Subject: [AVT] rfc2833. rtp for dtmf. sending dtmf packets

From: tk <[email-masked]>
Date: Wed, 27 Jul 2005 16:40:39 +0800

hi,

I have a question on sending dtmf packets.  under Section 3.6 
of Rfc2833, it says that "...start transmitting event packets as soon
as it recognizes an event and every 50ms thereafter or the packet interval
for the audio codec used for this session, if known. ..."

Assuming that the packet interval has been negotiated
to be 20ms. Under section 3.1 "DTMF digits...MUST use the same sequence
number and time-stamp base as the regular audio channel..."
Given the above sentence, I don't see how one is able to fulfill the above
requirement (sending audio and event packets when the 2 packet sizes
are different.

pls advise.

thanks.
tk

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
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From: "Tom-PT Taylor" <[email-masked]>
Date: Wed, 27 Jul 2005 08:01:13 -0400

I think the assumption behind the RFC2833 requirement is that the events 
are being transmitted in the same stream as the audio.  There are two 
possibilities:

1) The sender stops transmitting audio and starts transmitting events. 
In this case, if the sequence number of the last audio packet was N, 
that of the first event packet is N+1.  The timestamp behaves similarly, 
even if the increment from the last audio packet to the first event 
packet is larger than the increment between successive audio packets.

2) The more complicated case is that the sender continues to transmit 
audio even as it sends events.  The logic still works.  Here's an 
example, where T is in ms units and timestamp increments at the rate of 
8000 per second.  Assume that we're talking about a single tone 
continuing through the period being depicted.

Time       Packet content      Seq No.     Timestamp

T          Audio                N             X

T          Event                N+1           X

T+20       Audio                N+2           X+160

T+40       Audio                N+3           X+320

T+50       Event update         N+4           X

T+60       Audio                N+5           X+480

etc.