On Dec 6, 2007, at 4:39 PM, Timothy J. Salo wrote:
Depends on the link-layer energy management protocol. It is
possible to
heavily duty-cycle radio while supporting a broadcast communication.
Should I interpret your first sentence to mean: "It depends on
whether the node sleeps?" If it means more than that, please explain.
The general point is that the notion of "I send a packet when the
node is off" assumes a particular energy approach. For example, it
could be that the sender transmits a long PHY-layer preamble *before*
the packet, and receivers periodically wake up for a few ms to listen
for such a preamble. From the transmitter's perspective, this is just
"sending a packet" but the receiver's radio is asleep, nevertheless
the packet can arrive successfully.
The assumption is that the link layer provides a broadcast primitive
that reaches nearby nodes, taking into consideration the power
management policy. More specifically, the number of nodes reached by
a broadcast should be independent of the radio duty cycle.
Do any of the 802.15.4 chips support a "wake on receive"
capability? Does this capability (assuming it exists)
require that the radio be powered on continuously? That is,
does this capability power-down _only_ the processor, but
not the radio? Does this capability really save enough
power to meet the needs of many battery-powered, hopefully
long-lived wireless sensor networks?
This doesn't work like you think it does. To hear a signal, your
radio has to be on. The energy cost of demodulation is not the
principal issue.
If this capability actually exists, then we can talk
about whether it is actually useful.
Please provide an outline of how you think broadcast might
work in networks where most of the nodes sleep (really sleep,
as in powering down the radio and most of the microprocessor)
most of the time. This is an important discussion to start
soon.
Node transmits long preamble. Node transmits packet many times.
Here's a paper in submission that quantifies two approaches on a
particular 802.15.4 chip. Note that it actually references two other
approaches (BMAC and XMAC) which are themselves extensions of earlier
schemes. The author of this paper develops wireless sensor networks
for various branches of the U.S. government.
http://csl.stanford.edu/~pal/share/spots08.pdf
Well, I also don't believe that we have yet specified a complete
solution, much less one that works and is useful in common
environments.
You could argue that layer 2 interoperability is outside the scope of
6lowpan. For example, not all 15.4 devices work on 2.4GHz; I assume
we aren't trying to get 868MHz and 2.4GHz nodes to interoperate?
Phil
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