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Re: [Sip] Silence Suppression in SDP for VoIP

In some cases, the codec itself contains specific silence packets, and so their usage would be negotiated using the codec specific parameters. You can also use the specific payload format for comfort noise, and so support of that is negotiated using the SDP codec negotiation methodology in offer/answer. However, when silence suppression is done generally, by sending nothing, no signaling support is needed since all RTP receivers need to be prepared to receive nothing and treat it as silence. That is why there are both RTP timestamps and sequence numbers.

In the case of fax, I still do not see the issue. The sending side would know that it is sending g.711 encoded fax, and therefore never detect silence.

-Jonathan R.

Steve Silverman wrote:
I would have thought that if one side is using silence suppression, the other side needs to understand this.
If true, how is the use of this communicated? Steve Silverman

-----Original Message-----
*From:* sip-admin@ietf.org [mailto:sip-admin@ietf.org]*On Behalf Of
*Alex Agranov
*Sent:* Tuesday, February 04, 2003 12:30 PM
*To:* 'David R. Oran'; 'sip@ietf.org'
*Subject:* RE: [Sip] Silence Suppression in SDP for VoIP

Hi,

I see your point.
But if this is right, then why does H323 protocol support silence
suppression and echo
cancellation parameters?

On the other hand, in case of fax fallback to G711 coder (if T38 is
not supported), there is a
need to specify to remote device that "echo cancelation is ON, and
silence suppression is OFF".
AFAIK fax will not be properly passed if silence suppression is not
turned off.
And this, I beleive, is the reason why
draft-ietf-sipping-realtimefax-00.txt makes use of
"ecan" and "silenceSupp" SDP attributes.

Also there is a following sentence in H323-SIP interworking draft :
"A fmtp SDP attribute for silence suppression SHOULD be
defined if
silence suppression is on."
Is it not valid any more?

Best regards,
Alex Agranov

> -----Original Message-----
> From: David R. Oran [mailto:oran@cisco.com]
> Sent: Tuesday, February 04, 2003 5:53 PM
> To: Alex Agranov; 'sip@ietf.org'
> Subject: Re: [Sip] Silence Suppression in SDP for VoIP
>
>
> There are no such parameters in SDP nor SIP because both of these
are
> algorithm options for one end of a media stream. The source
> of the stream
> does these by itself and therefore they have no end-to-end
> significance. As
> a consequence there is no need for the receiver to know
> anything about Vad
> or ecan parameters of the transmitter, or vice versa.
>
> The reason these exist in MGCP and Megaco is because those are
device
> control protocols intended for comand and control of an endpoint, as
> opposed to peer signaling protocols like SIP.
>
> --On Tuesday, February 04, 2003 12:24 PM +0200 Alex Agranov
> <sagranov@COMGATES.co.il> wrote:
>
> >
> > Hi,
> >
> > I tried to ask this question in sip-implementors maillist but
nobody
> > could answer it there...
> >
> > What is the proper way to indicate Silence Suppression and Echo
> > Cancelation coder parameters in SDP? In MGCP these parameters are
> > passed in LCO element, but for SIP Offer/Answer model these
> parameters
> > should be passed in SDP. Right?
> >
> > There is RFC3108 that defines "ecan" and "silenceSupp"
> attributes, but it
> > applies to VoATM. Is it applicable to VoIP too?
> >
> > Some late drafts, e.g.
> draft-ietf-sipping-realtimefax-00.txt, seem to
> > assume RFC3108 attributes are valid for VoIP. Is this correct?
> >
> > Best regards,
> > Alex Agranov
> > ---
> > Senior Software Engineer
> > COMGATES Ltd.
> > 15 Hagalim Avenue
> > Herzliya, 46725
> > Israel
> > Tel. +972.9.950.0404, Ext: 228
> > Fax. +972.9.950.0385
> > Mobile. +972.54.928435
>
> ------------------------
> David R. Oran
> Cisco Systems
> 7 Ladyslipper Lane
> Acton, MA 01720
> Office: +1 978 264 2048
> VoIP: +1 408 571 4576
> Email: oran@cisco.com
>

--
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
jdrosen@dynamicsoft.com                     FAX:   (973) 952-5050
http://www.jdrosen.net                      PHONE: (973) 952-5000
http://www.dynamicsoft.com

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