GSM channel structure and TDMA frame efficiency

Channel structure of GSM

GSM has 124 carriers of 200 kHz in GSM-900 (890–915 MHz uplink, 935–960 MHz downlink). Each carrier is divided into 8 time slots; one slot on one carrier is a physical channel (a burst of 156.25 bits every 4.615 ms frame). Logical channels are mapped onto these physical channels.

 Logical channels
 +-- Traffic (TCH): TCH/F 22.8 kbps, TCH/H 11.4 kbps
 +-- Control (CCH)
      +-- BCH : FCCH, SCH, BCCH        (down)
      +-- CCCH: PCH, AGCH (down), RACH (up)
      +-- DCCH: SDCCH, SACCH, FACCH     (both)

• TCH: carry speech (13 kbps full rate, ~6.5 kbps half rate) or data; organised in the 26-frame multiframe (24 TCH, 1 SACCH, 1 idle; 120 ms).
• BCH: FCCH (frequency correction), SCH (frame sync and BSIC), BCCH (cell and network information).
• CCCH: PCH (paging), RACH (random access request, slotted ALOHA), AGCH (channel assignment).
• DCCH: SDCCH (call setup, authentication, location update, SMS), SACCH (measurement reports, power control, timing advance), FACCH (urgent signalling by stealing TCH bursts).
• Control channels use the 51-frame multiframe (235.4 ms) on TS0 of the beacon carrier.

Why TDMA frame efficiency cannot reach 100%

Frame efficiency is the fraction of transmitted bits that carry user data:

η_f = (1 − b_OH / b_T) × 100%

Each normal burst must contain overhead bits that carry no data: 3 + 3 tail bits, 26 training bits and 8.25 guard bits (stealing flags are counted with the data here).

Bits per slot   = 156.25
Overhead/slot   = 6 + 26 + 8.25 = 40.25
Per frame: b_T  = 8 × 156.25 = 1250 bits
           b_OH = 8 × 40.25  = 322 bits
η_f = (1 − 322/1250) × 100% = 74.24 %

(If the 2 stealing flags are also treated as overhead, η = 114/156.25 = 72.96 %.)

Since training bits (for equalization), tail bits (for the decoder) and guard time (for timing differences and power ramping) are essential, b_OH can never be zero, so η < 100%; for GSM η ≈ 74%.