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GSM BCH and CCCH operation

Short answer

BCH are downlink broadcast channels on TS0: FCCH gives frequency correction, SCH gives synchronization and BSIC, BCCH carries system information. CCCH are shared: PCH pages mobiles, RACH carries uplink access requests, AGCH grants dedicated channels. They let idle mobiles camp and initiate calls.

Explain the operation of the Broadcast control channel (BCH) and Common control channel (CCCH) in GSM.

Broadcast Control Channels (BCH)

BCH are downlink-only, point-to-multipoint channels sent on TS0 of the beacon carrier (C0) in the 51-frame multiframe. They let an idle mobile find the cell, synchronise and read cell information.

 51-MF on TS0 (downlink), frames:
 0  1  2-5   6-9  10 11 ...  40 41 ... 50
 F  S  BCCH  CCCH  F  S  ...  F  S  ... idle
 (F=FCCH, S=SCH)
  1. FCCH (Frequency Correction Channel) – an all-zero burst which, with GMSK, gives a pure tone 67.7 kHz above the carrier. The mobile detects it to find the beacon and lock its oscillator frequency. Sent in frames 0, 10, 20, 30, 40.
  2. SCH (Synchronization Channel) – the frame after each FCCH. Carries the BSIC (base station identity code) and the TDMA frame number, so the mobile aligns its timing with the frame/multiframe structure.
  3. BCCH (Broadcast Control Channel) – carries system information: cell identity, location area identity, network code, neighbour cell BCCH list, CCCH configuration, cell selection and access parameters.

Operation: on switch-on the mobile scans carriers, finds FCCH, then reads SCH, then BCCH, and "camps" on the best cell. In idle mode it keeps reading BCCH and measuring neighbours listed there.

Common Control Channels (CCCH)

CCCH are shared by all mobiles in the cell and are used to set up a connection. They also sit on TS0 of C0 (frames not used by BCH).

  1. PCH (Paging Channel) – downlink. The network pages a mobile (by IMSI or TMSI) for an incoming call or SMS. Each mobile listens only to its paging group, saving battery (discontinuous reception).
  2. RACH (Random Access Channel) – uplink. The mobile sends a channel request (short access burst with long guard time since timing advance is not yet known) using slotted ALOHA. Used to answer a page, start a call, update location or send SMS.
  3. AGCH (Access Grant Channel) – downlink. The network replies to the RACH request and assigns a dedicated channel (SDCCH), also giving the initial timing advance.
 Network --PCH: page-->        MS
 Network <--RACH: request--    MS
 Network --AGCH: SDCCH no.-->  MS
 then signalling continues on SDCCH

(CBCH, the cell broadcast channel, also uses CCCH resources for broadcast text messages.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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