Due to the severe energy constraints of large number of densely deployed sensor nodes, it requires a suite of network protocols to implement various network control and management functions such as synchronization, node localization, and network security. A routing protocol is a set of rules used by routers to determine the most appropriate paths into which they should forward packets towards their intended destinations.
Routing Protocol Overview
A routing protocol is considered adaptive if certain system parameters can be controlled in order to adapt to current network conditions and available energy levels. Routing in wireless sensor networks differs from conventional routing in fixed networks in various ways. There is no infrastructure, wireless links are unreliable, sensor nodes may fail, and routing protocols have to meet strict energy saving requirements. Part of the job of the routing protocol is to specify how routers report changes and share information with the other routers in the network in order to update their routing tables, thereby allowing networks to dynamically adjust to changing conditions (e.g., changes in network topology and traffic patterns).
Routing is the act of moving information from a source to a destination in an internetwork. During this process, at least one intermediate node within the internetwork is encountered. This concept is not new to computer science since routing was used in the networks in early 1970’s. But this concept has achieved popularity from the mid-1980. The major reason for this is because the earlier networks were very simple and homogeneous environments; but, now high end and large scale internetworking has become popular with the latest advancements in the networks and telecommunication technology. The routing concept basically involves, two activities: firstly, determining optimal routing paths and secondly, transferring the information groups (called packets) through an internetwork. The later concept is called as packet switching which is straight forward, and the path determination could be very complex.
Routing Protocol Significance
Traditional wireless communication networks require a fixed infrastructure over which communication takes place. Therefore, considerable resources and effort are required to set up such networks, even before they can actually be used. In cases where setting up infrastructure is a difficult or even impossible task, such as in military applications, disaster relief, or emergency operations, other alternatives need to be developed. Due to the dynamic nature of MANETs, designing communications and networking protocols for these networks is a challenging process. One of the most important aspects of the communication process is design of the routing protocols which are used to establish and maintain multi-hop routes to allow the data communication between nodes.
Routing Protocol Strategies
- Proactive Strategy: In proactive scheme, every node continuously maintains the complete routing information of the network. When a node needs to forward a packet, the route will be readily available; thus there is no delay in searching for a route. However, for a highly dynamic topology, the proactive schemes will spend a significant amount of scarce wireless resource in maintaining the updated routing information correct. Examples of these protocols based on this strategy are Destination Sequenced Distance Vector (DSDV) Routing and Optimized Link State Routing. The proactive protocols are not suitable for larger networks, as they need to maintain node entries for each and every node in the routing table of every node. This causes more overhead in the routing table leading to consumption of more bandwidth.
- Reactive Strategy: In reactive schemes, nodes only maintain routes to active destinations. A route search is needed for every new destination. Therefore, the communication overhead is reduced at the expense of route setup delay due to route search. These schemes are preferred for the ad-hoc environment since battery power is conserved both by not sending the advertisements as well as not to receiving them. The route discovery usually occurs by Flooding the route request packets throughout the network.
- Hybrid Strategy: In hybrid strategies, this protocol divide the network into zones (clusters) and run a proactive protocol within the zone and a reactive approach to perform routing between the different zones. This approach is better suited for large networks where clustering and partitioning of the network is very common. Hybrid routing protocols are proposed to combine the merits of both proactive and reactive routing protocols and overcome their shortcomings. Normally, hybrid routing protocols for mobile ad hoc networks exploit hierarchical network architectures. The proper proactive routing approach and reactive routing approach are exploited in different hierarchical levels, respectively.
Figure 1 Routing Protocols
 Patrick Stüdi, Gustavo Alonso, Quality of Service for Mobile Adhoc Networks, Diploma Thesis 2003.
 C.E. Perkins, P.R. Bhagwat, “Highly Dynamic Destination Sequences Distance Vector Routing (DSDV) for mobile computers”, Proceedings of ACM SIGCOMM, pp. 234−244, 1994
 Xiaoyan Hong, Kaixin Xu, and Mario Gerla, Scalable routing protocols for mobile ad hoc networks, 2002.