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

[Steve Underwood]

Chris Shaw wrote:

>>- Channel Support:
>>   IAX2 in asterisk
>>   IAX2 in libiax2
>>  Other IP channels in asterisk (RTP-based ones, I guess are all that is
>>    
>>
>left).
>
>CNG/VAD and DTX in SIP is a must if * is to be taken seriously as a complete
>solution... As much as we all hate it's complexity and wish that everything
>would speak IAX (I know I do) a large number of devices support (and will be
>supporting) SIP, making it equally as important as IAX2  in using * as a
>complete telephony solution...
>  
>
This is nothing to do with SIP. It is an RTP issue, common to everything 
which uses RTP - SIP and H.323 included. Sending no packets is perfectly 
valid, and normal, in RTP. If the receiving end takes no packets (other 
than, perhaps, an extremely long silence) as a disconnect it does not 
comply with the RTP spec. DTX is much despised, and CNG only slightly 
better. They just sound good (pun intende) on paper.

>>DTX Support:  Sending a single CN packet (in IAX2, this should probably
>> sent reliably)  would probably be good.
>>    
>>
>
>I second, third and fourth this one as does anyone who's tried to use
>BroadVoice with Voicemail... Currently when * is not making any noise (e.g.
>recording) absolutely NO packets are sent back to the proxy... A lot of
>proxies take this as a sign that the far end has disconnected... Including
>BroadWorks! But they do recognize small CN packets as a sign that the SIP
>device (Asterisk) is still there...
>  
>
A lot of CNG spec. call for only one transmission, and then silence. 
Continued CNG has real benefits, but it certainly not the norm.

>PLC I think is somewhat implemented already in codecs that support it, but I
>could be wrong, I remember seeing mention of it in the code...
>  
>
PLC is seldom included in the codecs. If you read the specs they often 
mention PLC, but only in terms of how the codec mitigates the awfulness 
of a lost packet. Few codecs actually include it.

Regards,
Steve