Position-Based Routing Protocols for Ad Hoc Networks - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 48

MFR is probed to be a loop-free algorithm [ 23 ] since it always forces message
to make a step closer to the destination, and it is the only progress-based algorithm
competitive in terms of hop count [ 24 ]. However, choosing the node with the largest
progress as the next hop will increase the probability that the two nodes discon-
nected from each other before the packet reaches the next hop. So, the packet drop
rate increases greatly, especially in highly mobile environments. Such a situation is
very common due to neighbor table inconsistency [ 1 ].

4.3.2 DIR

Compass routing algorithms, such as DIR [ 9 ], try to minimize the spatial distance
that a packet travels and base on forwarding the packet to the neighboring node that
minimizes the angle between itself, the previous node, and the destination [ 6, 8, 12 ].
The source or intermediate node A uses the location information of the destination
D to calculate its direction. Then the message m is forwarded to the neighbor C,
such that the direction AC is closest to the direction AD. This process repeats until
the destination is, eventually, reached [ 24 ]. Consider the network in Fig. 4.3 , where
the transmission radius is as indicated in the figure. The direction AC is closest to
the direction AD among candidate directions AS, AB, AC, AG, and AF. So the path
selected by DIR method is SACD.
As a greedy protocol, DIR suffers from congestion created by frequent beacon-
ing and it may not always find the optimum route, even it may fail to find a path
between source and destination if it exists. The DIR method, and any other method
that includes forwarding message to neighbor with closest direction such as
DREAM [ 11 ], is not loop-free as shown in [ 23 ] using the counterexample in
Fig. 4.4 . The loop consists of four nodes denoted S, B, C, and A. The transmission
radius is as indicated in the figure. Let the source be any node in the loop, e.g.,
S. Node S selects node B to forward the message, because the direction of B is closer
to destination D than the direction of its other neighbor A. Similarly node B selects
C, node C selects A and node A selects S.

Fig. 4.3 DIR example