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An ad hoc network is a collection of wireless mobile nodes or routers dynamically forming a temporary network without the use of any existing network infrastructure or centralized administration. The routers are free to move randomly and organize themselves arbitrarily; thus, the network's wireless topology may change rapidly and unpredictably. Such a network may operate in a stand-alone fashion, or may be connected to the Internet.
AN ADHOC NETWORK
An ad hoc network is a collection of wireless mobile nodes (or routers) dynamically forming a temporary network without the use of any existing network infrastructure or centralized administration. MANETs are infrastructure less and wireless in which there are several routers which are free to move arbitrarily and perform management of routes.
The goal of MANETs is to extend mobility into the realm of autonomous, mobile and wireless domains, where a set of nodes form the network routing infrastructure in an ad-hoc fashion
Figure1. An adhoc network
MANETs are infrastructure less and wireless in which there are several routers which are free to move arbitrarily and perform management of routes..
The majority of applications of MANETs are in areas where rapid deployment and dynamic reconfiguration are necessary and wired network is not available. These include military battlefields, emergency search, rescue sites, classrooms and conventions, where participants share information dynamically using their mobile devices.
Multicasting can improve the efficiency of the wireless link when sending multiple copies of messages by exploiting the inherent broadcast property of wireless transmission. Hence, reliable multicast routing plays a significant role in MANETs.
The goal is to support mobile ad hoc networks with hundreds of routers and solve challenges in this kind of network.
Some challenges that ad hoc networking faces are:
Limited wireless transmission range.
Hidden terminal problems.
Packet losses due to transmission errors.
Mobility-induced route changes.
Mobile ad hoc networks could enhance the service area of access networks and provide wireless connectivity into areas with poor or previously no coverage (e.g., cell edges). This feature gives the access from "anytime anywhere" into "all the time, everywhere."
The following are the applications of ad hoc wireless networks:
Emergency response network
II.ROUTING PROTOCOLS IN MANET
An MANET routing protocol is a convention, or standard, that controls how nodes decide which way to route packets between computing devices in a mobile adhoc network and they work on network layer.
The MANET routing protocols are mainly developed to maintain route inside MANET, and they do not use any access points to connect to other nodes in the network and Internet. Routing protocols can be classified into three categories depending on their properties.
The classifications in are :
Centralized versus Distributed
Static versus Adaptive
Reactive versus Proactive
In centralized algorithms, all route choices are made by a central node, while in distributed algorithms; the computation of routes is shared among the network nodes. In static algorithms, the route used by source destination pairs is fixed regardless of traffic condition. It can only change in response to a node or link failure.
"Proactive routing protocol" is the constant maintaining of a route by each node to all other network nodes. The route creation and maintenance are performed through both periodic and event-driven messages. If a route has already existed before traffic arrives, transmission occurs without delay.
The various proactive protocols are Destination-Sequenced Distance-Vector (DSDV), Optimized Link State Routing (OLSR), and Topology Broadcast Reverse Path Forwarding (TBRPF).
In Reactive (On-Demand) Routing Protocols, a node initiates a route discovery throughout the network, only when it wants to send packets to its destination. The different types of reactive routing protocols are: Dynamic Source Routing (DSR), Ad-hoc On-Demand Distance Vector routing (AODV) and Temporally Ordered Routing Algorithm (TORA).
III. SOME SELECTED ROUTING PROTOCOLS:
(i) AODV (Adhoc on-demand distance vector algorithm)
AODV is a reactive protocol that is a combination of DSR and DSDV. Route discovery and maintenance is similar to DSR, and uses the hop-by-hop routing of DSDV. It also uses sequence numbers for loop prevention, with the goals of quick adaptation under rapidly changing link conditions [3,5].
Figure2. Example of aodv.
ADVANTAGES AND DISADVANTAGES OF AODV:
The advantages of AODV protocol are that it favors the least congested route instead of the shortest route and it also supports both unicast and multicast packet transmissions even for nodes in constant movement. It also responds very quickly to the topological changes that affects the active routes. AODV does not put any additional overheads on data packets as it does not make use of source routing.
The disadvantage of AODV protocol is that it expects/requires that the nodes in the broadcast medium can detect each others' broadcasts. It is also possible that a valid route is expired and the determination of a reasonable expiry time is difficult which causes high latency. Also it causes higher consumption of bandwidth and power[6,7,8].
(ii)TORA (Temporally ordered routing algorithm)
TORA routing protocol is based on "link reversal" algorithm. Every node has information regarding it adjacent nodes. In this way TORA provides multiple routes for any pair of nodes.
