GSM SACCH: Slow Associated Control Channel Explained

This page describes the GSM SACCH (Slow Associated Control Channel). In the full rate traffic channel case, one SACCH burst appears once in every 26 frame multi frame structure. It takes four bursts to transmit complete SACCH information in four consecutive 26 frame MF. It takes about 480ms. One 26 frame MF is of duration 120ms.

SACCH is always available between the Mobile Station (MS) and Base Transceiver Station (BTS) in GSM when a dedicated traffic link is active. The Slow Associated Control Channel is called “associated” because it’s always associated with either a TCH (Traffic Channel) or SDCCH (Standalone Dedicated Control Channel). It also maps to the same physical channel (T0, F0) as TCH/SDCCH, where T0 is the time slot and F0 is the frequency.

In the downlink (BTS to MS), SACCH is used to convey transmit power level control instructions and timing advance information to the GSM mobile. In the uplink (MS to BTS), SACCH carries received signal strength, TCH quality information, and measurement reports of neighboring cells, among other things.

GSM mobiles use SACCH to carry signaling information while connected to the BTS during a call flow. Importantly, SACCH reports neighbor cell information, such as signal level, to the serving cell, which helps in making handover decisions.

SACCH Processing

Here’s an image illustrating SACCH processing:

Figure depicts GSM SACCH data processing through physical layer.

As shown, 184 bits of SACCH information are first passed through fire codes. This fire coder adds 40 parity bits, resulting in 224 fire-coded bits. After this, 4 zero tail bits are added to form 228 bits.

The next module in SACCH processing is a 1/2 rate convolutional encoder, utilizing two polynomials. This results in 456 coded bits. A block interleaver is then used after the convolutional encoder (C.E.), interleaving the data across all four bursts.

Therefore, a total of 456 bits of information are carried by four consecutive bursts after encryption (cyphering) is applied.

As mentioned, SACCH carries TA (Timing Advance), MS PC (Mobile Station Power Control) commands, and measurement reports from the mobile.