GSM FACCH: Fast Associated Control Channel Explained

This page explains the GSM FACCH (Fast Associated Control Channel). In GSM, the FACCH is utilized to exchange information between a GSM Mobile Station (MS) and a Base Transceiver Station (BTS) more rapidly than what’s achievable with the SACCH.

FACCH information messages can be transmitted swiftly in a 20 ms duration, which is the same as the traffic channel (TCH). The SACCH, on the other hand, is typically transmitted once every 26 frames. So, when rapid information conveyance is needed, the FACCH comes into play.

A single signalling block of the FACCH replaces one TCH speech block. When higher data rates become necessary, the TCH time slot is often “stolen” to be used by the FACCH. The FACCH is employed in both the uplink and downlink directions.

The figure depicts FACCH channel information through the physical layer.