JOURNAL OF INTERNATIONAL ACADEMIC RESEARCH FOR MULTIDISCIPLINARY Impact Factor 2.417, ISSN: , Volume 3, Issue 11, December 2015

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1 AN OVERVIEW ON CLASSIFICATION OF VARIOUS ROUTING PROTOCOLS IN VANET PARUL GUPTA* *Dept. Of Computer Science, P.I.G.G.C.W. Jind (Haryan), India ABSTRACT Vehicular connectivity is most important for an Intelligent Transportation System (ITS) in order to provide a wide spectrum of services, including safety-related, traffic control, and entertainment. Vehicular ad hoc networks (VANETs) are a subclass of mobile ad hoc networks (MANETs) having special characteristics of high dynamic topology and fast movement of nodes. Hence, protocols designed for mobile ad hoc networks (MANETs) cannot be used in vehicular ad hoc networks (VANETs). In this paper, an overview of VANETs, its characteristics, types of communication and applications are presented. The main of this paper is to provide a detailed survey of routing protocols used in VANET along with their classification on different basis. KEYWORDS: VANETs, ITS, Routing Protocols, Position based, Topology based, Unicast, Multicast, Broadcast 1. INTRODUCTION Vehicular Ad hoc Networks (VANET) is the type of Mobile Ad Hoc Networks (MANETs) [1]. VANET is the wireless ad hoc network in which communication takes place through wireless links mounted on each vehicle (node). Each vehicle within VANET plays the role of participant as well as router of the network. Some roadside units are also used which act as fixed units to facilitate the vehicular communication. Vehicular ad hoc networks are the self organizing networks aiming to provide safety and traffic-control along with entertainment. Rest of the paper consists: Section 2 presents types of communication in VANETs. Sections 3, presents applications of VANET, in Section 4, characteristics of VANET are explained and in Section 5, Routing protocols are classified and explained in brief. Finally, in section 6, we draw conclusions. 2. Types of communication in VANETs There are two types of communication possible in VANETs [2]: 123

2 Vehicle to Vehicle Communication (V2V) Vehicle to Roadside Communication (V2R) In V2V communication, Vehicle to vehicle communication takes place only through wireless devices present in vehicles. V2V communication is important for safety related applications as communication between them is required in no time. In V2R communication, communication takes place between vehicles and roadside installed units. V2R communication is important for traffic control and entertainment applications as roadside units provide a gateway to internet etc. Figure 1: Overall Structure of VANET 3. Applications of VANET VANET is one of the influencing areas for the improvement of Intelligent Transportation System (ITS). Applications of VANET are broadly classified in two categories: Safety Applications [3] to avoid road side accidents to avoid traffic jams speed control warning for curved road lane changing warning signal violation warning overtake vehicle warning free passage of emergency vehicles and unseen obstacles Comfort Applications to provide [4] weather information mobile e-commerce Internet access and other multimedia applications 124

3 The most well known applications include, Crash Avoidance Matrices Partnership (CAMP) developed by General and Ford Motors, Network on Wheels (NOW) that were developed under collaboration of various governments and major car manufacturers to improve the ITS [5]. 4. Characteristics of VANET There are some characteristics that distinguish VANETs from other types of ad hoc networks are explained below: High Node Mobility and Rapidly Changing Network Topology: In VANETs, Vehicles move with high speed especially at highways due to which communication link between two vehicles lasts for very short period of time. High mobility leads to extremely dynamic topology, this makes classic protocols used for MANET unsuitable for VANETs [6]. Location Information Availability: Vehicles are equipped with on-board navigation sensors like GPS which gives location and spatial environment information. This information can be used for routing purposes of vehicles. Time Delay Constraints: Applications in VANETs mainly safety applications like Crash Avoidance warning, Lane changing warning etc. require messages to be delivered with no delay otherwise results will be crucial. Inconsistent Node Velocity and Density: Nodes have different mobility speed ranging from zero to over 200km/hr. Nodes which are in traffic jam or RSUs have zero speed but while moving on highways have higher speed [7]. Similarly, density of neighboring nodes varies from zero to hundreds depending on the type of road as well as time. Therefore, while designing a protocol for VANETs all these aspects are to be considered. Restricted Node Movement Patterns: In VANETs vehicles move at high speed but their movement is regular and predictable. Vehicle movements are constrained by roads layout and other vehicles movement with limitations on driving speed and in addition vehicles also have to follow traffic signs and traffic signals [8]. No Power and Storage Constraint: Vehicles have sufficient power, computing and storage capacity as recharging of batteries from vehicles is possible in VANET but in MANET each mobile node has limited battery power and storage [9]. 125

