Wireless Network Design: Optimization Models and Solution ...

9 Routing in Mobile Ad Hoc Networks 209
flows. After traffic forwarding is achieved, the routing information is maintained for
a fixed time before it is purged from the routing table.
When a mobile node wishes to send packets to a destination, it starts checking if it
has an available route in its routing table. If this is not the case, it will initiate a route
discovery by sending a Route Request packet (RREQ) to locate the mobile recipient.
This packet is distributed to its neighbors. The mobiles receiving the RREQ will
look to determine if they have a route available to the recipient in their routing tables.
If this is not the case, they in turn rebroadcast the RREQ packet to their neighbors
and keep a trace 1 . RREQ packets are flooded throughout the network. When such a
packet reaches the destination, or a mobile node that has a route to this recipient, a
response packet (RREP) is generated by the mobile and sent back again, thanks to
information stored in the caches traversed by the RREQ. During the flooding of the
RREP packet, each mobile node will update its routing table by keeping the identity
of the destination according to the selected route.
For example, in Figure 9.5, the mobile node X, which has no route to mobile node
Y, initiates a route discovery process. As we can see, the RREQ generates many
redundancies and uses a high number of resources. From the MAC layer point of
view, many nodes will compete in order to win a slot within the conflict resolution
protocol. When the number of requests is very high, this protocol might result in
very poor network performance. In a dense network (high number of neighbors per
node), the uncontrolled flooding is a disaster for network performance.
Fig. 9.5 Route discovery
flooding
9.4.2.2 DSR (Dynamic Source Routing)
RREQ(1)
RREQ(3)
X Y
RREQ(1)
RREQ(2) RREQ(6)
RREP
RREP
RREQ(4) RREQ(1)
RREQ(7)
RREQ(5)
DSR is also a reactive protocol and it works in a very similar way to AODV. The
main difference is in the routing procedure. AODV is table driven and DSR uses
source routing. Nodes in DSR are idle and when a node decides to send traffic to a
destination, it will initiate a route discovery in an identical procedure to AODV. Then
to indicate the route, instead of creating local routing tables, each intermediate node
1 Using sequence numbers on packets avoids infinite loops.