Geographic routing protocols for Vehicular Ad hoc NETworks (VANETs): A survey
Introduction
Vehicular Ad hoc NETworks called VANETs are an application of Mobile Ad hoc NETworks (MANETs) [34]. They form the core of an Intelligent Transport System (ITS) designed to rationalize the operation of vehicles in order to improve road safety. Their main advantage is that they bypass the need of an expensive infrastructure since wireless technology becomes pervasive and cheap. Indeed, due to devices installed inside vehicles or placed at roadside, vehicular communications allow drivers to be warned early enough of potential dangers and situations. In addition to improve the road safety, Vehicular Ad hoc NETworks also offer new services to users such as traffic conditions, weather conditions (e.g. ice), and Internet services, making hence the trip more comfortable.
In a VANET (Fig. 1), vehicles are autonomous and move in a self-organized way along roads and, exchange information with other vehicles and road infrastructures within their radio range. They allow, on the one hand, a direct Vehicle-to-Vehicle communication (V2V) and on the other hand, a Vehicle-to-Infrastructure communication (V2I). V2V communication operates in a decentralized architecture, and it is a particular case of mobile ad hoc networks. It is based on the simple inter-vehicle communication without access to any fixed infrastructure. Indeed, a vehicle can communicate directly with another vehicle if it is within its radio range, or through a multi-hop wireless communication using neighboring nodes as relays. It can be used to provide information on traffic conditions and/or vehicle accidents via wireless communication. V2V communications are very efficient for the transfer of information services related to road safety, but they do not ensure a permanent connectivity between vehicles due to the high mobility of the vehicles. A V2I communication, in which vehicles send and receive data to/from fixed road infrastructures, can provide real-time information on traffic conditions, weather, and basic Internet services by communicating with the backbone networks. V2I communication environment makes better the use of shared resources and leverages the services provided through access points RSUs (Road Side Units) deployed on roadsides. However, this communication mode is inadequate for applications related to road safety because the infrastructure networks are not efficient regarding to the delivery times.
A VANET is formed of vehicles (cars, buses, and so on) that are equipped with positioning systems (i.e., GPS devices), wireless communication devices (such as IEEE 802.11p/WAVE network interfaces), and digital maps. The set IEEE 802.11p and WAVE (Wireless Access for Vehicular Environment) form the DSRC (Dedicated Short Range Communication) standard [1] for VANETs communications. The WAVE standard describes the set of standards IEEE 1609.x (.1/.2/.3/.4) deployed at the MAC layer (Layer 2) and the network layer (Layer 3) of the OSI model. At the physical layer (Layer 1), the IEEE 802.11p standard is used. DSRC is actually considered as the most appropriate standard for wireless communications in vehicular ad hoc networks. Its first objective is to provide high data transfers and low communication latency in small communication zones. Hence, using the DSRC standard, it is possible to establish a vehicle-to-vehicle (V2V) communication and a vehicle-to-infrastructure (V2I) communication. It supports a vehicle speed exceeding 200 km/h, it offers a wireless range between 300 and 1000 meters, and provides a theoretical bandwidth up to 6 to 27 Mbps.
A VANET has unique characteristics, including the frequent changes of the topology and the high vehicle speeds. Therefore, routing and forwarding packets in VANETs is a challenging task. Traditional ad hoc routing protocols have difficulties in dealing with the high mobility specific to the vehicular ad hoc networks as demonstrated in many studies [8][9][31][32] which compare the performance of the topology-based routing (such as AODV [28] and DSR [13]) against position-based routing strategies in urban as well as in highway traffic scenarios. The requirements imposed by vehicular ad hoc networks are slightly different from other forms of mobile ad hoc networks [38][25]. On one hand, memory storage and energy consumption are not a constraint since in modern vehicles, the battery power and the storage space are unlimited, and each vehicle can get its own geographic position since nowadays vehicles can be equipped with a positioning system (GPS). On the other hand, the network is highly dynamic due to the high mobility of the vehicles which do not move randomly but follow a particular mobility pattern compared to those of general ad hoc networks.
