Say you have a mesh network of sensors that measure something like ambient temperature. The nodes are out of range of the base station; however, there happens to be one intermediate node close enough that can relay their messages to the base. These sensors sample the temperature say every 30min (however they're not in sync) so ideally to conserve energy we'd like to put them in a lower power state the rest of the time. The problem is, if that one intermediate node isn't on then the rest of the nodes messages will either be lost or they'll have to stay on until the intermediate node turns on to sample temperature at which point it could relay other sensors messages if they resend their message before it goes back to sleep.
Is there some way a node could send out a beacon that excites a component, tuned to a certain frequency, on the other sensors (would this be a tuned antenna?) attached to a wake up interrupt line. This would enable an out of range sensor to wake up the intermediate sensor.