The formation of ad hoc wireless networks is stimulated by the requirement of a temporary communication infrastructure for quick communication with minimal configuration among a group of heterogeneous devices. Devices used for such applications could typically be enhanced personal digital assistants (PDAs), laptops with add-on wireless interface cards, or mobile devices with high processing power. Thus, this collaborative and distributed computing environment demands interoperatibilty among the devices. The additional features never come for free as they make the routing and other services more challenging and causes vulnerabilities in network services. Resource constrained, battery powered wireless mobile nodes not only have to self configure and self monitor them but also generates a very accommodating, trustworthy and affable environment. A classification of routing protocols and their brief description, based on their operating principles and underlying features is explained in this paper.
: Node mobility, mobility factor, routing latency, energy consumption, network traffic
In MANETs every node may function as a router and forward packets through routing paths. Co-operation among nodes during path discovery and packet relaying is of primary concern and should be supported for correct functioning of the network. Communication in a MANET occurs in a discrete and disperse environment with no centralized management which arises a main issue in MANET that is the breakage of link at certain moment and re-generation of link at certain state as it consists of routers which are mobile in nature i. e. are independent to roam in an arbitrary motion. A MANET is a dynamic multi-hop wireless network which is established by a group of mobile and independent nodes on a shared wireless channel by virtue of their proximity to each other. Generally low configured nodes are used in mobile adhoc networks to support mobility to user, so limited resources, dynamic network topology and link variations are the major issues with MANET. The number of link breakages observed by a node in an adhoc network can be used as a mobility metric so that each individual node can adjust its routing behavior based on the environment around it which improves the overall routing protocol performance
Requirements of Ad Hoc Networks
Ad hoc networks should give more emphasis and should also meet the following requirements to support a wide range of applications including military operations, outdoor emergencies, and natural disasters.
1. Scalability: The routing protocols employed for packet forwarding should be capable to scale for a network with a large number of nodes where the nodes keep on adding into the network dynamically. Routing should efficiently adapt itself to the network size.
2. Distributed Nature: The routing, computation and maintenance approaches in an ad Hoc wireless network should be fully distributed as a centralized approach in these domains may consume a large amount of bandwidth.
3. Communication Capabilities: The lack of any centralized support should not hinder the communication among the nodes.
4. Flexibility: Adhering to a same set of nodes to a destination throughout the routing process isn’t supposed to be valuable. Freedom to select suitable nodes in terms of their reliability and computing power offers flexibility in the network.
5. Efficient Routing: The prerequisites of an efficient routing scheme are the involvement of a minimum number of nodes in route discovery and maintenance and minimum connection set up time. The multicasting of packets should make a minimum number of transmissions to all the group members.
6. Bandwidth and Resource Availability: The shared wireless link and stringent resources like transmission power, battery energy, processor power and device power must assure their maximum availability to cope up with such a dynamic environment.
7.Multifence Security Scheme: A multi-hop connectivity is provided in Manets through distributed protocols in both the network and link layers, the ultimate multifence security solution must span both layers with each layer contributing to a line of defense.
The multi-hopping behavior of MANETs is as shown in Figure 1.2 The routing information and data packets travels from one hop to another in the network, if a node A wants to send a data packet to node D, it can do so via B which is in the common range of both the nodes. However if B moves away and is beyond the range of A, the link is broken and a different route has to be established.
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Fig 1.2 Multi-hopping behavior of nodes in Mobile Ad Hoc Networks
2. Strategies for Routing in Mobile Ad Hoc Networks (MANETs)
The most important networking operations include efficient routing and adequate network management. Based on the routing information update mechanism, ad hoc wireless network routing protocols can be classified in three major categories. These are:-
Proactive or Table driven Routing: In table driven routing protocols every node in the network maintains routing information and periodically exchange it with other nodes, which add a subsequent overhead in the network as the routing information is generally flooded in the whole network. Sequence numbers are used to distinguish recent information from the stale data. This category of routing suffers from excessive control overhead and keeps on increasing as the network scales to larger number of nodes and when the environment is highly dynamic. The nodes exchange the routing information either through incremental updates or in full dumps. Destination sequence distance vector (DSDV), wireless routing protocol (WRP) belongs to this category and offers availability of routes.
Reactive or On demand Routing: Reactive protocols obtain the necessary path to the destination only when it is required uses a connection establishment process. The routing information is propagated to the nodes only when necessary. Reactive protocols out performs proactive ones but high mobility in the network leads to degradation of performance. These protocols eliminate the need to periodically flood the network with table update packets and thus control the bandwidth requirement. The control overhead becomes low if we limit the search area for finding a path to the destination. Adhoc on Demand Distance Vector (AODV) and Dynamic Source Routing (DSR) are the quintessence of reactive routing.