Routing in ad hoc networks: proactive, Reactive & Hybrid networks

Ad hoc networks are decentralized wireless networks where nodes communicate directly without relying on a fixed/central infrastructure like routers or access points. They are also known as Mobile Ad Hoc networks (MANETs). Routing protocols in Ad Hoc Networks are categorized into three main types:

Proactive, reactive and hybrid networks. Each category depends on the network size, mobility of nodes and application requirements

1. Proactive Routing Protocols

Proactive routing protocols, also known as table-driven protocols, maintain fresh lists of destinations and their routes by periodically distributing routing tables across the network. These protocols work similarly to traditional wired network protocols like OSPF (Open Shortest Path First) or RIP (Routing Information Protocol) i.e. Every node maintains up-to-date information about all possible destinations in the network, so a route is readily available whenever data transmission is initiated thus reducing the delay in data transmission.

Examples of proactive protocols:

 Destination Sequenced Distance Vector routing protocol (DSDV): is a table driven protocol that extends the distance vector routing of the wired networks. This protocol is based on the Bellman-Ford algorithm, where each node maintains a table of the shortest paths to all other nodes. DSDV solves the "count-to-infinity" problem by incorporating sequence numbers to ensure up-to-date routing.

 Optimized Link State Routing (OLSR): OLSR uses link-state algorithms to maintain routing tables. It optimizes message flooding by using multipoint relays, reducing overhead i.e. It optimizes the standard link-state algorithm by reducing the amount of information broadcasted and the number of nodes involved in the routing process.

Advantages

 Low latency in route discovery where there is no waiting time needed in route discovery when data needs to be transmitted

 Suitable for low-mobility networks: Where the topology is relatively stable, proactive protocols ensure that the communication remains reliable with minimal delays.

Disadvantages:

 High overhead: Maintaining up-to-date routing tables for all nodes generates significant overhead, particularly in large or highly dynamic networks.

 Inefficient in high-mobility environments: In networks with frequent topology changes, proactive protocols waste resources in maintaining routes that may become outdated quickly.

2. Reactive Routing protocols

They are also known as on-demand routing protocol. In this type of routing, the route is discovered only when it is required i.e. they create routes only when desired by the source node. The process of route discovery occurs by flooding the route request packets throughout the mobile network. It consists of two major phases: Route discovery and route maintenance.

Examples of reactive protocols:

 Ad hoc On-Demand Distance Vector (AODV): AODV creates routes on an on-demand basis using a route request (RREQ) and route reply (RREP) mechanism. Routes are maintained as long as they are actively used i.e. It maintains only the routes that are actively used, reducing the need of periodic updates. In AODV, the source node doesn’t store complete path information, instead it stores it stores information for its previous and next node.

 Dynamic Source Routing (DSR): It is a reactive routing protocol where the route is discovered only when it is required. The process of route discovery occurs by flooding the route request packets thought the mobile network. The source node stores the complete path information and intermediate nodes do not need to maintain routing information, that is, DSR uses source routing where the entire route to the destination is included in the packet header.

Advantages

 Lower overhead since routes are only established when needed thus minimizing bandwidth consumption caused by continuous route updates.

 More efficient for highly dynamic networks: Reactive protocols perform well in networks with frequent topology changes because they do not waste resources maintaining unused routes.

Disadvantages:

 The time taken to find a route when data transmission is initiated can introduce delays, making reactive protocols less suitable for real-time applications which causes high latency in route discovery:

 The process of discovering routes requires broadcasting route requests to the network, which can lead to congestion in large networks causing route discovery flooding.

3. Hybrid routing protocols

Hybrid routing protocols combine the advantages of both proactive and reactive approaches to achieve a balance between low-latency route discovery and reduced control overhead. Hybrid protocols typically divide the network into zones or clusters. If the source and destination mobile nodes are present in the same zone, then proactive routing is used for transmission of data packets between them. And if the source and destination mobile nodes are present in different zones, the reactive routing is used for the transmission of data packets between them i.e. proactive routing is used within local region of a network and reactive routing for communication between these regions

Example of Hybrid routing protocols:

 Zone Routing Protocol (ZRP): ZRP divides the network into overlapping zones. Within each zone, proactive routing is used, while reactive routing is employed for communication between zones. This reduces the control overhead while maintaining low latency for intra zone communication.

 Hybrid Wireless Mesh Protocol (HWMP): HWMP combines elements of both proactive tree-based routing and reactive on-demand routing to provide efficient and flexible routing.

Advantages:

 Optimized performance: Hybrid protocols balance the trade-offs between proactive and reactive approaches, achieving better performance in medium to large-scale networks.

 Scalability: By using proactive routing in localized areas and reactive routing for longer-distance communication, hybrid protocols reduce the overhead and scalability issues present in purely proactive or reactive methods.

Disadvantages:

 Complexity: Hybrid protocols are generally more complex to implement and manage, as they require the maintenance of both proactive and reactive components.

In Conclusion, The choice of routing protocol depends on the specific requirements of the network, such as node mobility, network size, and application demands. Proactive protocols offer best performance in stable environments with low-latency needs but struggle under the weight of high overhead in dynamic scenarios. Reactive protocols offer efficient on-demand route discovery in fast-changing networks but can introduce delays during communication. Hybrid protocols provide a balanced approach, combining the advantages of both proactive and reactive methods to enhance scalability and efficiency in large networks.