Sensor networks consist of a certain number (which can
be very high) of sensing nodes communicating in a wireless
multi-hop fashion. Usually nodes report the results
of their sensing to a small number (in most cases, only
one) of special nodes called sinks. A large scientific literature
has been produced on sensor networks in the recent
past, addressing several problems at all layers of the protocol
stack [22]. Design objectives of the proposed solutions
are energy efficiency (which is the scarcest resource in
most of the scenarios involving sensor networks), scalability
(the number of nodes can be very high), reliability (the
network may be used to report urgent alarm events), and
robustness (sensor nodes are likely to be subject to failures
for several reasons).