[Asterisk-Dev] video in iax2 spec

[Derek Smithies]

Hi,

> >I think the RTP header used in H323 for video works very well, and should 
> >be used in the video packets. Yes, I know RTP headers take up bytes.
> >However, given that we are sending compressed video, an extra couple of 
> >bytes are not going to "break the bank".
> >  
> >
> I'm not sure that encapsulating RTP inside of IAX is necessarily going 
> to be a good idea; you basically will then end up with multiple 
> timestamps, and stuff like that. I'd just get the data portion (payload) 
> to be the same, and then it should be as easy to translate the headers 
> as it is for audio.
> 

the advantage of putting the rtp header in the video frame is when you are 
changing voip protocols (eg, from h323 to iax2) It becomes easy then to 
get the conversion to work reliably. Which is what we want - is it not - 
reliability of conversion?
 The second advantage of using the rtp header is that the packet format 
(across all video formats) becomes consistant.



> >I think also that the video packetization should be as  consistant  as 
> >possible for the major video codecs.
> >  
> >
> Hmm, I was aiming for inconsistency. I figured that the payload format 
> would be some random number of bytes from one frame, then some random 
> data, and then some random number of bytes from another frame. This 
> should change with each frame. Sometimes, you should XOR the frame's 
> data with some other random data too, just for fun.
> 

Excuse me - aiming for inconsistency?
Why?
All the audio codecs are handled in a relatively consistent fashion.
iax2 header, audio frame data. This makes a decoder simpler to write.

Surely, video should be handled in as consistant a fashion as possible. 
For H263, there are control packets (which are sent reliably). these can 
be sent with video IEs (that are yet to be specced.

If the video codecs are handled in a consistant fashion, the code becomes 
far more readable, easier to maintain, and should be more reliable.


======


My question when evaluating a video packetization algorithm is:
  a)how will it work when doing protocol conversion (iax-h323)
  b)is it consistant and simple to understand
  c)is there duplication of information



Derek.

-- 
Derek Smithies Ph.D.                         
IndraNet Technologies Ltd.                
Email: derek at indranet.co.nz         
ph +64 3 365 6485                          
Web: http://www.indranet-technologies.com/