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Bug #4487

revisit fn-advance / rts-advance default settings

Added by laforge about 2 months ago. Updated 24 days ago.

Status:
In Progress
Priority:
Normal
Assignee:
Category:
osmo-bts-trx
Target version:
-
Start date:
04/07/2020
Due date:
% Done:

0%

Spec Reference:

Description

We currently use a fn-advance default of of 20 frames, and a rts-advance of 5, resulting in a total of 25 frames (equalling 115ms) of downlink frame nubmer advance.

This will cause
  • significantly increased RTT for GPRS user plane data
  • increase latency of RLC/MAC signaling, specifically
    • tbf establishment
    • potentially cause window stalls if we don't poll for ACK/NACK a lot sooner than our window filling up.
  • probably mess with LAPDm timing

I would guess that on modern hardware, particularly with SCHED_RR on TRX + BTS, we can reduce the fn_advance drastically. The rts_advance likely needs to remain in place without too many changes, as this is the amount of time the PCU has to prepare downlink data (i.e. schedule DL).

As a second step, we could possibly even think of something like a dynamically sized fn-advance, similar to dynamic jitter buffers work in RTP.

Screenshot_20200408-183109.png View Screenshot_20200408-183109.png 196 KB osmo-bts-trx ping with fn-advance 20 daniel, 04/08/2020 04:46 PM
Screenshot_20200408-183218.png View Screenshot_20200408-183218.png 276 KB osmo-bts-trx ping with fn-advance 3 daniel, 04/08/2020 04:46 PM
4080
4081

History

#1 Updated by daniel about 2 months ago

4080
4081

So far on my laptop I reduced fn-advance to 3 and pings look a lot better.

#2 Updated by daniel about 2 months ago

https://gerrit.osmocom.org/c/osmo-bts/+/17766

Please test with those or even lower values and report back what still works.

#3 Updated by laforge 24 days ago

  • Assignee changed from daniel to pespin

pespin, please take over

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