GSM traffic and control channels

GSM logical channels are mapped onto the physical TDMA time slots. They are of two kinds: traffic channels (TCH) that carry user speech/data, and control channels (CCH) that carry signalling for synchronisation, access, call setup and mobility.

Traffic channels (TCH)

Carry user speech or data; they use the 26-frame multiframe (24 TCH frames, frame 12 for SACCH, frame 25 idle).

• Full-rate TCH (TCH/F): 22.8 kbps gross. Speech at 13 kbps (TCH/FS); data at 9.6, 4.8, 2.4 kbps (TCH/F9.6, F4.8, F2.4).
• Half-rate TCH (TCH/H): 11.4 kbps gross, uses alternate frames so two users share one slot. Speech at about 6.5 kbps (TCH/HS); data at 4.8 and 2.4 kbps.

Control channels (CCH)

Use the 51-frame multiframe, mostly on time slot 0 of the beacon carrier (C0).

(a) Broadcast Channels (BCH) – downlink only, point-to-multipoint:

• BCCH (Broadcast Control Channel): broadcasts cell identity, location area code, network code, list of neighbour cell frequencies, control channel structure, access parameters.
• FCCH (Frequency Correction Channel): a burst of all zeros that gives a pure tone (carrier + 67.7 kHz); the mobile uses it to correct its oscillator frequency. Sent in frames 0, 10, 20, 30, 40.
• SCH (Synchronization Channel): sent in the frame after each FCCH; carries the BSIC (base station identity code) and the TDMA frame number so the mobile can synchronise with the frame structure.

(b) Common Control Channels (CCCH) – shared, for call setup:

• PCH (Paging Channel), downlink: pages a mobile for an incoming call or SMS.
• RACH (Random Access Channel), uplink: mobile requests a channel (to answer a page or start a call) using slotted ALOHA and the short access burst.
• AGCH (Access Grant Channel), downlink: base station replies to RACH and assigns an SDCCH.
• (CBCH: cell broadcast messages.)

(c) Dedicated Control Channels (DCCH) – bidirectional, for one user:

• SDCCH (Standalone DCCH): signalling during call setup: authentication, ciphering, location update, SMS, before a TCH is assigned.
• SACCH (Slow Associated CCH): tied to a TCH or SDCCH. Downlink: power control level and timing advance; uplink: signal strength/quality reports of serving and neighbour cells (for handoff).
• FACCH (Fast Associated CCH): urgent signalling (e.g. handoff command) sent by stealing TCH bursts; stealing flags mark these bursts.

How they work together (mobile-terminated call)

 MS                                   BTS/Network
 |  FCCH, SCH, BCCH (sync, cell info) <-----|
 |  PCH: page (incoming call)        <-----|
 |-----> RACH: channel request             |
 |  AGCH: SDCCH assigned             <-----|
 |<====> SDCCH: authentication, ciphering, |
 |        call setup messages              |
 |  TCH assigned (on SDCCH/FACCH)    <-----|
 |<====> TCH: speech; SACCH alongside      |
 |  FACCH: handoff command / release <-----|

1. On power-on, the mobile finds FCCH (frequency lock), then SCH (frame sync), then reads BCCH.
2. An incoming call is announced on PCH.
3. The mobile sends a request on RACH; the network assigns an SDCCH on AGCH.
4. Authentication and call setup are done on SDCCH.
5. A TCH is assigned; speech flows, with SACCH carrying measurements and power control.
6. Handoff and release commands go on FACCH.