C. Network Model
WSNs for railway applications follow the open systems
interconnection model [137]. A typical WSN has five layers
in a protocol stack with three planes [5], [6] to transmit data
from the sensor nodes to the base station. The five layers
from lowest to highest are the physical layer defines how the
sensors transmit their data to the network. The data link layer
specifies the network topology and connects nodes to each
other. It uses MAC to control data access and permissions and a
logical link control to control error checking and data packet
synchronization. This is a key layer in railway monitoring
WSNs as they may be monitoring difficult environments such
as tunnels or monitoring moving trains. The topology must be
carefully designed. The network layer routes the data through
the network as packets. Railway WSNs are energy constrained;
thus, routing has to be carefully designed to work within the
available energy supply. The transport layer controls the sending
and receiving of data. Finally, the application layer allows
application software to access the data.
The size of the transmission varies from bits in the lowest
(physical) layer, bytes, and frames in the data link layer,
packets in the network layer, segments in the transport layer,
and data in the highest (application) layer.
The power management plane manages the power consumption
of sensor nodes. The mobility management plane
identifies and records the movement of sensor nodes to ensure
that data can be routed from the node to the control center
and so that sensor nodes are aware of their neighbors for communication.
This is important for WSNs monitoring moving
trains. The task management plane balances and coordinates
network regions (groups of nodes) and ensures that they generate
data according to their power level.