The route maintenance process at DSR is divided into two messages: rounte error and
acknowledgment. Route error message is generated when a node encounters fatal
transmission problem, while the acknowledgment message is created when correct operation
of link route is achieved.
2.4.4 Hybrid routing protocol
Many challenges face the routing protocols proposed for mobile ad hoc networks such as
frequently changed topology, low transmission power and asymmetric link. Proactive and
reactive routing protocols were proven to be insufficient to overcome these challenges when
operate separately. Hybrid ad hoc routing protocols are proactive and reactive routing
protocols at the same time, which are designed to work as proactive routing protocols for the
close destinations from the source and act as reactive routing protocols for far destinations.
These types of routing protocols are designed to reduce the overhead caused by route
discovery through allowing the close nodes to work together. These kinds of routing
protocols are also called Zone-Based routing protocols as they divide the network into zones
and nodes in each zone work together and communicate with other zones on demand. Zone
Routing Protocol (ZRP) [32] is an example of this kind of unicast ad hoc routing protocols.
This next section will describe ZRP as an example of routing protocols related to this type.
2.4.4.1 Zone routing protocol (ZRP)
ZRP [32] is developed to combine the advantages of reactive and proactive routing
protocols into one type of routing protocols i.e hybrid routing protocols.
In ZRP, a routing zone parameter (of radius P) is defined for each node; this parameter
defines the distance in number of hops from each node where within this zone the routing
information is available. For destinations outside this zone the ZRP performs a route
discovery using any of on-demand unicast routing protocols exploiting the information
available from the local zone. Figure 2.6 shows the node A with routing zone P=2.
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