Subject: [Megaco] Duplicated in-band and out-band tones

From: Richard Sun <[email-masked]>
Date: Wed, 15 Feb 2006 07:32:36 -0500

Is there a standard on dealing with duplicate tones
from in-band (RTP) and out-band (MGC request).

For example, take this scenario from T.38 (MGC directed
method):
1. MGC asks MG1 to detect CNG fax tone.
2. MG1 detected CNG and notify MGC
3. MGC asks MG2 to play CNG tone

The question is: how does the T38 fax/phone connected
to MG2 deal with the duplicated tones: one from
in-band RTP, and one from MGC request?

Thanks for any info.
-Richard

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From: "Tom-PT Taylor" <[email-masked]>
Date: Wed, 15 Feb 2006 11:13:03 -0500

I would think the basic position would be that unless MG2 is asked to 
detect CNG tone, it doesn't in theory understand the contents of the 
media stream. Hence there is no problem from the MG's point of view: it 
plays out CNG as requested, and this signal displaces the audio received 
from the far end while it is being played out.

That, of course, wasn't your question, but gives us a basis for tackling 
it. If the CNG tone received by the T38 fax/phone violates the start-up 
protocol (e.g. by coming after V.8 CI or CM), then the appropriate 
protocol error procedures are applied. This may mean that the two ends 
eventually synchronize, or it may mean the call fails.