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

[Kevin Walsh]

Michael Manousos [manousos at inaccessnetworks.com] wrote:
> Kevin Walsh wrote:
> > Michael Manousos [manousos at inaccessnetworks.com] wrote:
> > > a) The transmitter detected silence and sent nothing but the last CN
> > > packet was lost. According to the above interpretations, the receiver
> > > will try to conseal a packet loss, which is wrong.
> > > 
> > 
> > I would propose that after x lost packets, Asterisk should treat
> > all further lost packets as CN.  The proceeding x packets should be
> > interpreted as RTP packet loss and run through the concealment routine.
> >
> Well, no matter what kind of concealment algorithm is used, just the
> first one or two packets will be concealed. The rest losses will result
> in no-playback. No CN interpretation, just absolute silence.
> 
That's true - unless there's some logic to say that after x lost
packets, the line state should switch to CN generation instead of
silence.

The line state would switch back once a fresh RTP packet is received.

> > >
> > > b) The transmitter sent an RTP packet, that packet was lost and the
> > > last packet correctly received at the receiver was a CN packet. Again,
> > > following the above interpretation, the receiver will do nothing (or
> > > more accurate, will play some background noise), while it should
> > > conseal the packet loss. 
> > > 
> > In this case, there is nothing to conceal anyway, as the last received
> > data was a CN packet.  In this case, the CN state should be continued
> > until an RTP packet is received and the line state can be changed.
> >
> Exactly. So the receiver, in case of no-receiption, should go back and
> see what was the last packet correctly received and act as I described
> above. 
>
Maintaining an audio state flag (CN/RTP) would be the key here.

> > 
> > The difficult part to handle would be late or out-of-sequence RTP
> > packets.  These should be ironed out by the jitter buffer.  Late,
> > lost and juggled packets are to be expected when dealing with UDP.
> > 
> Actually this is not so difficult, if there is a jitter buffer.
>
Right.

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