Routing Protocols

1. Introduction
There are some challenges that make the design of mobile ad hoc network routing protocols a tough task:
1. In mobile ad hoc networks, node mobility causes frequent topology changes and network partitions.
2. Because of the variable and unpredictable capacity of wireless links, packet losses may happen frequently.
3. The broadcast nature of the wireless medium introduces the hidden-terminal and exposed-terminal problems.
4. Because mobile nodes have restricted power, computing, and bandwidth resources, ad hoc networks require effective routing schemes.

2. Design Issues of Routing Protocols for Ad Hoc Networks
The major design issues in mobile ad hoc networks are discussed in this section.
2.1. Routing Architecture
The routing architecture of self-organized networks can be either hierarchical or flat. In a flat self-organized network, mobility management is not necessary since all of the nodes are visible to each other via routing protocols. Examples for flat routing algorithms like destination sequence distance vector (DSDV) and the wireless routing protocol (WRP), the routing tables have entries to all hosts in the self-organized network. In a flat routing algorithm, routing overhead increases at a faster rate when the size of the network increases. The hierarchical routing architectures are used to reduce the routing overhead. The idea behind hierarchical routing is to divide the hosts of self-organized networks into a number of overlapping or disjointed clusters. One node is elected as cluster head for each cluster. This cluster head maintains the membership information for the cluster. When these nodes want to send a packet, the nodes can send the packet to the cluster head that routes the packet toward the destination. Cluster head gateway switch routing (CGSR) and the cluster-based routing protocol (CBRP) belong to this type of routing scheme. Hierarchical routing involves cluster management and address/mobility management.
2.2. Unidirectional Links Support
Even though it is assumed that every routing protocol is bidirectional, a number of factors will make wireless links unidirectional:
• Different radio capabilities: Different nodes can have different transmit powers and receiving power within a network.
Interference: This is due to either hostile jammers or friendly interference, which will reduce a nearby receiver’s sensitivity.
Message broadcast requirement: For upward links, satellite-based transmitters are used. The upward links use different types of alternative paths.
Mute mode: An extreme instance, applicable only in tactical mobile networks, is when hosts cannot transmit due to an impending threat. In such a case, they still need to receive information, but cannot participate in bidirectional communications.
The state of link direction is time varying: the state of the wireless link may be either a persistent or a transient phenomenon. The duration of the stay in a particular state may depend on a function of offered traffic, terrain, and energy availability in the nodes.

2.3. Usage of Superhosts

It is true that in all the available routing protocols, all the nodes in a particular network will share same bandwidth available for whole network and other facilities. But in some cases, some hosts will include preponderant bandwidth, guaranteed power supply, and high-speed wireless links. Such hosts are referred to as superhosts. Some self-organized networks have two-tier network architectures: backbone area and subarea. The backbone area is composed of superhosts.



2.4. Quality of Service (QoS) Routing

In QoS routing, routing will be established between nodes according to resource availability in the network as well as the QoS requirement of flows. QoS routing means that the path selection will be based on availability of resources and efficient resource utilization. QoS routing will consider multiple constraints and provide better load balance by allocating traffic on different paths, subject to the QoS requirement of different traffic. In self-organized networks, there are many metrics to be considered: (1) most reliable path, (2) most stable path, (3) maximum total power remained path, (4) maximum available bandwidth path, etc. It is desirable to select the routes with minimum cost based on these metrics rather than only to provide the shortest path based on the hop distance.


2.5. Multicast Support
, but not all, destinations in a communication network. Multicast supports group communication, especially in the case of MANETs, where the network is self-organized, where bandwidth is limited, and where energy is constrained.
Having multicast routing in self-organized networks poses new challenges. Traditional multicast protocols are not suitable for this environment for the following reasons:
1. The source-oriented protocols are inefficient as the source originates the route request moves.
2. As the nodes in the self-organized networks move, they change the topology of the networks; because of this, routing may be difficult.
3. Transient loops may form during spanning tree reconfiguration.
4. Since the communication is to a group of nodes, maintaining too much multicast-related state information puts much pressure on both storage capacity and power, and these resources are severely limited in handheld devices in self-organized networks.