Subject: [AVT] Reg G.711 Silence Suppression Negotiation

From: Sharma Kanchinadham <[email-masked]>
Date: Mon, 24 Dec 2001 11:42:30 -0600

Hello Steven,

You had earlier suggested that the G.711 Silence Suppression is to be
negotiated using the PT=13 in the SDP. Hence if a node wanted to negotiate
Silence packets it could send the following m line in the SDP

m=audio 49230 RTP/AVP 0 13

The terminator can respond back with PT=13 if it wishes to recieve CN
frames. If there is no PT=13 in the response the terminator does not wish to
recieve CN frames.
I am OK with this approach. However i feel this approach needs to be
solidified. The PT=13 is currently listed as Reserved at the following site.

http://www.iana.org/assignments/rtp-parameters

The PT=13 has to be listed as 'CN Frame' or something appropriate. Also it
would be helpful if this approach to G.711 Silence Suppression is documented
in one of the Internet Drafts.

Thanks ,
Sharma Kanchinadham
IP Call Processing & Standards
Worldcom

====================================================================

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf
Of Stephen Casner
Sent: Friday, November 30, 2001 9:06 PM
To: Sharma Kanchinadham
Cc: 'AVT WG'
Subject: RE: Reg draft-ietf-avt-rtp-mime-05.txt

On Thu, 29 Nov 2001, Sharma Kanchinadham wrote:
> Can you please illustrate how G.711 Appendix II can be negotiated with an
> example.

As far as I know, I do not have access to ITU Recommendations,
therefore I can't say anything definite about G.711 Appendix II.  That
is why in my previous message I asked you to give a brief description:

    I'm not convinced that a generic problem exists, but it sounds like
    you may be concerned with a specific problem related to G.711 and
    G.729 interoperation between Cisco products and others.  Can you
    please describe the specific problem?  In particular, what is Cisco
    VAD and how does it compare to G.711 Appendix II or G.729 Annex B?

However, the CN payload format (draft-ietf-avt-rtp-cn-04.txt) claims
to be based on G.711 Appendix II:

   The payload format is based on Appendix II of ITU-T
   Recommendation G.711 [9] which defines a comfort noise payload
   format (or bit-stream) for ITU-T G.711 use in packet-based
   multimedia communication systems.

Therefore, I assume that the way to negotiate G.711 Appendix II is to
negotiate the use of the CN payload format in combination with the
PCMU (G.711) payload format.  This may be done with an SDP media line
like the following:

    m=audio 49230 RTP/AVP 0 13

An implementation that did not want to receive CN frames would have to
restrict the media specification to:

    m=audio 49230 RTP/AVP 0

As I said, there isn't any way to insist that audio packets be
transmitted continously, but I am not convinced that the need to
signal such a requirement is justified.

> I believe the appraoch you are suggesting above might conflict with
> the following para in SDP RFC2327.
>
> "Additional parameters may be defined in the future, but
>  Codecspecific parameters should not be added.  Parameters added to
>  an rtpmap attribute should only be those required for a session
>  directory to make the choice of appropriate media too to
>  participate in a session.  Codec-specific parameters should be
>  added in other attributes. "

No.  I am not proposing that any parameters be added either to the
rtpmap or the fmtp attribute.
							-- Steve

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
http://www1.ietf.org/mailman/listinfo/avt

---

From: Colin Perkins <[email-masked]>
Date: Mon, 24 Dec 2001 14:37:03 -0500

The registration of payload type 13 is in draft-ietf-avt-rtp-cn-04.txt,
which is expected to become an RFC shortly.
Colin

---

From: Sharma Kanchinadham <[email-masked]>
Date: Wed, 26 Dec 2001 09:34:45 -0600

Colin,

It is suggested that the approach outlined below for Silence Suppression
Negotiation for codecs that use PT=13 be also included in the Internet
Draft. Would you agree?

Thanks ,
Sharma Kanchinadham

=======================================================================

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From: Colin Perkins <[email-masked]>
Date: Wed, 26 Dec 2001 14:20:57 -0500

Why does the draft need to state this explicitly? Is this not normal SIP
behaviour when choosing a codec?

Colin

---

From: Sharma Kanchinadham <[email-masked]>
Date: Thu, 27 Dec 2001 10:39:27 -0600

Colin,

Since PT=13 is not a regular codec and it defines Comfort Noise Frames, i
believe it would help to define the negotiation mechanism explicitly to
avoid any confusion.

Thanks ,
Sharma

-----Original Message-----
From: Colin Perkins [mailto:[email-masked]]
Sent: Wednesday, December 26, 2001 1:21 PM
To: [email-masked]
Cc: 'Stephen Casner'; 'AVT WG'
Subject: Re: [AVT] Reg G.711 Silence Suppression Negotiation

Colin

---

From: Colin Perkins <[email-masked]>
Date: Sun, 30 Dec 2001 13:47:55 +0000

Sharma,

From the point of view of RTP, the comfort noise payload format is treated
exactly the same as any other codec. Specifying the negotiation separately
would only risk causing confusion, since readers would believe it is some-
how special.

Colin