GSM SACCH(Slow Associated Control Channel)
This page describes 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 available always between Mobile and Base station(BTS) in GSM when a dedicated traffic link is active.
Slow Associated Control Channel is called as "associated" as this channel is always associated with either TCH or SDCCH. It also maps on the same physical channel(T0,F0) as TCH/SDCCH. T0 is time slot and F0 is frequency.
In the downlink, SACCH is used to convey transmit power level control instructions and timing advance information to the GSM mobile.
In the uplink, SACCH carries received signal strength, TCH quality information and measurement report of neighboring cells etc. GSM mobile carry signaling information using SACCH, while being connected with BTS during call flow.
SACCH reports neighbour cell information such as signal level to the serving cell which helps make handover decision.
Figure depicts GSM SACCH data processing through physical layer. As shown 184 bits are SACCH information is first passed through fire codes. This fire coder will add 40 parity bits resulting into 224 fire coded bits, after which 4 zero tail bits are added to form 228 bits.
The second module in SACCH processing is 1/2 rate convolution encoder of two polynomials. This makes 456 coded bits. The block interleaver is used after C.E. which results into interleaving over all four bursts. Hence total 456 of information is carried by consecutive four bursts after encryption(cyphering) is applied.
As mentioned SACCH carry TA(Timing advance), MS PC (Mobile Station Power control) and measurement response.