Feature #4796

Downlink Repeated FACCH Support

Added by laforge 18 days ago. Updated about 9 hours ago.

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Spec Reference:
3GPP TS 44.006 Section 10


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.

Related issues

Related to OsmoBTS - Feature #4794: Downlink Repeated SACCH supportNew10/09/2020


#1 Updated by laforge 18 days ago

  • Related to Feature #4794: Downlink Repeated SACCH support added

#2 Updated by laforge 15 days ago

#3 Updated by laforge 11 days ago

See also: Chapter 7.3 of "GSM/EDGE Evolution and Performance", which also includes a detailed case study on handover performance.

#4 Updated by laforge 11 days ago

  • Tracker changed from Bug to Feature

#5 Updated by dexter about 12 hours ago

  • File GP-042668_Enhanced_ACCH.pdf added

#6 Updated by dexter about 9 hours ago

  • Assignee set to dexter

#7 Updated by dexter about 9 hours ago

  • File deleted (GP-042668_Enhanced_ACCH.pdf)

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