GSM frame structure, traffic and control channels

Frame structure of GSM

GSM uses FDMA + TDMA: each 200 kHz carrier is divided into 8 time slots, gross bit rate 270.833 kbps (bit period 3.69 µs).

 Hyperframe = 2048 superframes = 3 h 28 min 53.76 s
        |
 Superframe = 26 x 51-MF = 51 x 26-MF = 6.12 s
        |
 +------------------+--------------------+
 26-multiframe       51-multiframe
 (traffic) 120 ms    (control) 235.4 ms
        |
 TDMA frame = 8 slots = 4.615 ms
 | TS0 | TS1 | TS2 | TS3 | TS4 | TS5 | TS6 | TS7 |
        |
 Time slot = 156.25 bits = 576.9 µs

Normal burst (one time slot):

|T |  Data  |S| Training |S|  Data  |T |Guard|
|3 |   57   |1|    26    |1|   57   |3 |8.25 |

• Tail bits (3+3): all zeros; let the equalizer/Viterbi decoder start and end in a known state, and give time for power ramp.
• Data (2 × 57): coded speech/data bits.
• Stealing flags (1+1): show whether the data half-burst carries traffic or has been "stolen" for FACCH signalling.
• Training sequence (26): known pattern in the middle of the burst, used by the adaptive equalizer to estimate the channel.
• Guard period (8.25 bits ≈ 30 µs): prevents overlap of bursts from mobiles at different distances.

Other bursts: frequency correction burst (FCCH, all zeros), synchronisation burst (SCH, long 64-bit training), access burst (RACH, short, long guard of 68.25 bits), dummy burst.

• 26-multiframe (120 ms): frames 0–11 and 13–24 TCH, frame 12 SACCH, frame 25 idle.
• 51-multiframe (235.4 ms): carries FCCH, SCH, BCCH, CCCH and SDCCH control channels on TS0 of the beacon carrier.

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.

Working during a call: the mobile syncs using FCCH/SCH and reads BCCH; an incoming call is paged on PCH; the mobile requests access on RACH and gets an SDCCH on AGCH; authentication and setup run on SDCCH; then speech flows on TCH with SACCH reports, and handoff commands come on FACCH.