There are different aspects to classify the current ad hoc routing protocols proposed for VANETs. The type of information their design depends on, applications’
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demands, and the VANETs’ property they employ are examples of these aspects. Even though the aspects seem to be different, all categories have almost the same set of routing protocols. Here, we describe the main categories of ad hoc routing protocols proposed for VANETs based on the information they use and VANETs’ properties they utilise [56-58, 70].
2.3.1.1 Basic Solutions
Routing protocols belonging to this category do not use any specific information about the traffic environment such as traffic density, velocity limits, etc. They operate using the control messages received from neighbouring vehicles. Two classifications can be found in this category
Topology-based solutions. In this category, routing protocols depend on the network topology, which consists of vehicles and communication links, to perform the data packet routing. Furthermore, topology-based protocols can be classified as reactive and proactive protocols. Using the proactive approach, also called table-driven, the routing protocol maintains coherent and up-to-date routing table information even if there are no data packets to route. The advantage of this approach is that routes are already available to be used providing packets are delivered with low delay. However, the control messages needed to maintain routing table information on paths that might not be used waste a large amount of available resources unnecessarily. Optimized Link State Routing (OLSR) [59] and Destination-Sequenced Distance Vector (DSDV) [60] are examples of proactive routing protocols extended to VANETs. On the other hand, using the reactive approach, also called on-demand, the route is established only when there is data to send. The advantage of the reactive approach is the available resources are used only when they are needed. However, in a highly dynamic network, the desired route might not be available, so the communication is delayed until a new route is discovered. Ad hoc On-Demand Distance Vector (AODV) [61], Message Delivery Delay (MDD) [62], and Dynamic MANET On-Demand Routing (DYMO) [63] are examples of reactive routing protocols extended to VANETs.
Position-based solutions. In this category, routing protocols use information about the physical locations of network nodes in order to route the data packets. The position of the destination node can be obtained via location management service or by flooding in the expected destination area. When the source node has data to send, it includes the location of the destination node in the header of the data packet. Each node that receives this packet makes its routing decision based on its location, obtained via GPS or other positioning service, and the location of the destination found in the header of received data packet. The advantage of this approach is that the control overhead needed is small because nodes do not discover the route explicitly or maintain routing table information. However, the operations of updating/obtaining nodes location still require some extra overhead. Since these protocols rely on the position information, the inaccuracy of this information, when the network dynamic increases, leads to false routing decisions and consequently, degrades the performance significantly. Another disadvantage is that position-based routing protocols should cope with situations such as no node can be found in the current geographic area. GeoSpray [64] and Position-based Routing using Learning Automata (PBLA) [65] are examples of position-based routing protocols.
2.3.1.2 Map-based Solutions
In this category, routing protocols depend on a street-level map to set the junctions needed to get to the destination node. After that, geographic routing is applied to route the data packets through each street until they reach their destination. It is assumed that each node is equipped with a pre-loaded digital map that provides traffic statistics on the roads at different times of the day and traffic signal schedules at intersections. The enhanced Message Dissemination based on Roadmaps (eMDR) [66] and GeoSVR [67] are examples of map-based routing protocols. The advantage of this method is that the whole route is pre-computed and included in the header of every data packet sent from the source node. Thus, the control overhead is kept at a minimum level. However, a pre-loaded street-level digital map might not be available at every node.
2.3.1.3 Trajectory-based Solutions
Since vehicles are equipped with multiple sensors such as odometers and speedometers, the current trajectory of the vehicle can be obtained easily. Routing protocols in this category use trajectory information of neighbouring vehicles to route the data packets to the destination. If the current vehicle finds a neighbour whose trajectory goes closer to the destination than its own, it forwards the data packets to that neighbour. This approach implies using a store-carry-forward scheme while routing the data packets. The routing decision at each node is based on the trajectory information received from neighbouring vehicles. Thus, the accuracy of this information plays an essential role in routing the data packets correctly. In addition, when trajectory information is out-dated, a node carries the data packet until receiving updated information from neighbouring vehicles. Therefore, these routing protocols are more suitable for delay tolerant data. Geographical Opportunistic Routing (GeOpps) [68] and Motion Vector (MoVe) [69] are examples of trajectory-based solutions.
2.3.1.4 Mobility-based Routing Protocols
In this category, the mobility information is used by the routing protocol to predict the lifetime of available links while making the routing decisions. Mobility information includes relative distance, relative velocity, relative acceleration/deceleration, and direction of movement. Therefore, routing decisions can be taken on the basis of the lifetime of a communication link or the direction of mobility. However, this method has extra control messages overhead because vehicles should send mobility status update messages to their neighbours to keep them informed. Prediction-Based Routing (PBR) [54] and Receive on Most Stable Group-Path (ROMSGP) [71] are examples of mobility-based routing protocols.
2.3.1.5 Infrastructure-based Routing Protocols
The road infrastructure such as RSUs, cellular base stations, and even routine buses are used in routing protocols belonging to this category. RSUs are considered fixed reliable nodes connected together by high bandwidth, low delay, and low bit error rates links. Therefore, they can be used to relay data packets to the destination.
However, the deployment of such infrastructure is costly and limited to specific areas. Differentiated Reliable Routing (DRR) [72] and Bus [73] are examples of infrastructure-based routing protocols.
2.3.1.6 Probability-based Routing Protocols
Here, the probability theory is used to build a probability model of the wireless communication link between two vehicles. After that, it is used to describe the likelihood of certain events like the probability of link breakage or the probability that the wireless link will stay connected for a certain time interval. The routing protocol then chooses reliable links to route the data packets based on their probabilities. Reliable and Efficient Alarm Message Routing (REAR) [74], DeReq [16] and GVGrid [75] are examples of probability-based routing protocols.