Unbounded AGCH queue in OsmoBTS

Hi all, We're moving this discussion to the mailing list, as it seems it is more generic and complex than we've thought initially. The issue arose when I started doing load testing of the OsmoTRX transceiver and disabled all gating in it. As a result, all incoming noise was processed as valid Normal Bursts and Access Bursts and sent up to OsmoBTS. This leads to a situation, similar to a RACH flood, when there are more RACH requests coming, than a BTS could reasonably process. And this leads to an unbounded increase of the AGCH queue in the BTS - it consumes a few Mb per minute. I think that this is the root cause of the issue we've seen at a Netherlands festival installation, when 20K phones suddenly started connecting to our station after official networks went down. When the amount of RACH requests exceeded available CCCH capacity (took <5 seconds), mobile phones stopped answering out IMM.ASS messages. Hypothesis is that the AGCH queue became so long, requests were sent too late for a phone to receive it. And thus no phones answered to our IMM.ASS messages. Unfortunately, I wasn't able to collect enough data to check this hypothesis during that time and we don't have another big festival on hands atm. An attached is a quick fix for the unbounded queue growth. It uses a hardcoded value for the maximum queue length, which is fine for our load testing, but not flexible enough for the real life usage. We should make the AGCH queue long enough to keep high performance. At the same time, it must not exceed MS timeout or _all_ IMM.ASS messages will miss their target MS's. We could make this parameter user-configurable on a BTS side, but it seems more reasonable to automatically calculate it, depending on the channel combination and timeout values. But this should be done on the BSC side. So the questions are: 1) what is a good way to calculate it? 2) should we configure this queue length over OML, or move the queue from BTS to BSC? -- Regards, Alexander Chemeris. CEO, Fairwaves LLC / ООО УмРадио http://fairwaves.ru