On Aug 10, 2007, at 5:08 PM, Kris Pister wrote:
>> The 1.4% is a bit misleading. As 6.2 discusses, the transport
>> protocol in the paper achieves 94.8% of the layer 3 protocol's
>> throughput (when considering the 1/3rd effect you get from
multihop).
Phil - this is exactly the sort of thing that drives me nuts about
papers in this community. First off, section 6.2 discusses details
of single-hop transmission using a 14.4kbps radio, and claims 33%
efficiency. The actual deployment is multi-hop, and uses 250kbps
radios, and achieves 1.4%.
:)
The "1/3rd effect" that you allude to is a constraint imposed by
single-channel operation. If you use a channel-hopping protocol
the theoretical limit is 1, not 1/3.
Absolutely -- it's the 1/3 imposed by having a single-channel CSMA.
Try telling an end user that his measured 1.4% throughput is
misleading, and that he should really take into account all of the
artificial constraints that you have come up with to explain it.
Agreed -- from a complete system standpoint, 1.4% is 1.4%. The TinyOS
15.4 MAC is notoriously inefficient for throughput, as it uses really
big backoff interval. All I was trying to say is that if you improved
the MAC in this regard, then the protocol described there would
benefit from it. We should be focusing on optimizing individual
layers, as they benefit everything above and below, rather than just
exploring cross-layer approaches. Of course, as I said earlier, cross-
layer efforts have helped figure out *what* those layers need: your
comments a few months ago about schedule information is a great
example. This is one reason I think 6lowpan has real potential to
shake out the space and lead to technical excellence.
Phil
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