Subject: Re: [Ieprep] RE:MLPPoIP - Codecs with error correction

From: [email-masked]
Date: Wed, 26 Nov 2003 17:04:13 EST

In a message dated 11/26/2003 12:19:17 PM Eastern Standard Time, 
[email-masked] writes:

> >[MAP] I am not aware of any codec that adapts to packet loss by increasing 
> >its transmitted rate.
> 
> Any codec with some form of Packet Loss Correction does this very thing - 
> and a lot of ITU codecs do.
> 
> >I suppose there could be one. Can you give an example?
> 
> Some of these are in their specs,
> some are with proprietary extensions
> 
>          G.729
>          G.711
>          G.723
>          iLBC
>          the list is longer...
> 
> 
[MAP] I think we're talking about two different things. The initial comment 
was about codecs that detect packet loss and then start transmitting higher bit 
rates (forward error correction?) to accomodate the loss. G.729, G.711, G.723 
certainly don't do that. iLBC doesn't, as far as I can tell. It transmits at 
a fixed 13.33 or 15.2 kbit/s (for different packet rates).

Yes, there are Forward Error Correction schemes built into some codecs, but I 
don't know of any that change their operation based on detecting packet loss 
(through some feedback mechanism). They are preconfigured to handle a certain 
amount of loss and transmit a fixed rate. As I said, there may be some, but I 
can't see using them in this environment since they are contrary to the 
principle of backing off on data rate when loss occurs (since loss is primarily 
caused by congestion).

Let me know if there are any that work that way so I can put them on the list 
of codecs not allowed in the DSN.

In a message dated 11/26/2003 11:20:44 AM Eastern Standard Time, 
[email-masked] writes:

> There is a good overview of several Forward Error Correction schemes
> that take such an approach in:
> 
> C. S. Perkins, O. Hodson, and V. Hardman, A Survey of Packet-Loss
> Recovery Techniques for Streaming Audio, IEEE Network Magazine,
> September/October 1998. 
> http://csperkins.org/rat/publications/ErrorCorrection.ps
> 

[MAP] This paper provides an overview of various error correction schemes 
(including FEC), but notes the problem with simply inserting more data to 
counteract the loss caused by congestion. It does not describe any situation (as I 
read it) of changing the data rate in real-time based on detection of loss (and 
cautions against it).

Mike