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GSM frame structure and channels during a call

Short answer

GSM's TDMA frame has 8 slots of 4.615 ms each, with a normal burst containing tail bits, 57-bit data halves, a 26-bit training sequence, and a guard period. For a mobile-originated call, the MS synchronizes via FCCH/SCH/BCCH, requests a channel on RACH, gets an SDCCH for authentication and setup, then traffic channel TCH with SACCH reports and FACCH for handover.

Draw and explain the frame structure for GSM. Describe how various traffic and control channels are used while making a call in GSM system.

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.

Use of traffic and control channels while making a call

Mobile-originated call (user dials a number):

 MS                                   Network
 |<--- FCCH, SCH, BCCH (sync, info) ----|
 |---- RACH: channel request ---------->|
 |<--- AGCH: SDCCH assigned ------------|
 |<--> SDCCH: authentication, ciphering,|
 |          setup (called number)       |
 |<--- SDCCH: TCH assignment -----------|
 |<==> TCH: speech  (+ SACCH reports)   |
 |<--- FACCH: handoff / release --------|
  1. FCCH – the mobile finds the beacon carrier and corrects its frequency.
  2. SCH – it reads BSIC and frame number and synchronises.
  3. BCCH – it reads cell/network information and camps on the cell.
  4. RACH – it sends a channel request (access burst, slotted ALOHA).
  5. AGCH – the network assigns an SDCCH.
  6. SDCCH – authentication (RAND/SRES), ciphering start, and call setup messages with the dialled number; the network then assigns a TCH.
  7. TCH – speech flows at 13 kbps (22.8 kbps coded); SACCH runs with it carrying measurement reports (uplink) and power control/timing advance (downlink).
  8. FACCH – steals TCH bursts for urgent signalling such as handoff commands and call disconnection.

For a mobile-terminated call, the only change is that the network first sends a page on PCH; the mobile answers on RACH and the rest is the same.

✓ Answer checked by Claude Opus 5.5 (high effort)

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