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

From: "Jean-Francois Mule" <[email-masked]>
Date: Wed, 13 Sep 2006 16:27:26 -0600

Adam wrote:
> 2) In order to avoid transcoding (cuz it's evil) the proposal would be
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The text first recommends the obvious: VSPs should agree on common
codecs.

> to state that a UA registered to a VSP should have support for
> commonly
> deployed codecs, or at a minimum G711, BUT if G711 is not supported,
                   ^^^^^^^^^^^^^^^^^^^^
The proposed text does not say or mean this at all as it shifts the
burden of the requirement to the VSPs wanting to peer by basically
saying: rather than egressing your calls to the PSTN using a gateway
with G.711, just provide some media transcoding/relay with G.711.

> to
> support transcoding to G711?

It is basically acknowledging the fact that not all UAs support G.711
but in those cases, the VSP should provide some support for G.711
transcoding (rather than the default PSTN gw, it's just a minor step
forward).

> 
> #2 seems a little contradictory to me, or did I misunderstand the
> summary?!?!

Hope this helps clarify the intent. Any suggestions to improve the text
is welcome.

Jean-Francois.

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From: "Geoff Devine" <[email-masked]>
Date: Thu, 14 Sep 2006 06:28:20 -0400

> > 2) In order to avoid transcoding (cuz it's evil) the proposal would
be

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.

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 agree that transcoding is evil and should be avoided wherever
possible.  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
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