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Bug #4487

closed

revisit fn-advance / rts-advance default settings

Added by laforge over 2 years ago. Updated over 2 years ago.

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

100%

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.


Files

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

Related issues

Related to libosmocore - Bug #4074: LAPD timers completely brokenIn Progressmsuraev06/21/2019

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