Subject: RE: [AVT] RFC2833 Question

From: "O'Connor, Kevin" <[email-masked]>
Date: Tue, 20 Jun 2006 12:52:08 -0400

While two audio codecs may rarely overlap, it's still possible. Much
more likely is a situation where RFC 2833 packets overlap with more
common audio codec packets. The behavior doesn't appear to be well
defined when the rates of these codecs are not all the same. Consider
the following SDP:

      m=audio 12344 RTP/AVP 99 100 101
      a=rtpmap:99 G729/8000
      a=rtpmap:100 DVI/16000
      a=rtpmap:101 telephone-event/8000
      a=fmtp:101 0-15

Let's say that G.729 packets are received up to timestamp 5000. From
timestamp 5000 on, DVI4 data is received at a rate of 16Khz. If a DTMF
event is received with a timestamp of 5100, where does that event appear
relative to the DVI4 data? What if an event straddles the change in
rate?

BTW, this really isn't a contrived example. It's highly desirable, in
the VoIP market, to be able to dynamically switch between narrowband and
wideband codecs as bandwidth permits. 

-----Original Message-----
From: Tom-PT Taylor [mailto:[email-masked]] 
Sent: Tuesday, June 13, 2006 3:53 PM
To: O'Connor, Kevin
Cc: [email-masked]
Subject: Re: [AVT] RFC2833 Question

Yes, it is possible. I could see it happening in a transition to and 
from voiceband data. I'm not sure if it would really happen in practice.

O'Connor, Kevin wrote:
> Is this possible with any 'audio'? For example, if an RTP session is
> negotiated to allow G.711 and G.729, should the receiver be able to
> handle receiving both G.729 and G.711 packets for the same timestamp
> range?
> 
> -----Original Message-----
> From: Tom-PT Taylor [mailto:[email-masked]] 
> Sent: Tuesday, June 13, 2006 1:36 PM
> To: O'Connor, Kevin
> Cc: [email-masked]
> Subject: Re: [AVT] RFC2833 Question
> 
> You can have two packets in the same stream representing the same 
> timestamp range. Because the media type for RFC 2833 events is
'audio', 
> it can be offered in the same stream as your G.711 example. You need a

> policy at the receiving gateway to decide which payload type to use, 
> bearing in mind the need for a clean transition between tones and
other 
> content.
> 
> O'Connor, Kevin wrote:
>> Is it possible to have two RTP packets in the same stream that
>> represent the same timestamp range? For example, the audio
>> representation of a tone event as G.711 as well as the
>> telephone-event representation. Another example would be the
>> telephone-event and the tone as two different packets.
>>
>>
>>
>> Or, is this only possible via an explicit redundancy mechanism such
>> as RFC 2198?
>>
>>
>>
>>
>>
>
------------------------------------------------------------------------
>>
>> _______________________________________________ Audio/Video Transport
>> Working Group [email-masked] https://www1.ietf.org/mailman/listinfo/avt
> 
> 
> 
> 

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Colin Perkins <[email-masked]>
Date: Tue, 20 Jun 2006 18:36:13 +0100

On 20 Jun 2006, at 17:52, O'Connor, Kevin wrote:
> relative to the DVI4 data?

You could use a second telephone-event payload type with 16kHz clock  
during the periods when sending DVI at 16kHz, which would solve the  
problem.

> What if an event straddles the change in rate?

If the event is a single packet, with duration that takes it past the  
rate change, then I don't see a problem: the rate in force at the  
sampling instant of the packet applies. For an event that spans  
multiple RTP packets across a rate change, it's less clear how things  
would work.

> wideband codecs as bandwidth permits.

Provided the rate switch occurs at the beginning of a packet, there's  
no overlap between packets of different rates, and the rates are  
integer multiples of each other, then this is possible without  
difficulty. That covers the majority, although not all, of use cases.

Colin