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
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Gerrit-Change-Id: Iade3310e16b906efb6892d28f474a0d15204e861
Gerrit-Change-Number: 33063
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