GSM control channels and their purposes

GSM control channels carry signalling between the mobile and the network for synchronisation, access, paging, call setup and handoff. They are grouped into three types: Broadcast channels (BCH), Common control channels (CCCH) and Dedicated control channels (DCCH).

                 Control channels
       +----------------+------------------+
      BCH             CCCH               DCCH
  FCCH,SCH,BCCH   PCH,RACH,AGCH    SDCCH,SACCH,FACCH

(a) Broadcast channels – downlink only, to all mobiles

• FCCH (Frequency Correction Channel): an all-zero burst that gives a pure sine wave 67.7 kHz above the carrier. Purpose: the mobile finds the beacon carrier and corrects its local oscillator frequency.
• SCH (Synchronization Channel): sent in the frame after FCCH; carries the BSIC and the reduced TDMA frame number. Purpose: frame synchronisation and identification of the base station.
• BCCH (Broadcast Control Channel): carries cell identity, location area identity, network code, neighbour cell list, control channel configuration and access parameters. Purpose: lets an idle mobile camp on the cell and monitor neighbours.

(b) Common control channels – shared, used for access

• PCH (Paging Channel), downlink. Purpose: alerts a particular mobile (by IMSI/TMSI) of an incoming call or SMS.
• RACH (Random Access Channel), uplink. Purpose: the mobile requests a dedicated channel, using slotted ALOHA and the short access burst (to answer a page or make a call).
• AGCH (Access Grant Channel), downlink. Purpose: answers a RACH request by assigning an SDCCH (or TCH).

(c) Dedicated control channels – one per user, bidirectional

• SDCCH (Stand-alone Dedicated Control Channel). Purpose: signalling before a traffic channel exists – authentication, ciphering, location updating, call setup, SMS.
• SACCH (Slow Associated Control Channel): always paired with a TCH or SDCCH (one frame in each 26-multiframe). Purpose: uplink measurement reports (signal level/quality of serving and neighbour cells); downlink power control and timing advance commands.
• FACCH (Fast Associated Control Channel): steals bursts from the TCH (stealing flags set). Purpose: urgent messages such as handoff commands and call release.