TCH_H and TCH_F

Neels Hofmeyr nhofmeyr at sysmocom.de
Thu Jul 21 15:33:15 UTC 2016


I have this problem:

DCC <0001> gsm_04_08.c:1649 Cannot patch through call with different channel types: local = TCH_F, remote = TCH_H

(At first I thought it was a bug in dynamic TCH_F/TCH_H/PDCH, but it's actually
about any situation where both TCH/H and TCH/F are available. Tested with master:)

If I configure the BSC to have both TCH_H and TCH_F slots, my two test phones
try to call each other with mismatching TCH types. The CHANNEL REQUIRED
messages send TCH/H and TCH/F, respectively.

phones: Samsung S4 vs. Sony Ericsson SE K800i.

If the CN is configured with only TCH/H or only TCH/F, the two phones talk to
each other fine.

So is this a missing feature in openbsc? In this sense: If one side started off
in TCH/H, the other side should be urged towards TCH/H as well.

This breaks the dynamic TS feature #1776: with fully dynamic TS, we're
implicitly allowing both TCH/H and TCH/F, so if an MS asks for a TCH/F, it will
get one before even considering a TCH/H, even if that mismatches the MS peer.

So we should fix openbsc to "clamp" the callee to the caller's pchan type.

We could also offer a limited TCH/H_PDCH type (or some other config to disable
TCH/F on dyn TS, for example).

~Neels

-- 
- Neels Hofmeyr <nhofmeyr at sysmocom.de>          http://www.sysmocom.de/
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