1.1ADHOC NETWORK
Wireless technology is the new boom in the market i.e. we can say that the world is going to be wireless. A human being is always in favor to simplify or to reduce his work or may want to work while doing some daily activity and going to be wireless is good solution for this man kind. Also due to recent advances in technology it is very common to use portable computing devices such as computer notebook and Personal Digital Assistance (PDA). But these devices are currently dependant on the fixed infrastructure to communicate with each other. They communicate via internet router i.e. the wireless communication is only the last hop [1]. There are certain situation where establishing such fixed infrastructure is time consuming or practically not possible.
Consider a rescue operation, in which one can never expect when and where it will be required and the time in hand is also very less. Mobile devices can be easily moved to rescue sites, which will help to increase the speed of the rescue operation. But what is about communication between them? There may not be any fixed infrastructure and may physically not possible to provide it. It is obvious to form a temporary network of mobile devices in record time and start the rescue operation. Such a temporary network is called as ad hoc network.Other such situation where such a temporary network is required is public meetings, military use, airport, big organizations, university campus, etc.Mobile hosts should be able to form a temporary network without any established infrastructure or administration. This type of network is known as Ad hoc network
Routing comes into picture when two hosts are required to communicate which are not in range of each other. Such communication is impossible without taking the help of some other node. This is also called as multi-hop communication. A path is established for secure communication between two nodes. This is called as routing. For the secure, fast, and 100% communication some sets of rules are defined (protocol) that should must be obey by every node. A good routing scheme should must provide a shortest path, as and when required by any two nodes, in very less time. Utilization of resource power and network bandwidth is also dependant on such routing schemes.
CHALLENGES FOR ROUTING AD HOC NETWORK
There are some inherent characteristics of the ad hoc network which place it apart from using the conventional routing scheme on the conventional wired fixed networks [1]. These characteristics are:
· There are no centralized administrative servers in an ad hoc network so handling of addressing has to be done in a distributed manner.
· Since there are no centralized systems, all the hosts have to support the network by acting as routers, passing along message packets that are not meant for that particular host.
· Transmission between two hosts over a wireless network may not work equally well in both directions, i.e. the data communication is not bi-directional, unlike a fixed wired network.
· The data transfers are less reliable as compared to wired networks.
· There are dynamic topological changes in an ad hoc network as the hosts, which are also acting as routers can move in and out of a network and under a worse case the network can become partitioned in the midst of a communication session.
· Unlike wired networks, mobile hosts are more likely to have limited power sources, depending mainly on battery and hence there should be minimum redundant data transfers to conserve power.
· Ad hoc networks have lower bandwidth as compared to wired networks.
· Ad hoc networks are more vulnerable in terms of security if there is no link layer or network layer measures. Snooping in the network traffic is relatively simpler in such a network.
ROUTING IN AD HOC NETWORK
3.3 IP Address Configuration
This is a very simple addressing scheme. A host, when detecting network connectivity for the first time (i.e. joining a network) will first picks a random IP address in the range of 169.254.1.0 to 169.254.254.255. This range is possible as it is registered with the IANA (Internet Assigned Numbers Authority) for such purposes.
Initially the host will assume a temporary address from the range 1 - 2047 of the network 169.254/16. The host will then broadcast a Route Request (RREQ) for that address. If there is no Route Reply (RREP) from other hosts within a certain timeout period, the broadcast of the request is repeated for a certain number of retries. If still no RREP are received, the requesting host will then assume that the address can be taken for its own. Otherwise the node randomly picks another address and tries again.
3.4 Desirable properties of Routing Protocol
Below is a list of the desirable qualitative properties that a good, efficient protocol should must have
Distributed operation: since there is no centralized administrative server and any node can leave and join network at any time hence the routing should not depend on the small no of master hosts.
Demand-based operation: instead continuous route maintenance a path should be provided on the demand basis which reduces the network traffic and is the proper utilization of bandwidth.
Proactive operation: this is reverse to that of the demand-based operation in which a path should restore easily which is in frequent use.
Unidirectional link support: bi-directional data flow is not possible in ad hoc network due to physical reasons. The protocol should must support the situation where the data can be transmitted in one direction only.
Security: since ad hoc network is more liable to intrusion some sort security must be provided by the protocol.
Sleep time operation: as we know the devices of ad hoc network use battery power it may happen that the node may go in sleep period for a short amount of time and then again start the transmission. The routing protocol should must support such sleeping of hosts.
