F. Transmission and Routing
Sensor nodes use the greatest amount of energy during data
communication. The transceiver in the sensor node contains
frequency synthesizers, mixers, voltage control oscillators,
phase-locked loops, and power amplifiers [6]. These all consume
power. Thus, data transmission and routing has to be
carefully designed to minimize the energy usage.
When messages are transmitted through tree or mesh networks,
they have to be routed to ensure they pass from source
to destination (sensor node to base station). Message latency
increases as the number of hops required to transmit the message
increases; thus, the routing protocol must be carefully
designed for the particular WSN topology. A number of routing
protocols have been proposed recently (see [4]). If the base
station fails in any topology, then the routing also fails. Thus,
when Mahlknecht and Madani [81] used a daisy chain (tree)
topology, they ensured that the nodes in the chain could also
directly communicate with the global communication network
if no base station was accessible. Another important consideration
of multihop communication is interference prevention.
For train-mounted WSNs, if two trains, Train A and Train B,
are within radio communications range of each other, then data
from sensors on Train A must never be transmitted to Train B
and vice versa [104]. Reason et al. [104] used identifiers to bind
the sensors to the individual train’s network.