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If the source and destination is in the same zone then source uses a route from proactively cached routing table that is maintained by IARP. If the destination is outside the zone then route is reactively discovered by IERP . In route discovery, the source node broadcasts a route request packet to its peripheral nodes or border nodes, including its own address, destination address and a unique sequence number. Route request packets are sent to peripheral nodes by using the Bordercast Resolution Protocol (BRP). After receiving the request packet, peripheral nodes check their local zone. If the destination is outside the local zone, then peripheral node forwards packet to its peripheral nodes. If local zone containing a destination node then it send route reply back to source . New neighbor nodes and link failures are discovered by Neighbor Discovery Protocol (NDP). In fig. the node S wants to broadcast a packet to node X and the zone radius is r=2. The nodes from A to J belong to the routing zone of S, but not other nodes. The nodes G to J are called peripheral nodes (Gray in Figure).
Fig.- Routing Zone of S
2). TORA (Temporally Ordered Routing Algorithm)- TORA is highly adaptive, loop free and proficient distributed routing algorithm based on the link reversal concept. TORA is designed to operate in highly dynamic multi-hop wireless mobile networks .It provides multiple routes to destination. The protocol performs three essential functions:
Route creation in this protocol is done by using QUERY and UPDATE packets. To discover a route to destination, the node sends a QUERY packet to its neighbor nodes . Initially height of destination is set to 0 and all other nodes to NULL. The source node transmits a QUERY packet by including ID of destination node in it.
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Fig.-Propagation of the QUERY message
A node that has non -NULL height sends a reply with UPDATE packet that list its height .A node that receives a UPDATE packet sets its height to one greater than the height of that node from which it was received the UPDATE packet . Higher height node is considered upstream node and lower height is downstream. In route creation phase, nodes use a height metric to form Directed Acyclic Graph (DAG) routed at the destination. Flow between nodes in DAG from higher to lower height node .
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Fig.-Nodeâ€™s height is updated
When the nodes change its position then Directed Acyclic Graph is broken and route maintenance is required to form a DAG again for that particular Destination. If network partition is detected by node then it will generates CLEAR packets and broadcast throughout the network. Route erasure results in reset of all routes in network .
A routing protocol is required to send a packet from source node to destination node via number of nodes. Several routing protocols have been proposed for mobile ad-hoc networks . Mainly, routing protocols are classified into three types:
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Fig.- Classification of Routing protocols
Reactive or On-Demand Routing Protocols- In reactive routing protocols, routes are not predefined for routing. Routes are generated by route discovery mechanism only, when it is required by source to transmit a packet to destination . Reactive protocols have lesser routing overhead but higher latency because these protocols do not make the nodes to start a route discovery until a route is required . Examples:
AODV (Ad-hoc On-demand Distance Vector Routing Protocol).
DSR (Dynamic Source Routing).
Proactive or Table Driven Protocols- In proactive routing protocols, all routes are predefined in routing tables and source nodes uses that routes to forward a packet to destination. Each node needs to be update routing tables regularly. Proactive protocols have higher routing overhead due to periodic updates in routing tables and lower latency because routes are predefined ,. Examples:
DSDV (Destination-Sequenced Distance-Vector Routing).
OLSR (Optimized Link State Routing).
Hybrid Protocols- Hybrid protocols includes the features of both reactive and proactive routing protocols .These protocols are suitable for large scale networks and overcome the drawback of both reactive and hybrid routing protocols. Examples:
ZRP (Zone Routing Protocol).
TORA (Temporally Ordered Routing Algorithm Protocol).
In this review paper, we focused on the study of reactive, proactive and hybrid routing protocols. The two protocols from each category have been taken up for the further study. There are numerous routing protocols have been proposed. We conclude that each protocol works in a different way in ad-hoc networks and routing protocols plays a major role in network performance. It has been further concluded that proactive routing protocols are more efficient for ad-hoc network due to regular routing updates that helps in route identification and deals with dynamic topology. Moreover, hybrid routing protocols overcome the drawbacks of both reactive and proactive protocols and it is mainly used for large scale networks.