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Vehicular ad hoc network is one of the most promising application of MANET that an inter communication system. In VANET nodes which are Vehicles can move safety with high speed and generally must communicate quickly reliably. VANET is a sub class of Mobile Ad hoc Network which provides a distinguished approach for a Intelligent Transportation System(ITS). The survey of routing protocol in VANET is important and necessary for smart ITS. This technology will be useful for Police and fire vehicles to communicate with each other for safety purpose. Vehicular Ad hoc Network architecture and cellular technology to achieve intelligent communication and improve road traffic safety and efficiency. To organize their in vehicular computing system, vehicle to vehicle ad hoc networks, hybrid architecture with special properties such as high mobility network portioning and constrained topology. Vanet can perform effective communication by utilizing routing information. This paper discusses the advantages/disadvantages and the application of various routing protocols for VANET. This paper explore on focusing on significant features, performance improvement in comparisons of routing protocol for vehicular ad hoc network(VANET).
Keywords:VANET, Routing Protocol,ITS,ADOV,TORA.
Vehicular ad hoc network is special form of MANET which is vehicle to vehicle road side wireless communication network. It is autonomous and self-organizing wireless communication network, where nodes in vanet involve themselves as servers and clients for exchanging information. VANET allow vehicles to avoid problem either by taking any desired action or by alerting drivers. Besides the road safety enhancement that VANETs will bring, they also opens door too many application to enhance the driving and traveling comfort ,like internet access from car. The main goal of VANETis providing safety and comfort for passengers. Each vehicle equipped with VANET device will be a node in the ad hoc network and can receive and relay other message through the wireless network.Collison warning , road signal arms and in place traffic view will give the driver essential tool to decide the best path along the way event or bad traffic areas. VANET has unique characteristic like high mobility with constraint of road topology, initially low market penetration ratio, unbounded network size, infrastructure supports thar differentiate it from MANET. The network architecture of vanet can be classified into 3 categories: Pure cellular/WLAN, pure ad hoc, and hybrid. Due to new technology it has taken huge attention from government, academy & industry. There are many research project around the world which are related with VANET such asCOMCAR, DRIVE, FleetNet and NoW(Networks on Wheels) CarTALK 2000, CarNet. The Routing Protocol in VANET are Categorized into various types likes Topology based, Position based, Geocast Based, Broad cast based.
II. FEATURES OF VEHICULAR AD HOC NETWORK:
The nodes in VANET are vehicles and road side unit.
The movement of these nodes is very fast.
The motion patterns are restricted by road topology .
Vehicles act as transceivers i.e. sending and receiving at the same time while creating a highly dynamic network, which is continuously changing.
The vehicular density varies from time to time for instance their density might increase during peak office hours and decrease at night hours.
III.APPLICATION OF VEHICULAR AD HOC NETWORK:
Major application of VANET include providing safety information, traffic management, toll services, location based services and infotainment. VANET application can be divided into following category:-
Vanet provide ubiquitous connectivity on the road to mobile usres.
Intelligents Transportation System(ITS) provide efficient vehicle to vehicle communication in VANET.
ITS has variety of application like co operative traffic monitoring, control of traffic flows, blind crossing and collision prevention.
Comfort application is the application to allow the passenger to communicate with other vehicles and with internet host, which improves passenger comfort.
VANET provide internet connectivity to vehicular nodes while on the movement so that watch passenger can download music and send emails, online movies and can online chatting etc.
VANET provide the payment service application is very suitable for toll collection without even decelerating the car or waiting on line.
IV. NETWORK ARCHITECTURE AND CHARACTERISTIC OF VANET
Wireless ad hoc network do not depend on fixed infrastructure, access point or infrastructure less network for communication and dissemination of information. The architecture of VANET consists of 3 categories: Pure cellular /WLAN, Pure ad hoc and Hybrid. VANET may used fixed cellular gateways and WLAN/WiMax access points at traffic intersection or for routing purpose. This network architecture is pure cellular network and WLAN.VANET can compile both cellular network and WLAN to form the network stationery or fixed gateways area and the road side also provides connectivity to vehicles. In such a scenario all vehicles and road side device form pure MANET. Hybrid architecture consists of both infrastructure network and ad hoc network together.
TABLE 1 SHOWING VARIOUS FEATURES OF VANET
Highly Dynamic topology
Vehicle are moving VANET is always changing at high speed, by formed network topology.
