[e2e] Satellite Date Rates

Christian Huitema huitema at windows.microsoft.com
Mon Nov 29 20:38:23 PST 2004

> Normally you'd tailor your FEC for the usual channel conditions for
> original transmissions, and fall back to ARQ for unusual. Explicit
> ARQ, buffering at only one end, does have some advantages.

It also has some drawbacks. You can only do with "buffering at one end"
if you don't care about out of order delivery, or if you are content
with a simple Go-Back-N scheme. Go-Back-N schemes are very sensitive to
the combination of error rates with high bandwidth and long delays, and
are thus not a very good choice on satellite links. You want to apply a
selective ACK, which is very efficient but requires buffering twice the
bandwidth delay product at the sender and once more at the receiver, for
re-sequencing. In short, there is no free lunch there either -- you
trade better efficiency for large buffer and longer delays, i.e. an
average sojourn of an additional RTT in the re-sequencing buffer if
errors are frequent enough. 

-- Christian Huitema 

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