Subject: RE: RFC 2833 on Tones

From: "Bratakos, Maroula" <[email-masked]>
Date: Wed, 31 May 2000 13:58:48 -0700

Dan Wing wrote: 

> When used with RFC 2198, the number of redundant encodings also has an 
> impact on the total bandwidth used. 

I think the duplication has merit, but appropriate text is required 
explaining when such RFC2198 duplication of DTMF tones isn't appropriate. 

Perhaps a simple MAY is good enough, but only if we assume implementors 
will understand why it's a MAY instead of SHOULD or MUST.  I haven't read 
RFC2198 in awhile, but it may have text which explains some of the 
drawbacks of arbitrary duplication of packets in a bandwidth-constrained 
or policed network.  My quick skim of RFC2198 didn't turn up such text, 
however.

You're right.  I don't think it's there. 

Note that the usual situation involving the need to send DTMF tones outside
of 
the normal RTP payload implies that the tones inside the normal RTP payload 
are of no use, or at least should not be trusted.  Indeed, in many/all cases
you 
would want to strip those tones out of the normal RTP payload when sending 
via RFC 2833/2198 methods.  Consequently, the normal RTP payload has an 
element of mutual exclusivity with the RTP payload used to carry the RFC 
2833/2198 encodings.  Some benefit can be gained from this knowledge when 
the access network is bandwidth limited, as in the cable and DSL cases. 

RFC 2833 does not address the possibility of replacing normal RTP payloads 
with RFC 2833 payloads.  This would be a useful concept in bandwidth limited

networks.  What do you think? 

Rex Coldren 

 -Dan Wing  

I think RFC 2833 already allows for replacing normal RTP payloads with RFC
2833 payloads because it says you MAY send both.  This implies to me you
don't have to send both and could just send RFC 2833 payloads.  The
advantage of RFC 2833 is that it provides a more reliable transport of tone
information (DTMF or otherwise) that can not be achieved via the audio media
stream due to degredation of the tones in the vocoding process (PCM and G726
excluded).  
 
I'm just trying to point out that people need to be careful how they send
DTMF/tone information.  If only audio is sent there should be no problem
(except that the tone information may be garbled for low bit-rate vocoders).
If only the RFC 2833 events are sent then there should be no problem.  But
if both payload types are sent it should be in a redundant format (2198) to
avoid the receiver from playing out both packets which could result in
double digits, tones out of phase, etc.
 
-Maroula Bratakos

---

From: Dan Wing <[email-masked]>
Date: Wed, 31 May 2000 17:00:12 -0700 (PDT)

> double digits, tones out of phase, etc.

OH!  I had completely misunderstood.  Sorry.  I understand now.

-Dan Wing