openbsc[master]: DTX: store time instead of fn

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Harald Welte gerrit-no-reply at lists.osmocom.org
Fri Sep 16 13:27:20 UTC 2016


Patch Set 1: Code-Review-2

this is IMHO the completely wrong way.  The only relevant time base to anything in the GSM PHY is the TDMA time clock based on the frame number.  Anything else is irrelevant.

Yes, GSM voice frames are sampled from a real-time clock every 20ms.  Yes, the SID (in the BTS->PHY->Um->MS direction) needs to be sent nominally every 160ms.  But no, it is not _exactly_ 160ms, but it is of course at the time when the respective (8th) PH-RTS.req of the PHY is happening, which correpsonds to more or less 160ms later.  Due to the multiframe scheduling of the 26-multiframe, it is not always _exactly_ 160ms wall-clock time, but has some jitter.  On average, it is 160ms.

In the MS->Um->PHY->BTS->RTP direction, it is similar.  The MS will (have to) send a SID every 8 codec frames, which is on average 160ms, but not _exactly_. It is every 8 codec frames, as defined by the GSM multiframe structure.

Yes, that means that we will not neccessarily send a RTP SID every 160ms wall clock time.  But it is the same as with the regular voice-carrying codec frames.  nominally they appear every 20ms, and they contain RTP timestamps incrementing in 20ms intervals.  But the exact time the RTP is sent is not 20ms according to wall-clock time, but whenever the PHY reports a codec frame via PH-DATA.ind.

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Gerrit-MessageType: comment
Gerrit-Change-Id: Ia46836d20357b3cfb3ad94596f1c28e3159fe394
Gerrit-PatchSet: 1
Gerrit-Project: openbsc
Gerrit-Branch: master
Gerrit-Owner: Max <msuraev at sysmocom.de>
Gerrit-Reviewer: Harald Welte <laforge at gnumonks.org>
Gerrit-Reviewer: Jenkins Builder
Gerrit-Reviewer: Max <msuraev at sysmocom.de>
Gerrit-Reviewer: Neels Hofmeyr <nhofmeyr at sysmocom.de>
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