[e2e] How many transmission attempts should be done on wireless networks?
lachlan.andrew at gmail.com
Fri Sep 18 18:44:20 PDT 2009
2009/9/19 David P. Reed <dpreed at reed.com>:
> Queues and retransmissions are inseparable. You have to maintain a queue
> while retransmitting. Ideally (in coding theory) you never would
> retransmit the same thing twice. You would instead transmit a smaller and
> different piece of information the second time, so that the combination at
> the receiver would regenerate the lost information.
Very true. I believe Detlef is interested in HSPA systems which
already do hybrid ARQ, rather than retransmitting the same thing
twice. His question still applies.
In information-theoretic terms, Detlef's could have asked "how much
water-filling-over-time" should there be? If a channel is bad,
information theory tells us to delay sending information until the
channel becomes good. That is essentially what heroic hybrid ARQ
repeated retransmissions (with increasing delays) does. However, this
causes obvious problems to applications, which are ignored by
traditional information theory.
It would be interesting to separate this question into:
a) What is the optimum for running VJ's TCP?
b) What is the optimum for more general congestion control?
The answers may be very different.
One issue which I couldn't see in RFC3366 is the effect of the fading rate.
If fading causes outages much shorter than a RTT (unknown at the link
-- another story), then AIMD benefits from aggressive retransmission
so that its window is the right size for the average rate achieved
over a whole RTT. That is the fastest granularity that AIMD can
However, if the outages and resulting queueing become significant
compared to the propagation component of the RTT, then perhaps fewer
ARQs should be performed. (This is better than having aggressive
retransmission and a short queue, since it is drop-from-front, which
is better than drop-tail.)
Lachlan Andrew Centre for Advanced Internet Architectures (CAIA)
Swinburne University of Technology, Melbourne, Australia
Ph +61 3 9214 4837
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