Frequently disconnected network topology
Changing node density due to occurs when highly dynamic topology frequently disconnected network.
Unlimited Battery power and storage capacity
Nodes of VANET are not subject to power and storage limitation in sensor networks. Nodes have limited amount of energy and computing power.
On Board Sensor networks
VANET routing protocols consists of many nodes of sensors network which provide useful information for many GPS unit which provides location information of nodes.
V. ROUTING PROTOCOL IN VEHICULAR AD HOC NETWORK:
In VANET the Routing protocols are classified in to 5 categories :Topology based routing Protocol, Position Based Routing Protocol, Cluster Based Routing Protocol, Geocast Routing Protocol and Broadcast Routing Protocol. These protocol are characterized on the basis of area/application where they are most suit
A). Topology Based Routing:
Several MANET Routing Protocol have used topology based routing approach. Topology based Routing Protocols use link's information within the network to send the data packets from source to destination. Topology based Routing Protocol which discover the route and maintain routing information in a table , the sender starts transmission data. They are divides in to3 categories
Proactive Routing Protocol
Reactive Routing Protocol
Hybrid Routing Protocol
Proactive Routing Protocol:
These protocol are mostly based on shortest path algorithm. They keep information of all connected nodes in form of tables because these protocol are table based. furthermore, these tables are also shared with their neighbors. Whenever any change occurs in new topology every node update its routing table. All the nodes of the network in proactive protocol or table driven routing protocols periodically exchanging the knowledge of topology. The proactive protocol do not have initial route discovery delay but consumes lot of bandwidth for periodic updates of topology.
DSDV: Destination-Sequenced Distance-Vector Routing:
DSDV is table-driven routing scheme for ad hoc mobile networks based on the Bellman-Ford algorithm. It was developed by C.Perkins and P.Bhagwat in 1994. It eliminates route looping, increase convergence speed, and reduces control message overhead. In DSDV, each node maintain a next-hop table, which it exchanges with its neighbors.
OLSR: Optimized Link State Routing Protocol:
It is an optimization of a pure link state protocol for mobile ad hoc networks. Each node in the network selects a set of neighbor nodes called as multipoint relay(MPR) which transmits its packets. The neighbor nodes which are not in its MPR set can only read and process the packets. This procedure reduces the number of retransmission in a broadcast procedure.
STAR: Source-Tree Adaptive Routing:
STAR is another link state protocol. In STAR, preferred routes to every destination are saved in each routerIt reduces overhead on the network by eliminating periodic updates. There is no need of sending updates,unless any event occurs. This protocol can be suitable for large scale network but it need large memory and processing because it has to maintain large trees for whole network.
Reactive Routing Protocol:
On demand and reactive routing protocols were designed in such a manner to overcome the overhead that was created by proactive routing protocols. This overcome maintaining by only those routes that are currently active. Routes are discovered and maintained for only those nodes that are currently being used to send data packets from source to destination. These protocol are periodically update the routing table ,when some data is there to send. When use flooding process for route discovery, which course more routing overhead and also suffers from the initials route discovery process which makes them unsuitable for safety application in VANET.
AODV: Adhoc on Demand Distance Vector:
In AODV routing, upon receipt of a broadcast query(RREQ), nodes record the address of the node sending the query in their routing table. This procedure of recording its previous hop is called backward learning. Upon arriving at the destination, a reply packet(RREP) is then sent through the complete path obtained from backward learning to the source.
DSR: Dynamic Source Routing:
DSR uses source routing, that is, the source indicates in a data packet's sequence of intermediate nodes on the routing path. In DSR, the query packet copies in its header the IDs of the intermediate nodes that it has traversed. The destination then retrieves the entire path from the query packet, and uses it to respond to the source.
TORA: Temporally Ordered Routing Algorithm:
TORA routing belongs to a family of link reversal routing algorithms where a directed acyclic graph(DAG) toward the destination is built based on the height of the tree rooted at the source. The directed acyclic graph directs the flow of packets and ensures reach ability to all nodes. When a node has a packets to send, it broadcasts the packet. Its neighbor only broadcasts the packet if it is the sending node's downward link based on DAG.
Hybrid Routing Protocol:
Hybrid Routing combines characteristics of both reactive and proactive routing protocol routing protocol to make routing more scalable and efficient. Mostly hybrid protocols are zone based, it means the number of nodes is divided into different zone to make route discovery and maintenance more reliable for VANET.
