OsMux + MGW + SIP

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Neels Hofmeyr nhofmeyr at sysmocom.de
Fri Sep 7 00:27:27 UTC 2018


On Thu, Sep 06, 2018 at 03:32:00PM +0200, Keith Whyte wrote:
> was that in some cases we might like to avoid having our RTP go
> over the (sometimes variable quality) Wifi links between the BTS and the
> BSC.

Reminds me of the LCLS feature recently implemented -- Local Call Local Switch.
I think the idea of LCLS is to take out the MSC's MGW, and instructing the
BSC's MGW to loop directly back to the BTS. But I guess it's only a tiny step
from there to switching BTS directly back to BTS. osmo-bsc has full control
over that.

> A problem is that if you use "bypass media" you have no early media and
> no audible ringing signal

IIUC LCLS happens after bridge.
IIUC we instruct the MS to generate a ringing signal anyway?

> * Full support for SIP reinvite in osmo-sip-connector which would then
> send a MNCC_RTP_MODIFY which ends up sending (I think it's called a
> IPAC_CRCX?) to osmo-bts which will then switch the stream endpoints.

IPAC_CRCX is a CReate ConneXion, I think you mean IPAC_MDCX, a MoDify.
In the MDCX you can tell the BTS where to send its RTP stream.

Feels wrong to me to let osmo-sip-connector mess around with the BTS'
IPAC_MDCX, I think that's firmly osmo-bsc's realm. And sounds like LCLS is the
thing.

As so often LaF0rge will know a lot more about it...

> * Implement a no media gateway mode in osmo-msc and have the MSC control
> the media stream using SIP via MNCC/osmo-sip-connector instead of
> controlling
> the MGW using MGCP.

Currently we need an MGW both from MSC and from BSC. Again it seems to me LCLS
is the answer to take out the MSC's MGW, and osmo-bsc's LCLS decision might
then end up bypassing the BSC's MGW, if it doesn't do that yet?

I think osmo-msc doesn't support LCLS though, only osmo-bsc? And also I think
the MSC doesn't need to do much for LCLS anyway.

But before I ramble on about things I'm not sure about, I'm shutting up now.

~N
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