Wake-on-WLAN
Published by Rob Flickenger July 7th, 2006 in Hacks, Wireless
Marco forwarded me an interesting implementation of Wake-on-WLAN from India.
When the network is unused, power is completely removed from the routing hardware (a Soekris 4521 running Pebble). It is switched on again when wireless transmissions are detected. This allows for substantial power savings, which is critical in remote installations.
We use off-the-shelf 802.15.4-based sensor motes (Moteiv’s Tmote sky). We take advantage of the fact that both 802.11b/g and 802.15.4 work in the ISM 2.4GHz band. 802.15.4 specifies a CCA (clear-channel assessment) mode which requires the received energy to be below a (configurable) threshold. The radio (Chipcon’s CC2420) on the sensor mote allows this to be programmed, and can generate an interrupt when the energy level crosses the threshold.
It’s a very nice write-up, including hardware design and lots of analysis, using multiple radio cards.
The initial cold boot of a stock Pebble means a delay of about 50 seconds on a net4521. This could be reduced quite a bit by using a 2.6 kernel with an optimized boot process (such as Pyramid) or on faster hardware (at the possible expense of more current consumption). And you could certainly adapt this technique to cycle the power on other devices as well.
While this hack would be of limited use in modern cities (where 2.4 GHz transmissions hang like a thick smog across the sky) it could be extremely useful in very remote places with little radio traffic.
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