Subject: [Megaco] RFC 3108 passed without adequate scrutiny?

From: "Rajesh Kumar" <[email-masked]>
Date: Mon, 8 Apr 2002 18:12:01 -0700

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of
> Stephen Casner
> Sent: Wednesday, April 03, 2002 6:06 PM
> To: Tom-PT Taylor
> Cc: [email-masked]; [email-masked]
> Subject: Re: [MMUSIC] Silence Suppression Attribute

<snip>

> It may have been a mistake to include the a=silenceSupp attribute for
> ATM without considering the problem more generally. I suspect that
> RFC 3108 did not get much attention in MMUSIC because many of the
> participants don't care much about ATM.

Another point on "rfc3108 passing without due scrutiny": There was a lively
discussion on the ATM SDP issues both on 'cnfcntrl' mailing list (which
preceded the present one), and on a separate mailing list "atmsdp" dedicated
to this subject. I think I made it a point to cross-copy [email-masked]
on all messages. Please read on for a justification of the silence
suppression parameter in rfc3108, which should be obvious to all ATM folks:

Steve,
A little bit of history on why there is a silence suppression attribute for
ATM in RFC3108. Let us look at AAL2 first, and then at AAL1.

If you look at the AAL2 profiles in ITU I.366.2, it is possible to support
PCM or ADPCM without silence suppression. Supporting silence suppression is
not mandatory for all AAL2 connections along the lines of the "strong
requirement" you quoted for silence suppression in the RTP context.

Why would I.366.2 make it optional for AAL2, and not mandatory as you made
it for RTP? Here is a possible justification: The optional nature of silence
suppression might be taken advantage of to reduce DSP costs. PCM without
silence suppression does not need a DSP, ADPCM without silence suppression
can be implemented in a non-DSP hardwired processor.

Now, moving from AAL2 to AAL1: it goes without saying that silence
suppression is completely optional in the AAL1 context. It is rarely used
with PCMU/PCMA on AAL1, however, there is nothing to preclude its use
(except there exists no SID mechanism for AAL1, as there exists for AAL2).

Does the inclusion of this attribute in the ATM context seem more reasonable
in this light now?
Rajesh
------------
Rajesh Kumar
Cisco Systems

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