1. INTRODUCTION:

VANET is the technology of building a robust Ad hoc network among mobile vehicles and between mobile vehicles and roadside units. A Vehicular Ad Hoc Network is a collection of wireless mobile nodes forming a temporary or short lived network without any fixed infrastructure, where all nodes are free to move arbitrarily and where all the nodes configure themselves. Each node acts as a router and as a host even the topology of network changes rapidly. There are two types of nodes in VANET viz., mobile nodes as On Board Units (OBUs) and static nodes as Road Side Units (RSUs) and are shown in the Fig. 1-1. An OBU resembles the mobile network module and a central processing unit for on-board sensors and warning devices. The RSUs can be mounted in centralized locations such as intersections, parking lots or gas stations. They can play a significant role in many applications. VANET challenges are Unicast routing, Multicast routing , Dynamic network topology, Speed, Frequency of updates or Network overhead, Scalability, Mobile agent based routing, Quality of Service, Energy efficient and Secure routing [1].

On Board Unit (OBU)

Road Side Unit (RSU)

Figure 1.1 Node types in VANETs

VANET is emerging as the preferred network design for intelligent transportation systems and is based on short range wireless communication between vehicles and roadside units [2]. The Federal Communications Commission (FCC) has allocated 75 MHz in the 5.9 GHz band for licensed Dedicated Short Range Communication (DSRC) [3] aimed at enhancing bandwidth and reducing latency for vehicle-to-vehicle (V2V) vehicle-to-infrastructure (V2I) and infrastructure to vehicle (I2V) communication. VANET has unique characteristics like very high mobility, theoretically infinite extension, absences of a centralized control and intermittent connectivity through the sparse infrastructure.

1.1 Applications of VANET:

The purpose of VANET is to provide comfort and safety for passengers.’

Comfort Applications: It improves the traffic efficiency and passenger comfort. Traffic information system, gas station and weather information are example of comfort applications.

Safety Applications: Sharing emergency and safety data among vehicles improves the safety of passengers. Safety applications are emergency warning system, road condition and traffic sign violation warning.

2. VANET Characteristics:

Unlimited transmission power: Mobile device power issue is not a significant constraint in vehicular networks as the vehicle itself can provide continuous power to computing and communication devices.

High computational capability: Operating vehicles can afford significant computing, communication and sensing capabilities.

Highly dynamic topology: Vehicular network scenarios are very different from classic ad hoc networks. In VANET, vehicles can move faster and can join and leave the network much more frequently than MANET.

Predicable Mobility: Unlike classic mobile ad hoc networks, where it is hard to predict the nodes mobility, vehicles tend to have very predictable movements that are (usually) limited to roadways. The movement of nodes in VANET is constrained by the layout of roads. Roadway information is often available from positioning systems and map based technologies such as GPS. Each pair of node can communicate directly when they are within the radio range.

Potentially large scale: Unlike most ad hoc networks studied in the literature that usually assume a limited network size, vehicular networks can extended over the entire road network and include many participants.

Partitioned network: Vehicular networks will be frequently partitioned due to the dynamic nature of traffic that results in large inter-vehicle gaps in sparsely populated scenarios and also several isolated clusters of nodes.

Network connectivity: The degree to which the network is connected is highly dependent on two factors, the range of wireless links and the fraction of participant vehicles, where only a fraction of vehicles on the road could be equipped with wireless interfaces.

3. Routing protocols in VANET:

A routing protocol is needed whenever a packet needs to be transmitted to a destination via number of vehicular nodes. Numerous routing protocols have been proposed for such kind of Ad hoc networks that are shown in the Fig. 1.2

Figure.1.2. Routing Protocol

The protocols find a route for packet delivery in order to deliver the packets to the correct destination. Studies on various aspects of routing protocols have been an active area of research for many years. The routing protocols in VANET which can be classified by their properties. They are Topology Based Routing (TBR) and Geographic Based Routing (GBR). TBR uses the information about links that exists in the network to perform packet forwarding, whereas, GBR involves neighboring location information to perform packet forwarding. Since link information changes in a regular basis, topology based routing suffers from route breaks. The TBR is further classified as (a) Proactive (Table Driven) Protocols and (b) Reactive (On Demand) protocols [4].

3.1. Proactive or Table Driven Protocols: Proactive algorithms maintain routing information about the available paths in the network even if these paths are not currently used. The main disadvantage of this approach is that the maintenance of unused paths may occupy a significant part of the available bandwidth if the network topology changes frequently. In Table Driven routing protocols, each node maintains one or more tables containing routing information to every other node in the network. All nodes keep on updating these tables to maintain the latest view of the network. Some of the existing table driven protocols are DSDV, DBF, WRP and ZRP [4].

3.2. Reactive or On Demand Protocols:

Reactive routing protocols maintain only the routes that are currently in use, thereby reducing the burden on the network when only a small subset of the available routes is in use. In these protocols, routes are created as and when required [4].

