Subject: RE: [Fecframe] Why FEC?

From: "Mark Watson" <[email-masked]>
Date: Tue, 22 Nov 2005 16:46:35 -0500

Hi Caitlin,

To answer your question, I need to take a guess at what you mean by
"RAID striping" and "packet mirroring", or more precisely how you would
apply these to communication over a network with packet loss.

I would guess that by "packet mirroring" you just mean sending duplicate
copies of each packet, so that the chance of losing both copies becomes
very low.

I would guess that by "RAID Striping" you mean sending some packets
which are the XOR of some previous packets, so that if you lose any one
packet it can be recovered using the redundant XOR packet.

If I guess right, then I would consider both those techniques to be FEC
codes. In both cases you preemptively send additional data which allows
the receiver to recover from the loss of packets en route.

So, then your question becomes one about the merits of different FEC
codes.

Here there is a simple answer, in that different FEC codes have
different properties in terms of the pattern and number of errors they
can correct, the additional bandwidth they require, the computational
complexity at sender and receiver and the flexibility of the code to
adapt along these axes to different applications.

Different applications and different networks have different
requirements and constraints which mean there is a place for a variety
of different FEC codes.

Simple XOR and repetition codes such as described above tend to be
computationally efficient but have a very high cost in terms of
bandwidth (which is an ongoing cost compared to the one-off cost of CPU
power) and are relatively weak in terms of error protection, in many
cases being simply incapable of providing the protection needed,
especially when looking at the kind of quality targets considered for
IPTV.

Regards,

Mark Watson

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On
Behalf Of Caitlin Bestler
Sent: Tuesday, November 22, 2005 12:31 PM
To: [email-masked]
Subject: [Fecframe] Why FEC?

This is one of those fundamental questions that might have a simple
answer, but even if it does it needs to be stated.

> 
> Charter for FECFrame Working Group (Draft 051121)
> 
> The object of this group is to develop specifications for
> using forward error correction (FEC) codes with applications
> in the Internet to provide protection against packet loss.
>

The question is, why FEC?

Why not simple RAID striping?

What is the benefit of FEC over striping, or even packet mirroring?
The costs of FEC are increased complexity and greater processing
power requirements. So what is the justification?

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