Gugrajah_Yuvaan_ Ramesh_2003.pdf

Routing Protocols for Ad Hoc Networks
2.2.2.4. Discussion on Reactive Protocols
Chapter 2
A reactive routing protocol attempts to discover a route to a destination only when it
is' presented with a packet for forwarding to that destination. This increases the
latency of delivering the packet since discovery must be completed before the packet
can be sent. Also, without additional information, a protocol using on-demand
routing must search the entire network for a node to which it must send packets.
Discovering a new route therefore remains a costly operation. The use of some sort
of route cache can be used to avoid the need to re-discover each routing decision for
each individual packet. However, the cache itself may contain out-of-date
information indicating that links exist between nodes that are no longer within
wireless transmission range of each other. This stale data represents a liability that
may degrade performance rather than improve it.
Various comparative simulations of AODV and DSR have been published in the
literature. Broch et al [Broch98] at Carnegie Melon University (CMU) developed ad
hoc networking extensions for the ns2 network simulator [Ns02] to compare DSDV,
TORA, DSR and AODV. Another simulation was scenario-based [Johansson99]
which simulated DSDV, AODV and DSR in a conference setting, event coverage
situation and a disaster area. Das et al [DasOl] compared AODV to DSR.
[DasOl] concluded that in terms of throughput and delay DSR outperforms AODV
when average node speed and load is low while AODV outperforms DSR with
widening performance gaps with high load and higher node speeds. However, it was
found that DSR consistently generates less routing overhead than AODV. This is
because AODV made more frequent route requests. The aggressive caching used in
DSR and the inability to expire stale routes were recognized.as DSR's weaknesses.
[Johannson99] also found that DSR and AODV perform relatively similarly with
similar conclusions to [DasOl]. [Broch98] found that while DSR and AODV
performed relatively similarly, at higher node speeds the routing overhead of AODV
is more expensive. Both [Broch98] and [Johannson99] found that DSDV had
difficulties in maintaining valid routes when the average node speed was increased
due to DSDV being a proactive protocol. [Johannson99] concluded that the reactive
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