ZRP: Zone Routing Protocol:
In this the network is divided into overlapping zones. The zone is defined as a collection of nodes which are in a zone radius. The size of a zone is determined by a radius of length Î± where Î± is the number of hops to the perimeter of the zone. In ZRP, a proactive routing protocol(IARP) is used in intra-zone communication, source sends data directly to the destination if both are in same routing zone otherwise IERP reactively initiates a route discovery.
Position Based Routing Protocol:
Position Based Routing consists of class of routing algorithm, which is sharing the property of geographic positioning information in order to select the next forwarding hops. The packets is send without any map knowledge to the one hop neighbor which is doesn't to destination. Position based routing provide better performance because there is no need to be created and maintained global route from source node to destination node. Position based routing assumes that each node have knowledge about its physical/geographic position by GPS or by some other position determining services. In it each node also has the knowledge of source, destination and other neighboring nodes. As compared to topology based routing, position based routing uses the additional information of each participating node to applicable in VANET, that additional information is gathered through GPS, here we discuss some popular position based routing techniques:-
GPSR: Greedy Perimeter Stateless Routing:
In Greedy Perimeter Stateless Routing(GPSR) a node forwards a packets to an immediate neighbor which is geographically closer to the destination node. This mode of forwarding is termed greedy mode. When a packets reaches a local maximum, a recovery mode is used to forward a packet to a node that is closer to the destination than the node where the packets encountered the local maximum.
GSR: Geographic Source Routing:
Earlier GSR was used in MANET. Then it was improved to use in VANET scenario by incorporating in to it greedy forwarding of messages toward the destination. If at any hop there are no nodes in the direction of destination then GPSR utilize a recovery strategy known as perimeter node.
A-STAR: Anchor Based Street and Traffic Aware Routing:
A-Star is similar to GSR in that packets are routed through anchor points of the overlay. However, A-STAR is traffic aware: the traffic on the road determines whether the anchor points of the road will be considered in the shortest path. A-STAR routes based on two kinds of overlaid maps: a statically rated map and a dynamically rated map. A statistically rated map is a graph that displays bus routes that typically imply stable amount of traffic.
Geocast Based Protocol:
Geocast routing is basically a location based multicast routing used to send a message to all vehicles in a pre-defined geographical region. It is main objective to deliver the packet from source node to all other nodes within a specified geographic region Zone of relevance ZOR. In geocast routing vehicles outside the ZOR are not alerted to avoid unnecessary hasty reaction. It normally defines a forwarding zone where it directs the flooding of packets in order to reduce message overhead and network congestion caused by simply floodingpackets everywhere. In the destination zone, unicast routing can be used to forward the packet. One pitfall of Geocast is network partitioning and also unfavorable neighbors which may hinder the proper forwarding of message.
Cluster Based Protocol:
Each cluster has one cluster-head, which is responsible for intra and inter-cluster management function. Intra-cluster nodes communicate each other using direct links, whereas inter-cluster communication is performed via cluster headers. In cluster-based routing protocol the formation of cluster and the selection of cluster-head is an important issue. In VANET due to high mobility dynamic cluster formation is a towering process.
Broadcast based Routing:
Broadcast is based on hierarchical structure for highway network. In broadcast the highway is divided into virtual cells which move like vehicle. The nodes in the highway are organized into two level of hierarchy; the first level hierarchy includes all the nodes in a cell, the second level hierarchy is represented by cell reflectors, which are few nodes located closed to geographical center of cell. Some cell reflected behaves for certain interval of time as cluster head and handles the emergency message coming from same members of the cell or nearby neighbor. This protocol performs similar to flooding base routing protocols for message broadcasting and routing overhead.
Routing is an important component in Vehicle to Vehicle (V2V) and Infrastructure to Vehicle(I2V) communication. This paper discuss various Routing protocols of VANET. Designing an efficient routing protocol for all VANET application very difficult. Proactive based protocol may not be suitable high mobility nodes because distance vector routing takes much bandwidth to store routing information with neighbors. Due to high mobility of VANET node's proactive based routing protocol may fails in VANET due to consumption of more bandwidth and large table information.
As compared to proactive routing reactive routing protocol such as (AODV & DS) focus on features and comparison of different protocols require less space to store the routing information and also consumed less bandwidth to communicate among neighbors for the highly mobile adhoc network. Thus this paper focus on features and comparison of different categories of VANET routing protocols. Such as position based Geocast and Cluster based protocol are more reliable for most of application in VANET.