When Harald Welte visited me for the Osmocom User Group meeting, he took the chance to experiment with OpenBSC and my RBS2206, a large GSM BTS in a cabinet. This inspired me to do some more investigations to get the RBS2206 actually run with OpenBSC including voice calls.

When OpenBSC configures a channel for voice, the RBS closes the channel after a few seconds with a "remote transcoder failure" error. I found some information which seems to indicate that there are O&M TRAU frames involved to signal the usage of the correct codec between BTS and TRAU. So I started to analyze the traffic between the RBS and the Racal 6113, a BTS tester which is able to run the RBS including voice calls. To analyze the traffic, I used a Siemens/Tektronix K1103 which can trace E1/T1 traffic.

To my surprise I did not find any O&M TRAU frames in the trace. After doing some more experiments it turned out that the RBS expects to receive speech TRAU frames of the same type (e.g. FR or EFR) as soon as it starts to transmit speech TRAU frames after the channel was configured for voice. So far OpenBSC sends Idle Speech frames as long as the voice channel is not yet connected to the other phone. This worked fine for the Siemens BS-11 or Nokia MetroSite/InSite BTS, but obviously did not work for the Ericsson RBS.

After adjusting the idle TRAU frame generation in OpenBSC with a quick workaround and solving some minor problems when configuring the RBS (depending on the software versions in the RBS some configuration messages are slightly different) I was finally able to run OpenBSC including voice calls with the RBS2206 over an E1 line and an RBS2401 over T1. The modifications are not yet in OpenBSC, but should be there soon after cleaning them up.