Downlink Repeated FACCH Support
In 3GPP Rel-7 (?) of GERAN, the concept of "repeated FACCH" was introduced.
The rationale for SACCH improvement can be found in 3GPP TDoc GP-042668 Section 1/2 (even though if later sections have not been implemented as suggested there): Particularly with AMR as a voice codec, the voice quality performance is better than that of control channels (and estimated 5dB).
So in the end, even though the voice channel would still be acceptable, calls fail due to signaling failure, both on SACCH and FACCH.
Downlink repeated FACCH basically works by re-transmitting FACCH frames in downlink even before T200 expires. (this obviously steals additional voice frames, as FACCH always does). Those retransmissions happen 8/9 TDMA frames after the original transmission, i.e. after 33/37ms.
- LAPDm commands can always be re-transmitted even without the MS indicating any support.
- LAPDm responses can only be re-transmitted if the MS indicates the "Repeated ACCH Capability"
As outlined in Section 2 of GP-042668, it is expected that this would particularly help hand-over performance in frequency hopping networks, as the Handover Command there often requires two FACCH blocks to be correctly received by the MS.