Wireless Sensor Networks : Technology, Protocols, and Applications

ROUTING STRATEGIES IN WIRELESS SENSOR NETWORKS 223
Other strategies assume that sensor nodes do not need to have the capability of
knowing their location coordinates. These strategies use virtual, as opposed to physical,
coordinate systems [6.36–6.39]. Using a virtual polar coordinate system, for
example, it is shown that a labeled graph can be embedded in the original network
topology. In this system, each node is given a label that encodes its position in the
original network topology in terms of a radius and an angle from a center location
[6.38]. These virtual coordinates do not depend on physical coordinates and can
therefore be used efficiently in geographical routing by using only node labels. It
is worth mentioning that schemes based on virtual coordinate sensor nodes may
need to know the distance, in terms of hop counts, to certain reference points.
This, in turn, may require periodic updates to be exchanged among the sensor nodes
and the reference points.
Despite its simplicity, the greedy approach to geographical routing may either
fail to find a path, even when one exists, or produce inefficient routes. This typically
occurs when, due to obstacles, for example, no neighboring node is closer to the
destination than is the current packet holder. To illustrate this problem, consider
Figure 6.17, where node S 1 needs to forward a packet to the destination D. Based
on the greedy approach, S 1 must select the closest neighbor to destination as the
next hop to forward the packet. However, nodes S 2 and S 3 , are both farther away
from the destination than is node S 1 . The greedy approach is trapped in a local minimum
(i.e., node S 1 ) and fails to make forward progress.
In WSN environments, where sensors are typically embedded in the environment
or deployed in inaccessible areas, voids are likely to occur. To circumvent voids, the
well-known graph traversal rule referred to as the right-hand rule has been
D
S 4
S 5
Void Area
S 2
S 3
S 1
Figure 6.17
Greedy algorithm forward progress failure.