Re: DTMF timing (was Re: [Sip] INFO ...)

[Dean Willis <[e-mail]>]

On Nov 1, 2007, at 3:20 PM, Michael Procter wrote:

> Whilst having the RFC2833 arriving in the timestamped audio stream  
> is certainly beneficial, it is also true to say that the RTP stream  
> generally travels quicker than signalling.  One of the main reasons  
> for this is that the RTP usually travels point-to-point, whilst the  
> signalling path often includes a couple of proxies.  Even if the  
> proxies process the messages in zero time (which they don't), there  
> are still multiple network hops introduced as a consequence.   
> Additional delays incurred by the proxies/other intermediate  
> signalling elements swiftly add up too.

I agree that this sounds correct in theory.

What I'm wondering about is those deployments that use media-path  
control elements (aka SBCs that police media) and/or decoupled media  
and signaling processors.

I suspect that that SBCs slow down RTP about as much as they slow  
down SIP. COuld be more, could be less -- I don't really know.

Then there's the question of decoding the 2833 stream and backhauling  
it from the RTP-processing DSP element up to the application that's  
running at the control layer. From what the people who build  
decomposed gateways sometimes tell me, I've surmised that this may  
not be the easiest thing to do efficiently. This also applies to  
application built using a control "application servers" and a media  
server, since we don't generally bother to negotiate different  
endpoints for the 2833 stream, so it just follows the path to the  
media server.

We don't really use DTMF much between "traditional SIP endpoints" --  
the kind of all-in-one device that does everything in a dedicated  
general-purpose processor. It seems that in practice we use it more  
frequently in transactions that involve large-scale decoupled  
gateways and decoupled media servers, at least at one end of the  
call. I wouldn't be surprised to see the practical reality collide  
with the theory around 2833.

So, that's why I'd like to see real-world measurements in real-world  
large-scale applications that actually use DTMF.

--
Dean


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