Subject: [MMUSIC] SDP 'gpmd' (General Purpose Media Descriptor) attribute

From: Kreuter Ruediger <[email-masked]>
Date: Wed, 30 Oct 2002 15:35:19 +0100

I would like to support 
draft-rajeshkumar-mmusic-gpmd-00.txt !
This should be driven towards a RFC, 
especially the registration of the 
necessary vbd parameter(s).

In a VoIP network, replacing a TDM 
network, a mechanism to support fax 
and modem (vbd) calls via G.711 is 
necessary.
Therefore it is necessary to have a 
signalization from the called-side-gateway 
to the calling-side-gateway, that the 
called-side-gateway detected a 2100 Hz 
tone (CED, ANS, ANSam) and may be later 
on a phase reversal (/ANS, /ANSam) and 
therefore the calling-side-gateway has 
to change the DSP configuration.

Overloading a codec with several payload 
types (one for voice and one for vbd), 
and using these to trigger a DSP configuration 
change, is a fast and reliable way to 
implement this feature.

Using RFC2833 packets would be an 
alternative, but then these packets 
would be used to control the receiving
gateway, which is not really the intention 
of RFC2833.

Besides:

In certain cases it seems to be necessary 
to have a distinct handling of fax versus 
modem transmission via G.711. Especially
the echo canceller may be handled differently 
in both cases.
Therefore it would be necessary to have 
another parameter indicating echo canceller 
on or off.

Therefore I suggest to add the parameter 
"ecan" (as in RFC3108) with possible values 
"on" or "off" (as with vbd), in 
draft-rajeshkumar-mmusic-gpmd-00.txt and 
have it registered together with "vbd".

A example for this parameter would be
(using a semi-colon delimiter, as suggested in
http://www.ietf.org/mail-archive/working-groups/mmusic/current/msg00932.html)

    m=audio 3456 RTP/AVP 0 15 98 99  
    a=rtpmap:98 PCMU/8000   
    a=gpmd:98 vbd=yes 
    a=rtpmap:99 PCMU/8000   
    a=gpmd:99 vbd=yes; ecan=no
 
In this example with PT=98 the echo canceller 
would stay as is, whereas with PT=99 it must 
be switched off.

Regards

Rüdiger Kreuter
Siemens AG
[email-masked]

_______________________________________________
mmusic mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/mmusic