Subject: [Megaco] Re: When is automatic switching between codecs useful?

From: "Joe Stone \(joestone\)" <[email-masked]>
Date: Mon, 3 Oct 2005 13:42:14 -0700

> Raphael Tryster wrote:
> > ReserveValue=TRUE may be used to reserve resources for multiple
codecs 
> > in a call.  In this case, the local descriptor will list multiple 
> > codecs.  Common examples of where this might be done are where a low

> > bit rate codec should be switched to G.711 on detecting a DTMF digit

> > or fax or modem.  My question is whether there are other scenarios 
> > when this is useful, and what would typically be the trigger?  Would

> > it be automatically switching the transmit codec when the received 
> > codec changes?  In that case, what might have triggered the remote 
> > sender to switch to a different codec?  Are there any scenarios
where 
> > automatic switching between different low bit rate codecs is useful?

This is an excellent question. One example is V.152. With V.152,
specific payload types are identified as voiceband data (VBD) payload
types,

     a=rtpmap:98 PCMU/8000
     a=gpmd:98 vbd=yes

V.152 defines the semantics by which the transition to and from VBD
codecs is coordinated. Are there other "standards" which define similar
semantics in applications other than VBD (fax, modem, text, ...)? Tom
mentioned RFC 2833. I'm not sure if he's alluding to DTMF (0 - 15),
fax/modem (32 - 36), ... all of the above. Is there a standard that
defines the sematics involved in coordinating the switch between
transmit/receive codecs with any telephone-event?

On a related topic...

Is ReserveValue intended to apply to all payload types? In other words,
must ReserveValue=TRUE in order for a Media Gateway to return more than
one payload type (regardless of what the payload types represent)? For
example, must ReserveValue=TRUE in order for a Media Gateway to return
G.729 and telephone-event,

	m=audio 12345 RTP/AVP 18 96
	a=rtpmap:101 telephone-event/8000

Or is the scope of ReserveValue limited to audio payload types (which
would allow a Media Gateway to return payload types corresponding to
G.729 and telephone-event with ReserveValue=FALSE)?

If ReserveValue is intended to apply to all payload types, it becomes
rather meaningless. A media description will commonly involve non-audio
payload types (mandating ReserveValue=TRUE). We are left with no syntax
by which a Call Agent can request a Media Gateway to negotiate one and
only one audio codec (short of explicitly specifying the one and only
one code which would defeat the purpose of "negotiation").

Joe Stone

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From: "Joe Stone \(joestone\)" <[email-masked]>
Date: Mon, 3 Oct 2005 14:10:07 -0700

(typo corrected)

	m=audio 12345 RTP/AVP 18 96
	a=rtpmap:96 telephone-event/8000

Joe Stone

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From: "Joe Stone \(joestone\)" <[email-masked]>
Date: Tue, 4 Oct 2005 09:22:46 -0700

Kevin Boyle wrote:
> For alternative streams such as audio versus video, the use of
> ReserveGroup comes into play.  This will allow the specification of
> multiple property groups within the SDP.  Each of those can be forced
to
> a single codec alternative through the use of ReserveValue.
>
> See clauses 7.1.7 and 7.1.8.

I don't think that we're talking about alternate streams. We're talking
about alternate encodings for the same stream. How does the MGC
authorize the use of DTMF relay in conjunction with one and only one
audio codec without explicity specifying the audio codec?

Albrecht Schwarz wrote:
> Your examples are all related to H.248-controlled VoRTP endpoints.
> You may also have a look on H.248-controlled VoAAL2 with ITU-T I.366.2
> as SSCS.

Excellent point! I inferred VoIP from Raphael's post. Raphael was
referring to lists of multiple codecs. With VoAAL2, you'd be negotiating
lists of AAL2 profiles,

	m=audio $ AAL2/ITU 1 2 7

Joe Stone