On Thu, Dec 07, 2023 at 01:49:20PM +0100, Harald Welte wrote:
IMHO, the BSC and its colocated MGW should
"hide" all the RAN-internal
details from the MSC. A MSC should never have to know anything specific
about each and every BTS, or even how many BTS there are.
so AMR oa/be mode should be:
BTS <--whatever-bts-needs--> MGW@BSC <--fixed-as-in-3GPP-spec--> MGW@MSC
The AoIP interface is the only where the RTP format /
codec types /
payload types are specified. We should always use what's specified in
TS 48.103.
3GPP TS 48.103 references TS 26.102, which says
10.2 AMR
The bandwidth efficient mode of RFC 4867 [21] shall be used. CRC and robust sorting
shall not be applied.
So 3GPP says use octet-align=0 on the A-interface:
BTS <--whatever-bts-needs--> MGW@BSC <--octet-align=0--> MGW@MSC
In osmo-bsc, I read that all our BTS currently use octet-align=1.
I also see osmo-bsc indicating octet-align=1 to the MGW, unconditionally.
https://gitea.osmocom.org/cellular-infrastructure/osmo-bsc/src/commit/71f96…
Then we want this:
BTS <--octet-align=1--> MGW@BSC <--octet-align=0--> MGW@MSC
Which means our 2G BTS use a different align than specified to be used on the
A-interface in 3GPP. For AMR, we'd *always* convert each RTP packet between OA
(BTS) and BE (CN) at osmo-bsc's MGW, once per call leg.
So far, osmo-bsc indicates octet-align=1 to both sides, also the CN.
Are you sure we should change that?
In our osmocom world, it seems practical to instead agree on OA=1 from RAN up
to osmo-msc, even though it does not match 3GPP TS 26.102:
- OA=1 is the status quo of what osmo-bsc tells its MGW to pass to the CN.
- We can do IuUP->AMR conversion directly as OA=1 (for 2G and 3G alike).
- To PBX, there's no problem with flexible OA: if the PBX asks for
octet-align=0, osmo-msc can forward that to osmo-mgw. osmo-mgw will then
transcode to BE.
I think, best to add VTY options in osmo-bsc and osmo-msc for which fixed AMR
mode to use on the A-interface; Default would be current behavior: OA=1 on the
A-interface. A user can then configure conform to 3GPP instead, if desired.
Makes sense?
~N