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in most cases a shortest route passes through the nodes which are closer to the direc-
tion of the destination.
Whenever a forward ant enters a node from one of its neighbors, the identifier
of the neighbor, the sequence number of the ant, and the identifier of the destination
will be stored. Repeated forward ants will be destroyed. When a forward ant reaches
the destination, it is destroyed and a backward ant with the same sequence number
is sent back to the source. Moving from node B to node A, the backward ant increases
the amount of pheromone stored in edge AB. An evaporation process causes the
amount of pheromone deposited in each link to decrease with time.
The above stochastic strategy establishes multiple paths between the source and
destination. As a result, POSANT is a multipath routing algorithm. Multipath routing
reduces the chance of congestion in the network; on the other hand, they can lead
to out-of-order packet delivery problems.
If a node A realizes that the link to B is broken and there is a pheromone trail
corresponding to link AB for D in the pheromone table of A. In this case the sto-
chastic data routing will continue but if there is no pheromone trail for D in any of
the other outgoing links of A, A sends a message to its neighbors to inform them
that there is no route to D from A. Upon receiving this message, these neighbors do
the same as if the link to A is broken. If the only outgoing link of the source node
that has a pheromone trail for D breaks or a message from this link is received that
states there is no route to D, a new route establishment process will begin and sending
data packets will be suspended until a new route is found.
Simulations in [ 18 ] showed that POSANT has a shorter route establishment time
while using a smaller number of control messages than other ant colony routing
algorithms.
4.3.13
PAGs
In real applications, nodes may be distributed in 3D space. Abdallah et al. in [ 30 ] pro-
posed the Coordinate Face (CFace(3)), which is a heuristic using a projective approach
to adapt face routing [ 25 ] to 3D. The 3D points are first projected onto the xy plane and
the face routing is performed on this projected graph. If the routing fails, i.e., a loop is
detected, the points are then reprojected onto the second plane (the yz plane) and face
routing is performed again. If the routing again fails, the points are projected onto the
third plane (the xz plane) and the face routing is again performed. A simplified version
of CFace(3), called CFace(1), attempts face routing with the points projected once only
onto one of the xy, yz, or xz planes, randomly chosen.
In many applications, since wireless nodes are battery operated devices, they
need to conserve energy so that node life is maximized. So, the same authors in [ 19 ]
proposed three Power-Aware 3D Position-based Routing Algorithms for Ad Hoc
Networks (called here PAGs). Authors tried to maximize the delivery rate as well
as increasing network survivability (which can be measured by the remaining
power in the maximum used node during a set of consecutive routing messages).
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