multiframe
• Each block comprises 4 frames carrying 1 message
The BCHs include the following channels;
- Broadcast Control Channel (BCCH): General information, cell specific (local area code (LAC), network operator, access parameters, list of neighboring cells, etc). The MS receives signals via the BCCH from many BTSs within the same network and/or different networks.
- Frequency Correction Channel (FCCH): Downlink only; correction of MS frequencies; transmission of frequency standard to MS; it is also used for synchronization of an acquisition by providing the boundaries between timeslots and the position of the first timeslot of a TDMA frame.
- Synchronization Channel (SCH): Downlink only; frame synchronization (TDMA frame number) and identification of base station. The valid reception of one SCH burst will provide the MS with all the information needed to synchronize with a BTS
Common Control Channels (CCCH)
A group of uplink and downlink channels between the MS and the BTS. These channels are used to convey information from the network to MSs and provide access to the network. The CCCHs include the following channels;
1. Paging Channel (PCH): Downlink only; the MS is informed by the BTS for incoming calls via the PCH
2. Access Grant Channel (AGCH): Downlink only, BTS allocates a TCH or SDCCH to the MS, thus allowing the MS access to the network.
3. Random Access Channel (RACH): Uplink only, allows the MS to request an SDCCH in response to a page or due to a call; the MS chooses a random time to send on this channel. This creates a possibility of collisions with transmissions from other MSs
Dedicated Control Channels (DCCH)
Responsible for roaming, handovers, encryption, etc. The DCCHs include the following channels;
- Stand-alone Dedicated Control Channel (SDCCH); Communications channel between MS and the BTS; signaling during call setup before a traffic channel (TCH) is allocated
- Slow Associated Control Channel (SACCH); Transmits continuous measurement reports in parallel to operation of a TCH or SDCCH
- Fast Associated Control Channel (FACCH); Similar to the SDCCH, but used in parallel to operation of the TCH; if the data rate of the SACCH is insufficient, “borrowing mode” is used: Additional bandwidth is borrowed from the TCH; this happens for messages associated with call establishment authentication of the subscriber, handover decisions, etc.