TYPES OF AD HOC ROUTING PROTOCOL
Since the advent of Defense Advanced Research Projects Agency (DARPA) packet radio networks in the early 1970s [1], numerous protocols have been developed for ad hoc mobile networks. Such protocols must deal with the typical limitations of these networks, which include high power consumption, low bandwidth, and high error rates [2]. As shown in Fig. 4, these routing protocols may generally be categorized as:
• Table-driven
• Demand-driven (Source-initiated)
5.1 List of Table-driven Routing Protocol
1. DSDV: Destination-Sequenced Distance-Vector routing
2. WAR: Wireless Anonymous Routing protocol
3. CGSR: Cluster-head Gateway Switch Routing
4. OLSR: Optimized Link State Routing protocol
In the next section will take a brief description of Destination-Sequenced Distance-Vector Routing (DSDV).
5.2 Destination-Sequenced Distance-Vector Routing
Destination-Sequenced Distance-Vector Routing protocol (DSDV) described in [2] is a table-driven algorithm. Every mobile node in the network maintains a routing table, as shown in fig.5, in which all of the possible destinations within the network and the number of hops to each destination are recorded. Each entry is marked with a sequence number assigned by the destination node. The sequence numbers enable the mobile nodes to distinguish stale routes from new ones, thereby avoiding the formation of routing loops.
Routing table updates are periodically transmitted throughout the network in order to maintain table consistency. To help alleviate the potentially large amount of network traffic that such updates can generate, route updates can employ two possible types of packets. The first is known as a full dump. This type of packet carries all available routing information and can require multiple network protocol data units (NPDUs). During periods of occasional movement, these packets are transmitted infrequently. Smaller incremental packets are used to relay only that information which has changed since the last full dump. Each of these broadcasts should fit into a standard-size NPDU, thereby decreasing the amount of traffic generated. The mobile nodes maintain an additional table where they store the data sent in the incremental routing information packets.
New route broadcasts contain the address of the destination, the number of hops to reach the destination, the sequence number of the information received regarding the destination, as well as a new sequence number unique to the broadcast [2]. The route labeled with the most recent sequence number is always used. In the event that two updates have the same sequence number, the route with the smaller metric is used in order to optimize (shorten) the path.
DEMAND-DRIVEN ROUTING PROTOCOLS
A different approach from table-driven routing is source-initiated on-demand routing. This type of routing creates routes only when desired by the source node. When a node requires a route to a destination, it initiates a route discovery process within the network. This process is completed once a route is found or all possible route permutations have been examined. Once a route has been established, it is maintained by a route maintenance procedure until either the destination becomes inaccessible along every path from the source or until the route is no longer desired.
6.1 List of Demand–driven Routing Protocol
1. AODV: Ad hoc On-Demand Distance Vector Routing
2. DSR: Dynamic Source Routing
3. TORA: Temporally Ordered Routing Algorithm
4. ASR: Associativity-Based Routing
5. ARA: Ant Routing Algorithm
To send a packet to another host, the sender constructs a source route in the packet’s header, giving the address of each host in the network through which the packet should be forwarded in order to reach the destination host. The sender then transmits the packet over its wireless network interface to the first hop identified in the source route. When a host receives a packet, if this host is not the final destination of the packet, it simply transmits the packet to the next hop identified in the source route in the packet’s header. Once the packet reaches its final destination, the packet is delivered to the network layer software on that host.
Each mobile host participating in the ad hoc network maintains a route cache in which it caches source routes that it has learned. When one host sends a packet to another host, the sender first checks its route cache for a source route to the destination. If a route is found, the sender uses this route to transmit the packet. If no route is found, the sender may attempt to discover one using the route discovery protocol.
While waiting for the route discovery to complete, the host may continue normal processing and may send and receive packets with other hosts. The host may buffer the original packet in order to transmit it once the route is learned from route discovery, or it may discard the packet, relying on higher-layer protocol software to retransmit the packet if needed. Each entry in the route cache has associated with it an expiration period, after which the entry is deleted from the cache.
Route discovery allows any host in the ad hoc network to dynamically discover a route to any other host in the ad hoc network, whether directly reachable within wireless transmission range or reachable through one or more intermediate network hops through other hosts. A host initiating a route discovery broadcasts a route request packet, fig.7, which may be received by those hosts within wireless transmission range of it.
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