Figure3. Example of tora and link reversal.
Consider the network topology as shown in figure .when node 1 has data packets to be sent to the destination node 7,a query packet is originated by the node 1 with the destination address included in it. This query packet is forwarded by intermediate nodes 2,3,4,5 and 6 and reaches the destination node 7 or any other node which has a root to the destination. The node that terminates the query packet replies with an update packet containing its distance from the destination. Each node that receives the update packet sets its distance to a value higher than the distance of the sender of the update packet. By this, a set of directed links are created by which the query is originated to the destination. This forms the directed acyclic graph as shown in the figure above.
When an intermediate node(say node 5),discovers that the route to the destination node is invalid, as from the figure above, it changes its distance value to a higher value than its neighbors and originates an update packet. the neighboring node 4 that receives the update packet reverse the link between 1 and 4 and forwards an update message to node 5.this is done to update the DAG corresponding to destination node 7.this results in a change in the DAG. if the source node has no other neighbor, that has a path to the destination, it initiates a fresh query/update procedure. Since the confliction occurs due to link reversal, a partition in the network can be inferred .if the node detects the partition; it originates a clear message which erases the existing path information in that partition related to the destination.
ADVANTAGES AND DISADVANTAGES OF TORA
One of the advantages of TORA is that the multiple routes between any source destination pair are supported by this protocol. Therefore, failure or removal of any of the nodes is quickly resolved without source intervention by switching to an alternate route.
TORA is also not free from disadvantages . One of them is that it depends on synchronized clocks among nodes in the ad hoc network. The dependence of this protocol on intermediate lower layers for certain functionality presumes that the link status sensing, neighbor discovery, in order packet delivery and address resolution are all readily available. The solution is to run the Internet MANET Encapsulation Protocol at the layer immediately below TORA. This will make the overhead for this protocol difficult to separate from that imposed by the lower layer.
IV.Comparison of TORA and AODV
Figure4. Comparison between tora and aodv.
Now again looking at the disadvantages of aodv as mentioned above:
There are many disadvantages in AODV and here we are focusing on generation of too many periodic hello messages that results in unnecessary consumption of bandwidth, power and time delay. According to the existing AODV; each node broadcasts a hello message after regular intervals .This may add to overheads in AODV. Thus these large no of hello messages causes a lot of consumption of bandwidth which creates a large congestion at the node and the packet may even get lost in the network [11,12,17].
In AODV routing protocol, all the nodes present in the network generate "hello" messages periodically which indicates if they are active or not. in this way a lot of beacons are generated. This leads to additional overheads. This may degrade the quality of service of AODV. Therefore, to overcome this problem, we have proposed an algorithm. In our proposed solution, instead the source node can keep a track of active nodes. in this case each and every node need not generate a "hello" message. The source node itself can broadcast a data packet named "active packet". In this packet, whichever nodes are active, can include their sequence IDs and this packet is then routed back to the source node through the generation of reverse path as done in original AODV. This data packet now contains a list of sequence IDs of all the active nodes, which are present in the network. Thus now the source node is aware of all the active nodes and can broadcast request query packets only to the active nodes. This action can be repeated after a particular time interval, and this time period can be large. Hence, instead of implementing "hello" algorithm as mentioned in existing AODV routing protocol, the new algorithm proposed by us can implemented. Our proposed algorithm can help in reducing bandwidth and power overheads, thereby, improving the existing protocol and enhancing the quality of service in existing AODV protocol.
VI.CONCLUSION AND FUTURE SCOPE
The ad hoc-on demand distance vector routing and temporally ordered routing algorithm are analyzed in this paper. Also comparisons on the two are drawn. It can be concluded under different scenarios both the protocols behave differently. TORA is a highly adaptive protocol. Both TORA and AODV support multicasting, unicasting and broadcasting. TORA also supports multipath routing. TORA is based on link reversal algorithm. Under different scenarios different results are drawn .the result is analyzed in this paper. Also a solution is proposed to increase the quality of service in AODV. The proposed solution ensures reduction in bandwidth and power consumption. Also time delay is reduced. In existing AODV routing algorithm, each node broadcasts a "hello" message periodically. This leads to large overhead if the network is large. This in turn reduces the bandwidth capacity. Also the power consumption increases. These are the major problems in ad-hoc network. We are proposing a solution to overcome this problem in ad-hoc networks using AODV routing protocol. This can enhance the quality of service of the above mentioned routing protocol. Also implementation of the same will be carried out in the future and comparison of TORA and AODV will be done practically after implementing these two protocols. The implementation will be carried out Using NS2 or MATLAB.
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