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[MMUSIC] comments on draft-garcia-mmusic-sdp-cs-00
I really like this draft. I think it makes a lot of sense to do this,
and there are also other useful applications of it. Anywhere that has
low-bandwidth IP or low QoS IP, but enough for signaling, this can make
sense.
I think the draft could benefit from some explanation of WHY to do this.
For the various cases, why not just use pure PSTN? What value does the
SIP bring? I think it helps handoff, it can bring additional features
that SIP signaling brings (IM, presence, etc.). Its worth mentioning
these and giving specific examples.
On the requirements: I'd also add a requirement that it be possible to
negotiate who makes the PSTN call. Directionality is hugely important
since costs can vary a lot depending on who makes the call. Also I'd
suggest a requirement that says it works with any kind of PSTN interface
- from analog to ss7. So the PSTN mechanism cannot rely on
access-specific features.
For the protocol mechanism, I pretty much agree with what you proposed,
with a few differences.
Firstly, I'd suggest that the network name is not "CS". Its "PSTN". THe
reason is, I can think of use cases where we actually USE sip itself to
set up the pstn bearer. For example, consider a call agent that sets up
the bearer by sending an INVITE to a local pstn gateway. So the access
doesn't have to be circuit per se - it just needs to go to the PSTN.
I'm not sure listing codecs makes sense. The reason is that, the codec
is usually hidden - certainly hidden from the far end. So if I make a
call from my pstn landline to your mobile, YOU may be using AMR but its
g711 to me. Indeed, its actually analog voice to me, its the switch that
converts to 711. So I don't think it makes any sense at all to include
codecs in the m-lines as they are always only locally significant.
Probably the biggest technical issue is correlation - how to link the
incoming pstn call with the SIP signaling. There are several options and
they go from awful (dtmf) to just OK (caller ID) to really nice (ISDN
UUI indicators). The draft clearly needs to discuss this problem and
propose specific mechanisms, possibly more than one depending on the access.
Thanks,
Jonathan R.
--
Jonathan D. Rosenberg, Ph.D. 499 Thornall St.
Cisco Fellow Edison, NJ 08837
Cisco, Voice Technology Group
jdrosen at cisco.com
http://www.jdrosen.net PHONE: (408) 902-3084
http://www.cisco.com
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