4 5. Classification of Routing Protocols in VANET Due to high mobility and rapidly changing topology, restricted nodes movement and inconsistent nodes velocity and density are such characteristics of the VANET network that make routing decisions more challenging. Several other factors such as time constraints and different environments such as city and highway makes routing more challenging in VANET. Routing protocols can be classified on various basis such as on the basis of communication type i.e. V2V and V2I based protocols [10], on the basis of information used for routing [11] i.e. topology and position based routing, on the basis of communication strategy used for delivery of information from source to destination [12] i.e. unicast, multicast and broadcast. Some researchers combined basis of classification [13] and some surveyed only a specific type in detail [14], [15]. In this paper, classification is done broadly using both communication strategy and routing information as shown below in figure 2. Routing Protocols Communication Strategy Routing Information Broadcast Multicast Unicast Topology Based DV-CAST Mobicast ROVER Proactiv Reactiv Hybrid DT Position Based Non DSDV AODV ZRP VADD GPCR OLSR OLSR MOVE CAR DIR GSR A-STAR Figure 2. Classification of Routing Protocols of VANET 126

5 A. Routing Information Based Protocols On the basis of information used for the purpose of routing which is available in the network, routing protocols are divided in two categories: Topology Based Routing and Position Based Routing. 1) Topology Based Routing Protocols: Topology-based routing protocol uses topology information which is stored in the routing table as a basis to forward packets from source node to the destination node. They are further divided into three groups as Proactive, Reactive and Hybrid Protocols. Proactive Routing Protocols Proactive protocols allow a network node to use the routing table to store routes information for all other nodes, each entry in the table contains the next hop node used in the path to the destination, regardless of whether the route is currently needed or not. The table must be updated frequently to reflect the network topology changes. These protocols cause more overhead especially in the high mobility network as they share routing information with the neighbors. However, routes to destinations will always be available when needed. Proactive protocols usually choose the shortest path algorithms to determine which route will be chosen. Proactive based routing protocols may not be suitable for VANETs as they have high mobility nodes and these protocols use much of the bandwidth for sharing routing information with neighbors. Furthermore, size of the table is also quite big for large networks. DSDV and OLSR proactive routing protocols are discussed below: Destination Sequence Distance Vector Routing (DSDV) The destination sequenced distance vector routing protocol (DSDV) is a proactive routing protocol [16]. It is extension of classical bellman ford routing mechanism. In DSDV each node maintains a routing table that contains information about all destinations i.e. the total number of hops needed to reach these nodes, next hop to reach the destination and a sequence number initiated by the destination node. The route with the recent sequence number is considered as a fresh route. To maintain routes reliability, each node must periodically share its routing table with its neighbors. The routing table updates can be sent in two ways: a full dump or an incremental update. DSDV protocol guarantees the loop free routes; it also keeps only the optimal path to every node, rather than keeping multi paths which will help to reduce the total size of routing table. 127

6 Optimal Link State Routing (OLSR) OLSR is a table driven protocol and an optimization of classical link state protocol [17]. In OLSR each node selects a set of Multipoint Relays (MPR) from the set of neighbors with which it has symmetrical links. Thus OLSR requires bidirectional links. Each node has the knowledge as to for which node it acts as a MPR as they periodically announce this information in their control messages. Therefore overhead minimizes as only MPR retransmit the control messages. In OLSR, MPR nodes declare link state information in the network for the nodes to which it acts as a MPR used to provide the shortest route path to all the destinations. MPR nodes are also responsible for formation of routes from source to the destination. The protocol is particularly best for large and dense network as optimization is done by using MPR nodes. Reactive Routing Protocols On demand or reactive routing protocols were designed to overcome the overhead that was created by proactive routing protocols in case of large and highly dynamic network. Reactive routing protocols establish the route only when it is required for a node to communicate with another node. Only the routes that are currently in use are maintained which reduces the burden in the network. Only AODV and DSR routing protocols designed for reactive routing are explained below: Ad-hoc On Demand Distance Vector Routing (AODV) AODV routing protocol works purely on demand basis [18]. When a source node needs to communicate with another node, it starts route discovery process by broadcasting a route request message to its neighbor including the last known sequence number for that destination. Each node that forwards the route request also creates a reverse route for itself back to the source node. When the route request reaches a node with a route to destination node that node generates a route reply that contains the number of hops necessary to reach destination and the sequence number for destination most recently seen by the node generating the reply. The state created in each node along the path from source to the destination is hop-by-hop state; that is each node remembers only the next hop and not the entire route, as would be done in source routing. The main features of AODV are quick response to link breakage in active route and loop-free routes by using destination sequence numbers. 128