When a transmission occurs from source to destination, it invokes the route discovery procedure. The route remains valid till destination is achieved or until the route is no longer needed. It has Route Discovery and Route maintenance procedure. The existing protocols are AODV, DSR and TORA. There are several protocols available and this paper deals with the comparative study of Reactive based AODV and DSR protocol [5].

3.3. Ad Hoc On Demand Distance Vector(AODV):

Ad hoc On-demand Distance Vector is an improved version of DSDV, as the name suggests, AODV establishes the route only when demanded or required for the transmission of data or information. It only updates the relevant neighboring node(s) instead of broadcasting every node of the network i.e. it does not make source routing to the entire node for the entire network. This protocol performs Route Discovery using control messages route request (RREQ) and route reply (RREP) whenever node wishes to send packet to destination [5]. To control network wide broadcasts of RREQs, the source node uses an expanding ring search technique. The forward path sets up in intermediate nodes at its route table with a lifetime association using RREP. When destination or intermediate node moves, a route error message (RERR) is sent to the affected source nodes. When source node receives the RERR, it can reinitiate route discovery process if the route is still needed. Neighborhood information is obtained from broadcast Hello packet.

3.4. Dynamic Source Routing(DSR):

Dynamic Source Routing is an on-demand routing protocol like Ad hoc on Demand Multiple path Vector (AODMV). It maintains the source routing, in which, every neighbor maintains the entire network route from source to the destination [5]. This protocol has two mechanisms- Route Discovery and Route Maintenance. The source route is needed when some node originates a new packet destined for some node by searching its route cache or initiating route discovery using ROUTE REQUEST and ROUTE REPLY messages. On detecting link break, DSR sends ROUTE ERROR message to source for new route.

4. Comparison of AODV and DSR:

According to our survey in reactive routing protocols, AODV has the best performance and lowest control overhead. DSR suffers from a very high delay because source routes change continuously due to high mobility. DSR route overhead is higher than AODV. Since DSR keeps route information within the packet header. AODV is still better in route updating and maintenance. In AODV, data packets carry the destination address, whereas in DSR, data packets carry the full routing information [6]. As the network diameter increases, the amount of overhead in the data packet will continue to increase. In AODV, route reply packets carry the destination address and sequence number, whereas, the DSR route reply packets carry the address of each node along the route. The throughput of AODV increases suddenly up to a certain level and decreases abruptly for some time and then increases again [7].It is shown in previous research that, as we increase the number of nodes the throughput of AODV also increases while, in DSR, its throughput increases uniformly but less than AODV.

According to [7] ,Comparison between AODV and DSR protocols on various parameters is shown in the following-

4.1 Benefits and Limitations of AODV:

The benefits 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 change s 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 limitation of AODV protocol is that it expects/requires that the nodes in the broadcast medium are able to detect the broadcasts of other nodes. It is also possible that a valid route is expired and the determination of a reasonable expiry time is difficult. The reason behind this is that the nodes are mobile and their sending rates may differ widely and can change dynamically from node to node. In addition, as the size of network grows, various performance metrics begin to decrease. AODV is vulnerable to various kinds of attacks as it is based on the assumption that all nodes must cooperate and without their cooperation no route can be established.

4.2 Benefits and Limitations of DSR:

One of the main benefits of DSR protoco:l is that there is no need to keep routing tables so as to route a given data packet as the entire route is contained in the packet header. The limitations of DSR protocol is that this is not scalable to large networks and even requires significantly more processing resources than most other protocols. Basically, in order to obtain the routing information, each node must spend lot of time to process any control data it receives, even if it is not the intended recipient.

5. Performance Metrics to evaluate through simulation:

According to survey, following performance metrics are determined by simulation.

a) Throughput - This represents the total number of bits received from sender divided by the time taken by the receiver to obtain the last packet.

b) Packet Delivery Ratio- The ratio between the amount of incoming packets to that of actually received data packets.

c) Average Delay- This represents average end-to-end delay and indicates how long it took for a packet to travel from the source to the destination.

d) Routing Overhead- This describes how many routing packets for route discovery and route maintenance need to be sent so as to propagate the data packets.

The numbers of nodes are high in city model and low in highway model. The parameters are compared with low and high mobility model of AODV and DSR in table 2(a) and 2(b). Performance evaluation done by various researchers.

Table 2(a) Low Mobility and Low Traffic

Protocol	Packet Delivery ratio	Routing overhead	End-to-End delay	Throughput
AODV	High	Low	Average	High
DSR	High	High	Average	Average

Table 2(b) High Mobility and High Traffic

Protocol	Packet Delivery ratio	Routing overhead	End-to-End delay	Throughput
AODV	Average	Very High	Average	High
DSR	Average	Average	High	Average

Researchers have also compared the drawbacks of various Position based routing protocols in VANET same is tabulated in table 3. Geographic (position) routing assumes each node knows its location, and the sending node knows the receiving node’s location by the increasing popularity of Global Position System unit. Since geographic routing protocols do not exchange link state information and do not maintain established routes like proactive and reactive topology based routing, they are more robust and promising to the highly dynamic environments like VANET. In other words, route is determined based on the geographic location of neighboring nodes as the packet is forwarded. There is no need of link state exchange or route setup.

