Cryptography Reference
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the nearest sink node (Section 7.2.1). When a cluster head needs to send information to
the nearest sink, the aggregated data is encrypted using the identity (public key) of the
sink node, and the header of the packet is encrypted using the public key of the next
hop cluster head that is en route to the nearest sink node. The intermediate cluster head
decrypts the header, and depending on the destination field in the header, the packet
is forwarded to the appropriate neighboring cluster head or the sink node. In case the
sink node decides to send a query-based message, it should first locate the identities of
the cluster heads belonging to the desired sensing region. To retrieve the identities, the
sink sends an ID-request message encrypted with the public key of the immediate next
hop neighboring cluster head. The cluster head decrypts the packet and verifies if it is
the destination addressed. If not, it is forwarded to the appropriate destination in a way
similar to the route taken by the sink that initiated the request. Once the requested
packet reaches the destined cluster head, it promptly responds to the query by sending
its identity in an encrypted manner.
7.2.4 Path Redundancy
The design of WSN should not only consider scalability and energy efficiency but
also be robust in nature, which means that a network remains operational despite the
occurrence of sensor-node and link failures. The reasons for the failure could be inten-
tional (security attack) or unintentional (defective node or natural calamity). One of
the approaches to make the system more robust is to incorporate multipath routing.
In short, multipath routing implies the existence of multiple paths (disjoint or par-
tially disjoint) between source node and destination sensors (cluster heads or sink node)
(Ganesan et al. 2001).
Although maintaining alternate paths in a routing table introduces some overhead
and consumes more energy, multipath routing is an effective technique to improve
robustness when link failures occur. Link failures could occur for many different rea-
sons, such as frequent topological changes due to unreliable wireless communication
links. Moreover, multipath routing enables recovery from sensor and link failures and
provides the necessary resilience to the network at the cost of excessive redundancy.
7.2.5 Quality of Service (QoS)
WSN applications have varied requirements that are usually expressed in terms of some
metrics, such as delay, fault tolerance, and reliability. For example, real-time applica-
tions (video surveillance) are dependent on delay bounds.
Hence, for such applications, the sensed data must reach the sink within a certain
time. In addition, a desired property of WSN applications is fault tolerance, which
means that a network should remain functional in the event of sensor or link failures.
Another desired property is reliability, wherein the aggregated data should be received
by the sink as correctly as possible. This would ensure accurate decision making by
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