7 Dynamic Source Routing (DSR) Dynamic Source Routing protocol (DSR) is designed for multi-hop wireless ad hoc networks [19]. This protocol consists of two main mechanisms Route Discovery and Route Maintenance that makes it self-configuring and self-organizing. Route discovery is used to discover the routes from source node to destination. A node caches multiple routes to any destination which support rapid reaction to routing changes as another cached route can be tried if the one it has been using should fail. It also avoids the overhead of need to perform a new Route Discovery each time a route in use breaks. In DSR, data packets store information about all the intermediate nodes in its header to reach at a particular destination. Intermediate routers don t need to have routing information to route the data packets, but they save routing information for their future use. The intermediate node which detects broken link through route maintenance also notifies the source node using a route error packet identifying the link over which packet cannot be forwarded. Hybrid Routing Need of these protocols arises with the deficiencies of proactive and reactive routing and there is demand of such protocol that combines good characteristics of both reactive and proactive routing protocols to make routing more scalable and efficient. ZRP hybrid ad hoc routing protocols is discussed in following: Zone Routing Protocol (ZRP) ZRP for reconfigurable wireless networks is based on the idea of routing zones [20]. Each node has a predefined zone centered at itself including other nodes whose distance is in predefined limits in terms of number of hops. Each node has to maintain up-to-date routing information only for nodes in its zone that reduces the network overhead that is caused by proactive routing protocols. Route Discovery is done to communicate with nodes not present in the zone of a node by forwarding query messages selectively only to the nodes in its zone rather than all the nodes in a network. This causes route discovery mechanism to be much faster than that of global reactive route discovery mechanism. 2) Position Based Routing Protocols: Position based routing protocols use geographical positions of the node and its neighbor nodes determined using GPS or other positioning devices rather than the network address of nodes for routing. Position routing protocol doesn't maintain any routing table. Therefore, position-based routing protocols perform better and are more scalable for VANETs. Geographic routing protocols are classified 129

8 as: Delay Tolerant Network (DTN) Protocols and Non Delay Tolerant Network (Non DTN) Protocols. Delay Tolerant Networks Routing Protocols DTN is a wireless network in which continuous network connectivity is not available. In this network, nodes use store and forward scheme for delivery of packets from source node to destination node. Many DTN routing protocols have been designed but VADD and MOVE DTN routing protocols are discussed below: Vehicle Assisted Data Delivery (VADD) VADD protocol uses carry-and-forward strategy [21]. The most important issue is to select a forwarding path with the smallest packet delivery delay for data delivery from a moving vehicle to a static destination. VADD protocol assumes that vehicles are equipped with pre-loaded digital maps, which provide street-level map and traffic statistics such as traffic density and vehicle speed on roads at different times of the day and traffic signal schedule on intersection. In VADD protocol, vehicle transmits packets through wireless channels as much as possible, otherwise using the road on which vehicles are moving speedily. Using information provided by digital maps and delay model, VADD protocol performs dynamic path selection for packet forwarding process. VADD has three packet modes i.e. Intersection, Straight Way, and Destination based on the location of the vehicle carrying the packet. Motion Vector Routing Algorithm (MOVE) MOVE algorithm is designed for light networks to deliver data between static nodes whose positions are known globally [22]. This protocol assumes that each node knows its position through GPS and velocity of its neighboring nodes. Using the relative velocities of mobile node and its neighboring nodes it predicts the closest distance to the destination. While making forwarding decision the algorithm predict whether forwarding message will be progressed toward intended destination or not. MOVE algorithm gives high success rate and low end-to-end latency with less buffer space and communication overhead as compared to broadcast approach. However, Non DTN position-based routing could have better performance only if the routes are stable and consistent. Non- Delay Tolerant Networks Routing Protocols The non-delay tolerant network protocols do not consider non-continuous connectivity issue and assume that sufficient number of nodes is always available for successful communication. So, these protocols are suitable for highly connected VANETs. It uses 130

