Attention is currently required from: laforge, fixeria.
falconia has posted comments on this change. (
https://gerrit.osmocom.org/c/libosmocore/+/33063 )
Change subject: coding: gsm0503_tch_{fr,hr}_encode(): add ability to emit BFI
......................................................................
Patch Set 1:
(2 comments)
File src/coding/gsm0503_coding.c:
https://gerrit.osmocom.org/c/libosmocore/+/33063/comment/775d3030_55069b50
PS1, Line 1953: 185
> `sizeof(conv)` - this way it's clear that you're zero-initializing the whole
> buffer.
Will do.
https://gerrit.osmocom.org/c/libosmocore/+/33063/comment/7ff61124_85f3249c
PS1, Line 1955: osmo_con
> but how would this work for EFR? As we can see, this function is used to
> treat EFR and FR cases, dep […]
The BFI-inducing frame of all zeros at the u(k) level is the same for FR and
EFR. Strictly speaking it is neither valid FR nor valid EFR, but that's the
whole point: the intended effect is that the receiving MS will flag it as a
BFI, and that result will happen in both TCH/FS and TCH/EFS channel models. I
have also observed experimentally that what I implemented here is exactly what
sysmoBTS PHY does, and I run with EFR most of the time.
If you are wondering how I did the experimental observation of what sysmoBTS
puts out, I used the TCH DL sniffing feature of FreeCalypso, tapping the DL
path in the MS at the point between channel decoder output and Rx DTX handler
input - that's the a_dd_0[] array in the DSP's accessible NDB page. I see a
frame of 260 zero bits (including EFR) with the channel decoder bit error count
reading zero, but the BFI flag is set, telling me that the CRC3 is inverted.
Normal encoding of all-zeros would put 111 in CRC3, hence it is obvious to me
that the proprietary PHY is generating that bad-CRC3 frame simply by emitting
all zeros at the u(k) level.
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Gerrit-Project: libosmocore
Gerrit-Branch: master
Gerrit-Change-Id: Iade3310e16b906efb6892d28f474a0d15204e861
Gerrit-Change-Number: 33063
Gerrit-PatchSet: 1
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Gerrit-Reviewer: fixeria <[email protected]>
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