Subject: [Megaco] H.248-controlled V.152: VBD-over-RTP/UDP/IP - H.248 SDP Encodings

From: [email-masked]
Date: Mon, 21 Mar 2005 11:40:56 +0100

Hello Dominic & other V.152 experts!

ITU-T V.152 allows to use CHOOSE in the payload type list field of the
m-line:
e.g.
m=audio $ RTP/AVP 0 $

We think that the CHOOSE capability, according to V.152, in the payload
type list field of the m-line is ambiguous and therefore not generic
enough, as other services than VBD like RTP Clearmode or RTP NTE event may
use
this mechanism as well.

Therefore we think that the CHOOSE capability should be accompanied by
an appropriate a-line, which specifies the context for this request. In
case of V.152 this is implicitly assumed to be VBD, but other MGCs may
ask for other services.

Note: all examples assume ReserveValue=OFF, thus only a single dynamicPT
is selected.

Example 1)
MGC->MG
m=audio $ RTP/AVP 0 $
a=rtpmap:$ PCMU/8000
a=gpmd:$ vbd=yes
MG->MGC
m=audio 40000 RTP/AVP 0 98
a=rtpmap:98 PCMU/8000
a=gpmd:98 vbd=yes

Example 2)
MGC->MG
m=audio $ RTP/AVP 0 $
a=rtpmap:$ telephone-event/8000
MG->MGC
m=audio 40000 RTP/AVP 0 97
a=rtpmap:97 telephone-event/8000

Example 3)
MGC->MG
m=audio $ RTP/AVP $
a=rtpmap:$ CLEARMODE/8000
MG->MGC
m=audio 40000 RTP/AVP 97
a=rtpmap:97 CLEARMODE/8000

Moreover the MGC may ask the MG to select multiple dynamic PTs like:
Example 4)
MGC->MG
m=audio $ RTP/AVP 0 $
a=rtpmap:$ PCMU/8000
a=gpmd:$ vbd=yes
a=rtpmap:$ telephone-event/8000
MG->MGC
m=audio 40000 RTP/AVP 0 97 98
a=rtpmap:98 PCMU/8000
a=gpmd:98 vbd=yes
a=rtpmap:97 telephone-event/8000

Moreover in case of VBD, it is maybe desirable that the encoding is
selectable as well. In this case the a-line a=gpmd:$ vbd=yes clearly
indicates that the encoding is related to VBD.

a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
parameters>]

Thus the MGC may send that following SDP lines:

Example 5)
MGC->MG
m=audio $ RTP/AVP 0 $
a=rtpmap:$ $/$
MG->MGC
m=audio 40000 RTP/AVP 0 98
a=rtpmap:98 PCMU/8000
a=gpmd:98 vbd=yes

Best regards
Albrecht

ALCATEL

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