[opendtv] Re: tolerance Latency , Jitters & packet loss issues for MPEG-2 TS over IP network

John McClenny wrote:

> I think that the IPTV people using FEC are FECing at
> the UDP/IP packet level, not at the video stream level.
> So a discarded packet gets reconstructed.  I have not
> direct experience with this, but this is what I have
> been told.  Not my preferred mthod of handling this
> problem.

Kon Wilms wrote:

> Right, cause the IP layer will discard a corrupted
> packet (or it just won't arrive). Hence you are not
> really doing forward error correction, but rather
> erasure correction. With the entire packet missing you
> can't do your standard bit error reconstruction, you
> have to do block error reconstruction. Reed-solomon is
> also useless for this application. Also to do it
> correctly, one has to randomize the processed block
> and parity blocks (you don't want your parity blocks in
> sequence, it defeats the purpose) - the impact of this
> is latency in the decoder buffer...

Yes but ...

What you guys are describing is not a limitation of IPTV
per se. It is a limitation of using Ethernet framing at
the Link Layer, for the purpose of transmitting one-way
streams of packets, and expecting a vanishingly low
error rate end-to-end.

For example, if you transmit IP or LLC/SNAP encapsulated
frames over MPEG-2 TS over either DVB or ATSC, you will
benefit from convolutional as well as block FEC, not to
mention (except for DVB-T) interleaving.

This stuff has to be done below the IP layer, clearly,
if it is really needed. With a fiber optic cable plant,
perhaps FEC is not mandatory. With a wireless plant, FEC
certainly is, which is why no wireless standard uses
Ethernet as its link layer protocol.

Bert
 
 
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