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WSNs, which can be
considered as a special case of ad hoc networks with reduced or no mobility,
are expected to find increasing deployment in coming years, as they enable
reliable monitoring and analysis of unknown and untested environments. These
networks are "data centric", i.e., unlike traditional ad hoc networks
where data is requested from a specific node, data is requested based on certain
attributes such as,"which area has temperature over 35°C or 95°F".
Therefore a large number of sensors need to be deployed to accurately reflect
the physical attribute in a given area. Routing protocol design for WSNs is
heavily influenced by many challenging factors, which must be overcome before
efficient communication can be achieved. These challenges can be summarized as follows:
Ad hoc
deployment -
Sensor nodes are randomly deployed which requires that the system be able to cope
up with the resultant distribution and form connections between the nodes. In
addition, the system should be adaptive to changes in network connectivity as a
result of node failure.
• Computational capabilities -
Sensor nodes have limited computing power and therefore may not be able to run
sophisticated network protocols leading to light weighted and simple versions
of routing protocols.
• Energy consumption without
losing accuracy - Sensor nodes can use up their limited energy supply
carrying out computations and transmitting information in a wireless
environment. As such, energyconserving forms of communication and computation
are crucial as the node lifetime shows a strong dependence on the battery
lifetime. In a multi-hop WSN, nodes play a dual role as data sender and data
router. Therefore, malfunctioning of some sensor nodes due to power failure can
cause significant topological changes and might require rerouting of packets
and reorganization of the network.
Scalability - The number of
sensor nodes deployed in the sensing area may be in the order of hundreds,
thousands, or more. Any routing scheme must be scalable enough to respond to
events and capable of operating with such large number of sensor nodes. Most of
the sensors can remain in the sleep state until an event occurs, with data from
only a few remaining sensors providing a coarse quality.
• Communication range -
The bandwidth of the wireless links connecting sensor nodes is often limited,
hence constraining inter sensor communication. Moreover, limitations on energy
forces sensor nodes to have short transmission ranges. Therefore, it is likely
that a path from a source to a destination consists of multiple wireless hops
Fault tolerance - Some sensor
nodes may fail or be blocked due to lack of power, physical damage, or
environmental interference. If many nodes fail, MAC and routing protocols must
accommodate formation of new links and routes to the data collection BSs. This
may require actively adjusting transmit powers and signaling rates on the
existing links to reduce energy consumption, or rerouting packets through
regions of the network where more energy is available. Therefore, multiple
levels of redundancy may be needed in a fault tolerant WSN.
• Connectivity - High node
density in sensor networks precludes them from being completely isolated from
each other. Therefore, sensor nodes are expected to be highly connected. This,
however, may not prevent the network topology from varying and the network size
from shrinking due to sensor nodes failures. In addition, connectivity depends
on the, possibly random, distribution of nodes.
Transmission
media -
In a multi-hop sensor network, communicating nodes are linked by a wireless medium.
Therefore, the traditional problems associated with a wireless channel (e.g.,
fading, high error rate) also affect the operation of the sensor network. In
general, bandwidth requirements of sensor applications will be low, in the
order of 1-100 kb/s. As we have seen in Chapters 4 and 5 and in the previous
section, the design of the MAC protocol is also critical in terms of conserving
energy in WSNs.
• QoS - In some
applications (e.g., some military applications), the data should be delivered
within a certain period of time from the moment it is sensed, otherwise the
data will be useless. Therefore, bounded latency for data delivery is another
condition for time constrained applications.
• Control Overhead - When
the number of retransmissions in wireless medium increases due to collisions,
the latency and energy consumption also increases. Hence, control packet
overhead increases linearly with the node density. As a result, tradeoffs
between energy conservation, selfconfiguration, and latency may exist.
• Security - Security is
an important issue which does not mean physical security, but it implies that both
authentication and encryption should be feasible. But, with limited resources,
implementation of any complex algorithm needs to be avoided. Thus, a tradeoff
exists between the security level and energy consumption in a WSN.
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