Bug #5518
closedosmo-bts-trx: Downlink FACCH/H scheduling is not working properly
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Description
It was reported by a customer that no voice can be heard during the emergency calls, while the normal MO/MT calls are working fine. The MS originating an emergency call indicates RxQual=7 in the RR Measurement Reports, whereas the Uplink measurements generated by the BTS indicate RxQual=0. This problem is specific to TCH/H, full rate channels are not affected.
The key difference from normal calls is that emergency calls are established via the early assignment: a traffic channel gets activated in signalling mode and after the call establishment it gets modified to one of the speech modes. Also, the same problem with no voice can be reproduced in a setup with no SDCCH channels - this would force the BSC to use early assignment even for the normal calls.
After several hours of experiments with USRP B210 and after reading the source code, I found out that scheduling of the Downlink FACCH on TCH/H is completely broken. Attached you can find a PCAP file containing A-bis/RSL traces and GSMTAP Um frames showing the problem. One can see LAPDm errors and retransmissions during the call establishment. I tried downgrading osmo-bts down to 1.2.0 (Jan 2020) without any luck.
A reasonable question is why didn't ttcn3-bts-test catch this? Because we use trxcon as the MS side scheduler, which was written by looking at osmo-bts-trx as the reference implementation. There can be similar bugs in there, so in this regard we're testing the implementation against itself. Most likely it needs to be fixed too.
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