ROUTING

 Routing is the act of moving information across the network from a source to a destination. It is also referred as the process of choosing a path over which the packets are sent. The routing process usually directs forwarding on the basis of routing tables which maintain a record of the routes to various network destinations.

At least one intermediate node within the internetwork is encountered during the transfer of information. Basically two activities are involved in this concept: determining optimal routing paths and transferring the packets through an internetwork. The transferring of packets through an internetwork is called as packet switching which is straight forward, and the path determination could be very complex.

Routing protocols use several metrics as a standard measurement to calculate the best path for routing the packets to its destination that could be : number of hops, which are used by the routing algorithm to determine the optimal path for the packet to its destination. The process of path determination is that, routing algorithms find out and maintain routing tables, which contain the total route information for the packet. The information of route varies from one routing algorithm to another. The routing tables are filled with entries in the routing table are ip-address prefix and the next hop.

Routing is mainly classified into static routing and dynamic routing.

 1. Static routing refers to the routing strategy being stated manually or statically, in the router. Static routing maintains a routing table usually written by a networks administrator. The routing table doesn‗t depend on the state of the network status, i.e., whether the destination is active or not.

 2. Dynamic routing refers to the routing strategy that is being learnt by an interior or exterior routing protocol. This routing primarily depends on the state of the network i.e., the routing table is affected by the activeness of the destination.

 b) Routing in Mobile Ad-hoc Networks

Mobile Ad-hoc networks are self-organizing and  self-configuring multihop wireless

 networks, where the structure of the network changes dynamically. This is mainly due to the mobility of the nodes. Nodes in these networks utilize the same random access wireless channel, cooperating in an intimate manner to engaging themselves in multihop forwarding. The node in the network not only acts as hosts but also as routers that route data to / from other nodes in network.

 In mobile ad-hoc networks there is no infrastructure support as is the case with wireless networks, and since a destination node might be out of range of a source node transferring packets; so there is need of a routing procedure. This is always ready to find a path so as to forward the packets appropriately between the source and the destination.

 In infrastructure networks, within a cell, a base station can reach all mobile nodes without routing via broadcast in common wireless networks. In the case of ad-hoc networks, each node must be able to forward data for other nodes. This creates additional problems along with the problems of dynamic topology which is unpredictable connectivity changes.

c) Problems in routing with Mobile Ad hoc Networks

 i)             Asymmetric links: Most of the wired networks rely on the symmetric links which are always fixed. But this is not a case with ad-hoc networks as the nodes are mobile and constantly changing their position within network

 ii)             Routing Overhead: In wireless ad hoc networks, nodes often change their location within network. So, some stale routes are generated in the routing table which leads to unnecessary routing overhead.

 iii)             Interference: This is the major problem with mobile ad-hoc networks as links come and go depending on the transmission characteristics, one transmission might interfere with another one and node might overhear transmissions of other nodes and can corrupt the total transmission.

iv)            Dynamic Topology: Since the topology is not constant; so the mobile node might move or medium characteristics might change. In ad-hoc networks, routing tables must somehow reflect these changes in topology and routing algorithms have to be adapted. For example in a fixed network routing table updating takes place for every 30sec. This updating frequency might be very low for ad-hoc networks.

d) Classification of Routing Protocols

 Classification of routing protocols in mobile ad hoc network can be done in many ways, but most of these are done depending on routing strategy and network structure.

 The routing protocols can be categorized as flat routing, hierarchical routing and geographic position assisted routing while depending on the network structure. According to the routing strategy routing protocols can be classified as Table-driven and source initiated. The classification of routing protocols is shown below.

FSR – Fish Eye State Routing ;FSLS – Fuzzy Sighted Link state;

 OLSR – Optimized Link State Routing; DSR – Dynamic Source Routing

TBRPF – Topology broadcast based on Reverse – Path Forwarding     

AODV – Ad hoc On Demand Distance Vector; HSR – Hierarchical State Routing CGSR – Cluster Gateway Switch Rouing; ZRP – Zone Routing Protocol 

LANMAR – Landmark Ad hoc Routing; GeoCast – Geographic Addressing and Routing LAR – Location Aided Routing Protocol; GPSR – Greedy Perimeter Stateless Routing DREAM – Distance Routing Effect Algorithm for mobility

 Based on the Routing Information Update Mechanism :

 Ad hoc wireless network routing protocols can be classified into three major categories based on the routing information update mechanism. They are

 1. Proactive or Table driven routing protocols : In table drive routing protocols , every node maintains the network topology information in the form of routing tables by periodically exchanging routing network.

2.     Reactive or On-demand routing protocols : Protocols that falls under this category do not maintain the network topology information. They obtain the necessary path when it is required by using connection establishment process. Hence these protocols do not exchange routing information periodically.

 3.     Hybrid Routing Protocols : Protocols belonging to this cateogory combine the best features of above two categories. Nodes within a certain distance from the node concerned or within a particular geographical region are said to be within the routing zone of the given node. For routing within this zone, a table- driven approach is used. For nodes that are located beyond this zone is on-demand approach is used