Table 3. Drawbacks of position based protocols.

Routing Protocols	Drawbacks
GPSR	Frequent network disconnection. Routing loops. Too many hops. Routing in wrong direction.
GSR	End to end connection is difficult in low traffic density.
GPCR	End to end connection is difficult in low traffic density.
A-STAR	Routing paths are not optimal and results in large delay of packet transmissions.
MDDV	Large delay if the traffic density varies by time
VADD	Large delay due to varying topology and varying traffic density.
PDGR	Too many hops Large delay if the traffic density is high. Low packet delivery ratio. Frequent network disconnection.

The GPSR works better in highway condition but is not much suitable in city model owing to obstacles whereas the GPCR is suitable with city model [9] . Here, a restrictive greedy algorithm is simply followed when nodes are in street and an actual routing decision is taken when at the junction of streets. Here the packet is forwarded to a node in the junction rather sending it across the junction. A-STAR [9] is suitable for city with obstacles like hi-rise buildings. In addition to GPCR techniques, it uses statistically or dynamically related maps to assess traffic condition and identify anchor path with high connectivity for packet delivery.

4. Simulation:

For simulation of VANET protocols, a simulator is needed which is able to simulate 1) Communication and Network protocols (i.e., network simulation), 2) Road and Vehicle traffic (i.e., traffic simulation), and 3) Tightly integrate both network and traffic simulations so that there is a feed back loop between them during simulation. Nowadays, few network simulators or traffic simulators meet all of these requirements. We found the NCTUns (National Chiao Tung University network simulator) network simulator and emulator, which provide all the requirements of user [10]. At the same time, GUI facility is there, can run on Linux or window. It is used for specify network configurations quickly and efficiently. Important parameters that should be set are number of vehicles, vehicle speed, bandwidth, packet type, packet size, simulation time, etc.

5. CONCLUSION

Purpose of VANET is to provide comfort and safety to the passengers. In this paper description, classification, characteristics, routing mechanism, their comparison and simulation aspects of VANET related protocols are discussed as done by various researchers. Overview given in this paper is expected to give the researchers useful information for further improvement in VANET protocols.

6. REFERENCES

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4. Vishal Kumar ,Narottam Chand, “Data Scheduling in VANETs” A Review”, International Journal of Computer Science and Communication,Vol1,No.2,July-December 2010,pp 399-403.

5. Kevin C.Lee,UCLA,USA,Uichin Lee,UCLa,USA,Mario Gerla,UCLA,USA, “Survey of Routing protocols in Vehicular Ad Hoc Networks,2010.

6. Sunil Taneja and Ashwani Kush ,” A Survey of Routing Protocols in Mobile Ad Hoc Networks”, International Journal of Innovation Management and Technology, 2010.

7. Pallavi Khatri, Monika Rajput, Alankar Shastri and Keshav Solanki, ”Performance study of Ad Hoc Reactive Routing Protocols”, Journal of Computer Science, 6(10):1159-1163,2010.

8. K.Prasanth, K.Duraiswamy, K.Jayasudha and C.Chandrasekar, ”Efficient Packet Forwarding Approach in Vehicular Ad Hoc Networks Using EBGR Algorithm”, International journal of Computer Science Issues ”, Vol 7,Issue 1,No.3,January January 2010.

9. Arnab Kumar Banik, ”Routing Protocol with prediction based mobility model in Vehicular Ad hoc Networks”, April 2010.

10. Shie Yuan Wang and Chih Liang Chou ,”NCTUns simulator for Wireless Vehicular Ad Hoc Network Research”, Nova Science Publishers, 2002.

11. The NCTUNS simulator –http://NSL.csie.nctu.edu.tw/nctuns.html

12. S.Y Wang, C.L. Chou, Y.H.Chiu, Y.S.Tseng, M.S.Hsu, Y.W.Cheng, W.L.Liu, and T.W.Ho, ”NCTUns 4.0: An integrated simulation platform for vehicular traffic, communication and network researches.” 1st IEEE WiVee, 2007.

Received on 24.03.2011 Accepted on 28.03.2011

Research J. Engineering and Tech. 2(2): April-June 2011 page 72-76

Protocol	Multiple Routes	Multicast	Periodic broadcast	Requires sequence data	Expiry of routing information	Summary
AODV	No	Yes	Possible	Yes	Yes	Route Discovery, Expanding ring search ,setting forward path
DSR	Yes	No	No	No	No	Route Discovery, Snooping