DL TBF abnormal release

historical

Dear Jacob,

Finally I managed to get everything in a working order, so I can finally start testing the GPRS part of OpenBSC/Osmo-PCU with an USRP B200.

I am using a Cat 12 device with a 4 PDCH configuration.

In the past I wrote a little script to test the PRACH stability of OpenBTS, which sends a single ping to the gateway address, waits until the session goes into idle, and then tries again. The goal is to generate as many PRACH as possible and check if there is a response.

I used this script and it turns out that around 1 out of every 20 requests the ping is not reaching the gateway, and the UE indicates an abnormal DL TBF release.

I attached the UE side log, so you can take a look.

During this issue there is no reasonable frequency nor timing variation indicated on the UE side.

What I noticed is the very high number of RLC resent and RLC restarted. After a few minutes of pinging, this is the result from the PCU:

 RLC Sent             :    18015 (0/s 0/m 0/h 0/d)
 RLC Resent           :    17596 (0/s 0/m 0/h 0/d)
 RLC Restarted        :    15669 (0/s 0/m 0/h 0/d)
 RLC Stalled          :        0 (0/s 0/m 0/h 0/d)

Please note that the ARFCN I use is completely clean, I filter the uplink with proper duplexer and the TRX is not complaining about anything.

Also interesting that during constant pinging (the system does not need to do PRACH), this problem is not popping up.

Will update my UHD stack from 3.8.0 to 3.9.0 just to test with the latest, but I don't think itt will change anything about this issue. And to be hones, we've seen similar PRACH related issues with OpenBTS GPRS too.

If you need any more logs or you have an idea what to try, I am happy to do that. :-)

Regards,
Csaba
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