9 greedy approach to forward a packet to the neighbor which is closest to the destination. If there is no neighbor closer to the destination than the node itself then this strategy fails and the packet has reached the local maximum as it has made the maximum progress at the current node. The routing protocols use their own recovery strategy to deal with this problem. GPCR, CAR, DIR, GSR, A-STAR non DTN routing protocols are discussed below: Greedy Perimeter Coordinator Routing (GPCR) Greedy Perimeter Coordinator Routing (GPCR) is a position-based routing protocol [23]. GPCR does not use any global or external information such as a static street map but takes advantage of the fact that streets and junctions form a natural planar graph. GPCR consists of two parts: a restricted greedy forwarding procedure and a repair strategy. In restricted greedy forwarding if forwarding node is located on a street and not on a junction the packet is forwarded along the street towards the next junction. But at junction coordinator decides which street that the packet should follow using restricted greedy approach, the neighboring node with the largest progress towards the destination is chosen. GPCR avoids local optimum using repair strategy which implements graph planarization in which routing decisions are made on the basis of streets and junctions instead of individual nodes and their connectivity. Connectivity Aware Routing (CAR) Connectivity-Aware Routing (CAR) protocol was designed for inter-vehicle communication in a city or highway environment [24]. CAR locates destination and find the connected paths between source and destination by broadcasting path discovery packet. Each forwarding node records it s ID, hop count, and average and minimum number of neighbors in searching packets. CAR protocol also sets the anchor points for junctions in the broadcasted packet. After receiving the path discovery packets, the destination chooses a routing path with better connectivity and lower delivery delay time. Destination node sends the reply packet to the source through recorded anchor points in the selected routing path. After receiving the route reply, source node starts transmitting data packets in a greedy manner toward the destination through the set of anchor points. Route maintenance is done with the help of guards which maintains enough state information to allow efficient communication between two moving nodes. Diagonal Intersection Routing (DIR) Diagonal-intersection based routing (DIR) protocol is designed to improve CAR protocol [25]. CAR works with anchor points while DIR protocol constructs a series of diagonal intersections between the source and destination vehicles. The DIR protocol is a geographic 131

10 routing protocol. Based on the geographic routing protocol, source vehicle geographically forwards the data packet toward the first diagonal intersection, the second diagonal intersection, and so on, until the last diagonal intersection, and finally geographically reaches to the destination vehicle. For given a pair of neighboring diagonal intersections, two or more disjoint sub-paths exist between them. The novel property of DIR protocol is the autoadjustability; while the auto-adjustability is achieved that one sub-path with low data packet delay, between two neighboring diagonal intersections, is dynamically selected to forward data packets. To reduce the data packet delay, the route is automatically re-routed by the selected sub-path with lowest delay. Geographic Source Routing (GSR) GSR deals with high mobility of nodes on one hand and uses topological structure of the city on the other hand to perform position based routing [26]. GSR uses Reactive Location Service (RLS) to find the current position of destination node. Using map of the streets, the sending node computes a path to the destination as a sequence of junctions that the packet has to traverse in order to reach the destination. The protocol calculates the shortest path between source and destination using Dijkstra shortest path calculation based on the street map. To forward a packet between two successive junctions greedy forwarding approach is used. Anchor based Street and Traffic Aware Routing (A-STAR) A-STAR routing uses anchor-based routing with street awareness which means spatial awareness and traffic awareness [27]. A-STAR like GSR uses the street map to compute the sequence of anchors through which a packet must pass to reach its destination. But unlike GSR, A-STAR also refers to the traffic awareness. A-STAR uses statically and dynamically rated maps to find the number of junctions. In statistically rated maps, street map used by the vehicle is assumed to be loaded with routine bus route information and anchor path is computed using Dijkstra s least-weight path algorithm. In dynamically rated maps, A- STAR dynamically monitors and assigns weight to a street based on its latest traffic condition, which helps in selection of anchor of higher quality. A-STAR uses a new recovery method. When a packet reaches the local maximum and unable to pass then that junction is marked as out of service temporarily and this information is distributed to the all the nodes in the network. Therefore, nodes do not use that junction during anchor computation until that junction becomes operational. 132

