This week I had to make parts of OpenBSC work with TCH/H and use AMR. This work is needed for On Waves and when I say parts I mean the strict BSC subset of OpenBSC (in contrast to make the MSC code we have work as well).

The first part is to make TCH/H work and that was easy as LaF0rge did almost everything to make it work. You have to change the OpenBSC configuration to use TCH/H instead of TCH/F for the given timeslots. The next thing was to make channel assignment work. The Mobile Station (MS) comes on the Random Access Channel (RACH) and is asking for a channel and gives a random number (so it can identify the response). Now depending on a global indicator (NECI) the MS will ask for different channels.

So the next step was to add a NECI configuration to our VTY configuration code and then change the code that decodes the channel request to know about the NECI and pick the right channel. On top of that a small hack to assign a TCH/H in case of a MS requesting “any” channel as part of paging.

Now that TCH/H should work one has to focus on the speech. GSM 08.08 and GSM 04.08 have different enums for speech. GSM 08.08 is differenting speech version 1,2,3 for full and half rate totalling in six different values, for GSM 04.08 there is a TCH mode that includes speech version 1,2,3, various data modes and signalling (but no differentation full/half rate channel). After getting this right and selecting speech version 3 it still didn’t work. It turned out that one has to fillout the optional Multirate Configuration when one is using speech version 3. This multi rate configuration needs to be present in the GSM 04.08 RR Assignment Command, Modify Channel but also in the RSL messages for Modify Request and Channel Assignment.

After this AMR on a TCH/H should work (when the BTS is supporting it too). The next step for someone else is to make the MSC code in OpenBSC work with TCH/H and other audio codecs. This would require to stop to ask for a TCH/F, change the channel requested decoding again..