[e2e] Satellite Date Rates

David P. Reed dpreed at reed.com
Mon Nov 29 20:02:26 PST 2004

Actually, Lloyd, I was proposing something based on the simplest, most 
basic digital fountain idea.

If you remember Luby's original papers, the basic idea is that the 
digital fountain stream is an infinite sequence of packets that encodes 
a source file that is N packets long, such that if you receive ANY 
subset of N packets correctly, you can reconstruct the original file.   
I.e., it's pretty close to the information theoretic efficiency of the 
packet-at-a-time channel.   You may also remember that the first N 
packets of the fountain are just those of the source file, while 
subsequent packets are combinations of the preceding frames.

Also, I was not proposing an idea to optimize the *utilization* of the 
satellite link, but to avoid such links' high variance in latency. when 
packets are dropped by processes that may be far from poisson or other 
simple distributions that can be handled by short-span FECs.

The problem with ARQ  on geosync satellite links is that anticipating 
which packet you need to retransmit is not useful, so the recipient must 
buffer at least a full round-trip's worth of data in a jitter buffer if 
he wants reliable isochronous transmission at minimal latency.   If 
instead the sender encodes each half-round-trip of data as a digital 
fountain, and dribbles the tail into the succeeding epochs to anticipate 
lost packets, you have a scheme that might help reduce the jitter 
buffer, and reduces the worst-case latency.   The protocol would use the 
reverse channel to adjust the rate of dribbles and stop the dribble of 
each successively received fountain.

This is analogous to an adaptive FEC, and becomes one, but based on a 
somewhat different theory and loss model, and with a quite different 
optimization goal - to wit, minimizing end-to-end latency and jitter 
given a requirement of reliable, sequenced packet delivery within a 
flow, not maximizing utilization.

Anyway, I was making no particular claims that this was the "best" way 
to do anything.   It was merely a suggestion of an interesting approach, 
at least one that was interesting to me, and possibly of interest to 
others.  Maybe it doesn't even work very well.  I wasn't intending it as 
equivalent to a published paper, just musing.  Perhaps the above 
paragraphs explain more why I think it was interesting, perhaps not.   
You are always free to ignore random ideas put forth by others.

PS: you persist in using disparaging words like "disingenuous" in this 
latest posting (like your use of "specious claim" in the prior email).   
"Disingenuous" is usually used to characterize a person as an 
intentional liar.   Come on, man.   I may or may not say stupid 
things.   I may have dumb ideas.   I characterized my original 
suggestion as an interesting thought.   But what on earth makes you 
persist in derogatory accusations about my motives and morals?

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