[Asterisk-Users] PLC (Packet loss cancel) questions

[Steve Kann]

I'm also interested in this, as the other Steve knows.

Anyone in the re-worked jitter-buffer/PLC/DTX crowd besides me going to 
be at astricon?

We can at least start working there on requirements.  I think I've wrote 
this before, but here's what I'd _really_ like to see as requirements 
for a re-worked jitter-buffer:

- Channel Support:
    IAX2 in asterisk
    IAX2 in libiax2
    Other IP channels in asterisk (RTP-based ones, I guess are all that 
is left).

- DTX Support:  Sending a single CN packet (in IAX2, this should 
probably be sent reliably)
  would probably be good.
- PLC Support:   For codecs that support this natively, use that support 
(iLBC, speex, others). 
                           For those that don't, we can add some kind of 
hack to the codec.  For example,
                           in app_conference, I repeat even GSM frames 
when I detect loss.

- Configurability by applications:  It seems that some applications 
(app_fax) might do better without any of this; we should consider what 
different applications may want to configure in their use of this, and 
allow them to change settings while the channel is in the application.  
So, app_fax might want to disable PLC, and also disable any 
jitter-buffer shrinkage (and dropped frames that it would cause).

- For IAX2 (and RTP, I think) Interleaved frame support:  For 
single-channel calls, in IAX2, and a compressed codec, you have about 
100% overhead with 20ms frames.  Using larger frame sizes of course 
drops this considerably, but without interleaving, it makes PLC much 
less effective.  Using interleaved frames, where in a single packet you 
have, for example

Packet 1, frame 0,2
Packet 2, frame 1,3
Packet 3, frame 4,6
[...]

And PLC, means that if you drop packet 2, you can do a much better job 
concealing that loss.


-SteveK


steve at daviesfam.org wrote:

>On Fri, 27 Aug 2004, Michael Manousos wrote:
>
>  
>
>>I hope that the above issues will start a discussion and result to a
>>solution, no just for PLC, but also for the DTX operation.
>>    
>>
>
>Yeah - my goal for a reworked jitter buffer includes DTX and PLC.  And 
>other TLAs ;-)
>
>Steve
>
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