abis_rsl: Fix TCH-as-SDCCH allocation on Channel Request
On rsl_rx_chan_rqd(), so far osmo-bsc tried to preferably assign the lchan type
that was asked for in the RACH. Firstly, this contained a bug, and secondly,
it does not make sense to heed that preference, since we do late assignment.
Ignore the preference for the MS' TCH kind.
We do late assignment to avoid codec mismatches. In the "old days", we would
heed the MS' TCH channel kind, even if the MSC or BSC didn't actually allow
or prefer that channel kind. Hence, in the presence of both TCH/F and TCH/H,
the MS could ask for TCH/F (which we would grant on the MO side) and the BSC
or MSC could prefer TCH/H (which we would apply on the MT side), and hence
fabricate a codec mismatch. Instead, since quite some time now, we *always*
assign an SDCCH first, and only later on do another Assignment to hand out
a proper voice lchan that heeds the MS capability bits as well as MSC's and
BSC's preferences.
By completely ignoring the channel kind the MS asked for in the RACH, we
also eliminate this bug in rsl_rx_chan_rqd():
- If the first "lchan_select_by_type(GSM_LCHAN_SDDCH)" fails (all SDDCH in use),
we should try to fall back to any TCH instead, to serve as SDCCH.
- the first "if (!lchan && lctype != GSM_LCHAN_SDCCH)" was an attempt to prefer
a fallback to the lchan type the MS requested.
- the remaining two "if (!lchan && lctype == GSM_LCHAN_SDCCH)" were obviously
only applied if the MS asked for an SDCCH, and skipped if the type was TCH/*.
- hence, missing was: if the MS asked for a TCH, to also try the *other* TCH
kind that the MS did not ask for. (Example: all SDCCH in use, MS asks for
TCH/F, but BSC has only TCH/H lchans; we should assign TCH/H as SDCCH, instead
we said "no resources")
Change-Id: Ie3684cf071751f9528183d761c588102936e498c
Related: OS#3503
1 file changed