This method can guarantee the path only on a hop-by-hop basis,
but cannot guarantee an end-to-end path. Most of these protocols include
adaptive algorithms to get around obstacles or dynamically back track and try
a di?®erent set of nodes. Ad-Hoc on-demand Distance Vector Routing (AODV)
[18] is a classical example of a protocol designed to set up temporary paths in
an on-demand fashion. AODV is an on-demand version of DSDV. Instead of
setting up routing paths to every node in the network, a node in AODV only
initiates a path when it becomes necessary. Once a node needs to transmit
data, a route request message is broadcast with a unique ID.When a receiving
node has a path to the destination in its routing table with a higher ID than
the route request??™s ID, it broadcasts a route reply. Otherwise it broadcasts
the route request packet to its neighbors and increments the hop count.
On-demand routing also has disadvantages. In situations where the data
is time critical, each node does not have time to query its neighbors for the
information necessary to decide on the next hop. This infers that the node
already has the information necessary to figure out an optimum next hop. The
information could be stored and transmitted with periodic local broadcasts.
These periodic transmissions are overhead because several of the transmitting
nodes may not be used in a routing path. When the data is not time-critical,
this approach is very e?±cient, unless the path needs to back track from a dead
end.
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