Subject: RE: [Speermint] Speermint] requirements: codec

From: "Jean-Francois Mule" <[email-masked]>
Date: Thu, 14 Sep 2006 07:09:39 -0600

Geoff,

   See inline.

Jean-Francois.

Geoff Devine wrote:
> The use case I worry about is the case where a service provider has a
> temporary overload/congestion condition on a portion of their access
> network.  For example, it may take weeks to schedule a truck roll to
> perform a fiber node split on a DOCSIS network.  In the mean time, the
> service provider has to implement a local policy where all voice calls
> on that portion of the access network must use a compression codec.
> While G.711 is almost universally supported, there will always be
> cases
> where two endpoints don't share a common compression codec.  In those
> cases, you can't avoid the need for transcoding.
Noone is saying: transcoding *MUST NOT* occur; the consensus so far is
that it *should* be avoided.
 
> Another issue with codec selection is coping with FAX in a network
> with
> significant packet loss.  As long as you're running access networks
> over
> copper, you're going to have packet loss in the real world.  With even
> insignificant packet loss, FAX falls over so you need to run a codec
> with enough data redundancy and error correction capabilities to
> support
> imperfect access networks.  Codecs that cope with these issues such as
> T.38 are definitely not widely implemented and a service provider may
> very well need to transcode to deal with packet loss on their access
> network for FAX.
I won't comment on this as you're again referring to decision a VSP may
make based on local policies.

> I agree that transcoding is evil and should be avoided wherever
> possible.  
Ok; that's part of the proposed wording I made more or less.

> Since you can't avoid it in some corner cases, it has to be
> part of the architecture.

> In an ideal world, you only want to transcode once to avoid audio
                     ^^^ ^^^^         ^^^^^^^^^ ^^^^!
Tandem 2 and tandem 3 codec tests are common in voice codec assessment.
There is no way for a terminating VSP to know that media was already
transcoded once (to the best of my knowledge).

> degradation.  Unfortunately, that is a difficult problem to solve in
> some network topologies.  The Brian Rosen proposal to transcode to
> G.711
> as the common tongue is probably the best we're going to be able to do
> when we transcode in those cases.
> 
> Best practices should be:
> * Avoid transcoding if possible.
> * Transcode once if you have to transcode.
> * Use G.711 as the intermediate codec if you can't avoid multiple
> transcodings.
> 
> Geoff
> 

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---

From: "Geoff Devine" <[email-masked]>
Date: Thu, 14 Sep 2006 09:32:27 -0400

> > In an ideal world, you only want to transcode once to avoid audio

Audio degrades every time you do a codec conversion or echo cancellation
so you want to avoid it.  Tandem 2 and tandem 3 codec tests merely
measure how much they degrade.  

You could have the candidate transcoding points insert information in
the signaling to assist in avoiding multiple transcodings.  If we are
considering a network architecture where there is intelligence in the
middle rather than fully distributed to the endpoints, we need ways for
the entities in the middle to pass information and coordinate their
activities.  This codec discussion is just one example.

Geoff

-----Original Message-----
From: Jean-Francois Mule [mailto:[email-masked]] 
Sent: Thursday, September 14, 2006 9:10 AM
To: Geoff Devine; Uzelac, Adam; [email-masked]
Subject: RE: [Speermint] Speermint] requirements: codec

Geoff,

   See inline.

Jean-Francois.

                     ^^^ ^^^^         ^^^^^^^^^ ^^^^!