11 B. Communication Strategies Based Protocols On the basis of communication strategy used for delivery of information from a source node to a destination node protocols can be classified as unicast, multicast and broadcast routing protocols. 1) Unicast Protocols: The unicast routing protocols transmit data packets from a single source to a single destination using either multi-hop forwarding technique in which intermediate nodes are used to deliver data from source to destination or store and forward technique. There are many unicast routing protocols proposed for VANETs; most of the topology based and position based routing protocols are unicast such as VADD, AODV, DSR, CAR, DIR, GPSR, DSDV and many other, which are presented in the previous section. 2) Multicast Protocols: Multicast routing protocols transmit data packets from a single source to all other vehicles located in the specific geographical area. Mobicast and ROVER multicast routing protocols are explained below: Mobicast Mobicast is a multicast routing protocol which supports spatiotemporal coordination [28]. It ensures the delivery of information to all nodes that happen to be in a prescribed region of space at a particular point in time i.e. to deliver Mobicast packet to Vehicles inside a zone of Relevance (ZOR) at time t (ZORt). If any vehicle of ZOR accelerates or decelerates unexpectedly then it has different connectivity. So, some vehicles in ZOR may not receive the mobicast messages due to loss of connectivity. This condition is known as temporal network fragmentation problem. To overcome this problem Mobicast dynamically estimate the accurate Zone of forwarding (ZOF) using Zone of approaching (ZOA) to successfully disseminate Mobicast packets to all vehicles in ZOR. Robust Vehicular Routing (ROVER) ROVER is a multicast routing protocol which uses geographical addressing to form ondemand multicast tree which is used to forward multiple data packets within a zone of relevance [29]. ROVER like AODV protocol floods only control packets in the network and unicasts the data packets. It assumes that each vehicle have unique vehicle identification number, GPS and digital map. The source node starts route discovery by flooding ZOF by zone route request packet, this packet contains source ID, its location, its current ZOR, and a sequence number of the route. When a vehicle receives the zone route request packet, it accepts the packet if it is located inside the ZOF and close to the source. If the vehicle was 133

12 outside the ZOR, it doesn't send a reply. After a vehicle accepts the zone route request packet, it sends a zone route reply packet contains its ID to one-hop neighbor that generated the zone route request packet and also stores information in its routing table. After that it rebroadcasts the route request packet. ROVER differs from AODV as it sends reply back not to the source but to the node transmitting the route request packet. So, all nodes store the local information needed to build a multicast tree rooted at the source node. After the tree is formed, data is broadcasted in the tree. Therefore, ROVER can be used by a reliable transport protocol to ensure end-to-end Quality of Service. 3) Broadcast Protocols: Broadcast routing protocols enables transmission of data packets to all the available nodes inside the broadcast domain of the network by flooding. These routing protocols provide reliable packet transmission with higher network overhead. DV-CAST routing protocol is discussed below: Distributed Vehicular Broadcasting Protocol (DV-CAST) DV-CAST is a multi-hop broadcast routing protocol for vehicular ad hoc networks [30]. It uses only local connectivity information to make a routing decision i.e.1-hop neighbor topology based routing. While designing DV-CAST protocol, it is assumed that infrastructure is not available in the network; each vehicle has a Global Positioning System (GPS) and a wireless communication device to periodically send out beacon messages to its neighbors to collect local connectivity information and not every vehicle is member of VANET. DV- CAST takes action according to the state that the vehicle is operating in i.e. light traffic, moderate traffic and traffic jam. When vehicles are in traffic jam then are in a well-connected neighborhood so every vehicle uses the broadcast suppression mechanism. While vehicles in light traffic are either sparsely-connected or totally disconnected neighborhoods so they use store-carry-forward mechanism. However, vehicles in moderate traffic may be in any of these three neighborhoods, so each vehicle takes action accordingly to handle the broadcast packet. 6. Conclusion In this paper, an overview on Vehicular ad hoc networks (VANETs) is presented including types of communication possible in VANETs, its applications and characteristics. Classification of VANETs routing protocols has been done on the basis of information used for the purpose of routing i.e. either topology of the network or location of nodes in the network and communication strategy used to for communication between source and destination. A summarized overview of routing protocols belonging to each type of 134

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