Status of Traffic channel

Dieter Spaar spaar at mirider.augusta.de
Fri Aug 13 10:40:43 UTC 2010


Hello,

here is some information about the current status of traffic
channel support in Layer-1:

  - first of all its not yet complete and considered "alpha"
    because there are some stability problems which have to
    be solved and some further enhancments for a more general
    approach are needed.

  - What works: Signalling (FACCH/SACCH) for a Full Rate Traffic
    channel and voice (Full Rate or Enhanced Full Rate Codec).

  - TODO: The Layer-1 API has to be extended to allow switching
    the channel mode (e.g. Signalling only) or the Voice Codec.
    Also turning the audio path on and off is needed.

  - TODO: The Rx/Tx TPU Window has been modified to support Rx and Tx
    operation in the same frame. This always happens for a Traffic
    Channel but can sometimes also happen for other channels. For
    those cases its necessary that the Tx TPU Window is set
    differently. Some sort of state information that Rx is happening
    in the same frame has to be implemented, this currently only
    works for traffic channels.

If you want to try it out you have to use the "dieter/tch_f" branch
for Layer-1 and the Master branch for Layer-23 from Andreas (really
great work, Andreas).

With this combination you can use it with OpenBSC. Some minor
adjustment is needed: The Layer-1 code currently sets the voice
Codec to "Full Rate", however OpenBSC expects "Enhanced Full Rate"
You either have to modify OpenBSC to set "Full Rate" or change
"TCH_FS_MODE" to "TCH_EFR_MODE" in "layer1/prim_rx_nb.c", there
is only one line which has to be changed.

What you can do now is a MOC/MTC between OsmocomBB and another
phone. Sometimes it can happen that the firmware hangs, you have
to restart the phone in this case. However if you have a connected
call, its working pretty good (one of the test calls was over 20
minutes).

Maybe Andreas can give a short introduction how to use Layer-23
for this, I am not sure if there is some information in the Wiki
yet.

Best regards,
  Dieter
-- 
Dieter Spaar, Germany                           spaar at mirider.augusta.de




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