Fast-Associated and Slow-Associated Dedicated Control Channels (FACCH, SACCH) are used for supervisory data transmission between mobile station and the base station during a call.
A. Fast-Associated Dedicated Control Channel (FACCH):
FACCH carries urgent messages and contains essentially the same type of information as the SDCCH.
A FACCH is assigned whenever a SDCCH has not been dedicated for a particular user and thjere is an urgent message such as handoff request.
The FACCH gains access to a time slot by ‘stealin’ frames from the traffic channel to which it is assigned. This is done by setting two special bits calles staeling bits in a TCH forward channel burst, if stealing bits are set the time slot is known as to contain FACCH data not a TCH for that frame.
B. Slow-Associated Dedicated Control Channel (SACCH):
The SACCH is always associated with a Traffic Channel or a SDCCH and mapps onto the same physical channel. Thus each ARFCN systematically carries SACCH data for all of its cuurent users.
The SACCH carries general unformation between the MS and BTS. On the forward link the SACCH is used to send slow but regularly changing control information to the mobile such as transmit pwer level instructions and specific timing advance instructions from each user on the ARFCN. The reverse SACCHN carries information about the receiveed signal strength and the quality of TCH, as well as BCH measurement results from neighbouring cells.
The SACCH is transmitted during the 13th frame (and the 26th frame (when half rate traffic is used) of every speech or dedicated contril channel multiframe and within this frame the eight time slots are dedicated to providing SACCH data to each of the full-rate 9 or 16 half-rate) users on ARFCN.
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