FACCH in GSM

FACCH full form is Fast associated control channel.

FACCH carries important messages. It is used for the exchange of time-critical information between the mobile and the base station during the process of the cell.

The FACCH  is assigned whenever SDCCH is not dedicated to a user and there is an urgent message like handoff request.

The FACCH was transmitted control information by stealing capacity from the associated traffic channel. It is done by setting two special bits called stealing bits. In the TCH forward channel burst.

If a stealing bit is set, the time slot contains FACCH data for that frame.