In the literature, VANETs routing protocols are classified into five categories: ad hoc, cluster-based, broadcast, geocast and position-based. However, position-based or geographic routing tends to be the predominant one. Research [10][19] have shown that the Position-Based Routing (PBR) performs well with the high mobility of the vehicles. PBR uses the geographic position of vehicles to decide in which direction a data packet should be forwarded. Generally, this decision is based on a geometric heuristic which selects the direct neighbor that is the closest to the destination and which is called greedy forwarding [16]. The great advantage of the greedy forwarding mechanism is that it depends only on the direct neighbors. Thus, there are diverse requirements on the availability of position information. First of all, position-based routing requires that each node must be aware of: i) its own geographic position, ii) the position of its direct neighbors and iii) the position of the final destination. A node obtains its position by using the GPS. The position of its direct neighbors is transmitted through beaconing messages. Each node periodically broadcasts a beacon message containing its current position and probably other information like the direction and the speed. So, in order to make the routing decision, a source node also requires information on the current geographic position of the destination to include it in the packet header. The information about the destinations' position is provided by a location service [4].
The fact that greedy forwarding approach uses only local information could cause the risk that a packet gets stuck in a local optimum (void) i.e. no neighbor, that is closer to the destination than the current node itself, exists. To escape from a local optimum, a recovery strategy must be applied. The overall objective of a recovery strategy is to transmit the packet to a node which is closer to the destination than the node of the local optimum. Once a such node is found, the greedy forwarding can be applied. One of the most used recovery strategies is the right-hand rule [16] for crossing a graph. This rule states that when a node x reaches a local optimum, the next edge to cross is the node that is in the counterclockwise from the virtual arc formed by x and the destination. Once a node closer to the destination is found, the protocol is switched to the greedy mode. Another used approach is called the carry-and-forward [6]. When a local optimum occurs, the node carries the packet until that an eligible neighbor appears or it reaches itself the destination. Instead of using these recovery strategies, some algorithms recalculate a new path from the node of the local optimum. Several others recovery approaches were proposed in the literature giving rise to new routing protocols (see section 3).
In this paper, we explore and we present existing Position-Based Routing protocols in VANETs. Based on a main analysis of these protocols and according to the literature, Position-Based Routing can be divided into three categories: Non-Delay Tolerant Vehicular Ad hoc NETworks (Non-DTVANETs), Delay Tolerant Vehicular Ad hoc NETworks (DTVANETs), and Hybrid. The Non-DTVANETs protocols do not consider intermittent connectivity and are only practical in densely populated VANETs while the DTVANETs protocols do not consider disconnectivity and are designed from the perspective that networks are disconnected by default. Hybrid types combine the Non-DTVANETs and the DTVANETs to exploit partial network connectivity.
This paper is organized as follows: Section 2 provides the main challenges to consider for the position-based routing in VANETs. In Section 3, we present and we analyze some existing works in the literature. Finally, our paper is concluded in Section 4 with a comparative table.
Section snippets
Challenges in the design of position-based routing protocols for Vehicular Ad hoc NETworks
The key difference between VANETs protocols and any other form of Mobile Ad hoc NETworks is the design requirements. VANETs have some unique characteristics which make them different from MANETs. These characteristics must be taken into account when designing protocols for VANETs, it include:
High dynamic topology: Unlike ad hoc networks and sensor networks, the VANETs are characterized by a highly dynamic network situation due to the movement of vehicles at high speed making the topology of the
Geographic routing protocols for VANETs
Several routing protocols have been proposed in the literature for VANETs to address the recalled challenges. These protocols are categorized on the basis of the application where they are most suitable: Non-DTVANETs, DTVANETs, Hybrid.
Conclusion
In this paper, we discussed the challenges of designing routing protocols for VANETs and surveyed several geographic routing protocols dedicated for this kind of networks. Table 1 summarizes the characteristics of these geographic routing protocols by considering: their requirements (GPS, GLS, beacon or not), their forwarding strategy, the recovery strategies, the architecture (V2V or V2I), the type of applications where they are most practical (delay tolerant or not, Hybrid) and how they are
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