Cellular Network Infrastructure » Radio Access Network » OsmoBTS

Patch implementing DELETE IND is in https://gerrit.osmocom.org/6875 but we also need to reduce the extremely high queue length of 1000 entries.

FYI: I'm observing a rate of about 13 IMM.ASS messages (mac blocks) per second on my combined SDCCH/4 cell with BS-AG_BLKS-RES=1. This means a queue with 1000 entires would take 76 seconds to drain. Many higher-layer timers in the BSC will long have released a given channel by that time.

One idea would be to use 200 entries, like paging (equalling 15 seconds in above case). This is still quite long, but much more reasonable. Probably make sense to aling with some of the assignment related timers in the BSC.

Patch https://gerrit.osmocom.org/#/c/osmo-bts/+/9515/ enables sending DELETE IND for messages dropped in the BTS once already been queued. Also, as per https://gerrit.osmocom.org/#/c/osmo-bts/+/9514/ we also drop messages different than imm ass reject, so now all dropped messages should send a DELETE IND. Unit test agch_test shows indeed a bigger number of rejected messages and as a result the queue takes less time to drain.

Also hard limit of 1000 was dropped to 100 in https://gerrit.osmocom.org/#/c/osmo-bts/+/9513/

Number is selected according to agch_test output table, 100 being slightly bigger than any biggest value found there:

Testing AGCH queue length computation.
T               BCCH slots
        1(NC)   1(C)    2(NC)   3(NC)   4(NC)
3       20      9       20      20      20
4       28      11      28      28      28
5       40      13      40      40      40
6       59      19      59      59      59
7       79      25      79      79      79
8       21      9       21      21      21
9       28      12      28      28      28
10      40      13      40      40      40
11      60      20      60      60      60
12      80      26      80      80      80
14      22      10      22      22      22
16      30      13      30      30      30
20      42      14      42      42      42
25      63      21      63      63      63
32      83      28      83      83      83
50      28      14      28      28      28

I close this task as the required bits have been implemented.

• the number of IMM.ASS on Um
• the number of RSL DELETE IND on Abis
  when sending IMM.ASS at different rates.

I would expect us to write such a patch whenever introducing a new feature or fixing an issue.