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Some position-based routing algorithms, such as GPSR, fail to find a route from a
source to a destination (or they find a route that is much longer than the shortest path)
when the network contains nodes with irregular transmission ranges. On the other
hand, routing algorithms based on Ant Colony Optimization (ACO) guarantee mes-
sage delivery and converge to a route which is very close to the optimum route even
if the network contains nodes with different transmission ranges. However, ACO
algorithms use large number of messages and need long time before the routes are
established. POSition-based ANT colony routing Algorithm for mobile Ad Hoc net-
works (POSANT) [ 18 ] is a reactive routing algorithm which is based on ACO and
uses information about the location of nodes in order to reduce the route establish-
ment time while keeping the number of generated ants smaller in comparison to other
ant-colony-based routing algorithms.
In POSANT, to establish a route from a source node S to a destination node D,
neighbors of S are partitioned into three zones as shown in Fig. 4.9 . After that S
launches n forward ants with unique sequence numbers from each zone at regular
time intervals. POSANT assumes that each node maintains a table of the values of
pheromone trails assigned to its outgoing links for different destinations. Upon
receiving a packet for a specific destination, a node will check if there is at least one
pheromone trail for that destination, this pheromone trail will be used for making a
stochastic decision to select the next hop. If no such pheromone trail exists, a phero-
mone trail is initialized to each outgoing link. The amount of the deposited pheromone
on each link depends on the zone of the corresponding neighbor. The motivation is that
Fig. 4.9 Different zones of S for destination node D
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