The term MANET (Mobile Ad hoc Network) refers to a multi hop packet-based wireless network composed of a set of mobile nodes that can communicate and move at the same time, without using any kind of fixed wired infrastructure. MANETs are actually self-organizing and adaptive networks that can be formed and deformed on the-fly without the need of any centralized administration. This exclusive characteristic allows the use of MANETs in many particular civilian and military situations as well as in the emerging sensor networks technology.
As for other packet data networks, one-to-one communication in a MANET is achieved by unicast routing each single packet. Routing in MANETs is challenging due to the constraints existing on the transmission bandwidth, battery power, and CPU time and the requirement to cope with frequent topological changes resulting from the mobility of nodes. Nodes of a MANET cooperate in the task of routing packets to destination nodes since each node of the network is able to communicate only with those nodes located within its transmission radius R, while the source and destination nodes can be located at a distance much higher than R.
A wireless network that transmits from computer to computer. Instead of using a central base station (access point) to which all computers must communicate,
This peer-to-peer mode of operation can greatly extend the distance of the wireless network. To gain access to the Internet, one of the computers can be connected via wire or wireless to an ISP routing from one node to another on such a "mesh" network typically uses an on-demand routing protocol, such as PROACTIVE and REACTIVE, which generates routing Information only when
a station initiates a transmission.
In the past, much research effort has been devoted on Service Discovery in static networks, like the Internet. The emergence of wireless communications and small mobile computing devices has created the need for developing service discovery protocols and architectures targeted to mobile environments. Especially, the proliferation of Mobile Ad Hoc Networks (MANETs) has introduced new requirements to service discovery due to the inherent characteristics of these networks.
It is a collection of mobile computing devices equipped with wireless network interfaces which can connect together dynamically to create a multi-hop wireless network, without the requirement for any pre-existing infrastructure. The mobility of the nodes makes the topology of the network time-variant. The rate of change of the network topology depends upon the velocity of the nodes. The wireless network is characterized by low bandwidth links that are subject to harsh conditions of fading and interference. Thus, routing in such a network is difficult and challenging. A plethora of routing protocols has been proposed for wireless ad-hoc networks. These protocols may be mainly classified as either proactive or reactive
A first attempt to cope with mobility is to use specific techniques aimed at tailoring the conventional routing protocols to the mobile environment while preserving their nature. For this reason, the protocols designed around such techniques are referred to as table ÿ driven or proactive protocols. To guarantee that routing tables are up-to-date and reflect the actual network topology, nodes running a proactive protocol continuously exchange route updates and recalculate paths to all possible destinations.
When proactive routing protocols are employed a node would possess routing information to a destination before it would actually need to route data to that destination.
For this purpose routing tables are maintained. Route updates are exchanged periodically to reflect the changes in topological information. Popular proactive routing protocols for ad-hoc networks include
Destination Sequenced Distance Vector (DSDV) Protocol
Wireless Routing Protocol
Source Tree Adaptive Routing (STAR) Protocol.
The proactive routing protocols usually require the maintainance of routing tables and thus, in the dynamically changing mobile ad- hoc network, nodes would need to exchange routing updates periodically.This exchange of route updates would consume bandwidth and if the network is large, these control messages could contribute to a significant amount of overhead.
By continuously evaluating the known and attempting to discover new routes, they try to maintain the most up-to-date map of the network. This allows them to efficiently forward packets, as the route is known at the time when the packet arrives at the node.
If on demand routing protocols are used, when data is to be routed to a destination, a source node might be required to initiate a search for the destination. If the network is large, significant latency may be incurred before the destination is found .
A different approach in the design of a routing protocol is to calculate a path only when it is necessary for data transmissions. Protocols of this family are dubbed reactive protocols or on ÿ demand routing protocols (e.g., , , , ). A reactive protocol is characterized by a path discovery procedure and a maintenance procedure. Path discovery is based upon a query-reply cycle that adopts flooding of queries. The destination is eventually reached by the query and at least one reply is generated. Path discovery is triggered asynchronously on-demand when there is a need for the transmission of a data packet and no path to the destination is known by the source node. Discovered paths are maintained by the route maintenance procedure until they are no longer used.
Reactive protocols determine the proper route only when required, that is, when a packet needs to be forwarded. In this instance, the node floods the network with a route request and builds the route on demand from the responses it receives. The scalability of both the table driven and the on demand routing protocols is limited.
Numerous on-demand routing protocols have been proposed some of the on demand routing protocols .They are
Adaptive On Demand Distance Vector (AODV) protocol
Dynamic Source Routing (DSR) Protocol
Temporally Ordered Routing Algorithm (TORA)
1. There are a number of implementations of the Ad-hoc On-demand Distance Vector (AODV) routing protocol available for the Linux platform.
2. The AODV protocol consists of a number of messages which it uses for route discovery, route maintenance and repair, and neighbor detection.