Abstract: Recent advances in wireless communication technologies and auto-mobile industry have triggered a significant research interest in the field of VANETs over the past few years. Vehicular Network consists of vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications supported by wireless access technologies such as IEEE 802.11p. This innovation in wireless communication has been envisaged to improve road safety and motor traffic efficiency in near future through the development of Intelligent Transport Systems (ITS). Hence, government, auto-mobile industries and academia are heavily partnering through several ongoing research projects to establish standards for VANETs. The typical set of VANET application areas, such as vehicle collision warning and traffic information dissemination have made VANET an interesting field of mobile wireless communication. This paper provides an overview on current research state, challenges, potentials of VANETs as well the ways forward to achieving the long awaited ITS. Keywords: Vehicular communication, V2V, V2I, ITS, IEEE 802.11p, WAVE, IEEE 1609. Road accidents has been on an alarming increase despite the introduction of several innovative in-vehicle safety-oriented devices such as anti-locking braking system (ABS), seatbelts, airbags, rear-view cameras, electronic stability control (ESC). Several studies have maintained that 60% of the accidents that occur on motorways could be avoided if warning messages were provided to the drivers just few seconds prior to moment of crash [1] [2]. The possibility of direct exchange of kinematic data between vehicles over an ad-hoc network environment called a vehicular ad-hoc network (VANET) has been widely perceived by governments, car manufacturing industries and academia as a promising concept for future realization of intelligent transportation system (ITS) thereby achieving safety and efficiency in our nearly overcrowded motorways. The VANET is a sub-class of MANET where the mobile nodes are vehicles. When compared with Mobile Ad-Hoc Network (MANET) and other cellular systems, inter-vehicle communication (IVC) has four major advantages: broad coverage area, relatively low latency due to direct wireless communication, little or no power issue as well as no service fees. In the recent years, car manufacturing industries, academia and government agencies have started putting much joint efforts together towards realizing the concept of vehicular communications in wide scale. Some frameworks are already worked out with the first landmark of standardization processes made by US Federal Communications Communication (FCC) [15] adopted an amendment of the legacy IEEE wireless LAN standard done by an IEEE Task Group (TG). The amendment is denoted by IEEE 802.11p as the platform for Wireless Access in Vehicular Environment (WAVE) which will be used to enable wireless communications between moving vehicles within a coverage distance of 1000m in a free space (i.e. highway scenario) and 300m in non-free space (i.e. urban scenario). Fig. 1 shows the ASTM endorsed DSRC standard structure for DSRC link and data link layer. The rest of this paper is organized as follows. Section II presents a brief overview of VANET. Application of VANET is presented in Section III while the current VANET open research challenges and certain ideas on possible solutions are presented in Section IV. Final conclusion of this paper is presented in Section V. In VANETs, participating vehicles are equipped with set of wireless sensors and On Board Units (OBUs) to allow for possibility of wireless communication between the vehicles and their environs. These devices make each vehicle function as packet sender, receiver and router which enable  The concept of equipping future vehicles with sets of wireless sensors, on-board units, Global Positioning System (GPS) or Differential Global Positioning System (DGPS) receivers and network interfaces presents an ample opportunity to achieve intelligent transportation systems with wireless-enabled vehicles capable of sending and receiving kinematic data on the road. VANET is the bedrock upon which vehicles will be able to gather, process and distribute information both for safety-related and non-safety-related purposes on our motorways. Extensive areas of potential VANET applications have been listed and evaluated by several researchers through different projects and consortia. Typically, these applications are classified into either safety-related or non-safety-related applications. Safety-related VANET applications are classified into three basic categories, namely: driver assistance (co-operative collision avoidance, road navigation and lane changing), alert information (work zone and speed limit alert information) and warning alert (road obstacle, post-crash and other life-threatening traffic condition warning).The vehicular safety communications consortium has listed eight (8) potential safety-related applications [19] : pre-crash sensing, curve speed, lane-change, traffic signal violation, emergency electronic brake light and co-operative forward collision alert, stop sign movement and left turn assistant. Safety-related messages from these applications normally require direct communication owing to their stringent delay requirement. For instance, in the case of a sudden hard breaking or accident, the vehicles following those ones involved in accident as well as those in opposite direction will be sent a notification message. Major road safety applications are the primary measures taken to reduce (or eliminate) the probability of traffic accidents and loss of life in our motorways [10] [20-21]. Some of the traffic accidents that occur annually across the world are as a result of intersection, rear-end, head-on and lateral mobile vehicle collisions. The necessary precautionary measures (or traffic warning systems) required for the effective implementation and deployment of this road safety applications with their required use-case, mode of communication, minimum transmission frequency and acceptable latency are summarized in Table II . These active road safety-related applications offer assistance to drivers through the provision of time-sensitive, life-saving traffic information which enables drivers to avoid collisions with other mobile vehicles on the road. This is achieved through the timely and reliable exchange of safety-related kinematic information amongst vehicles through V2V communication system as well as amongst vehicles and other road infrastructures through V2I communication, which is processed to predict traffic accidents and collisions. This kinematic information contains the vehicle's current location, intersection position, speed, acceleration and direction of movement, to create the awareness of the presence of other vehicles on the road. Moreover, most of these life-critical messages in vehicular communications are broadcast-oriented, time-sensitive, life-saving, safety-related messages which must have deep penetration across the entire network and must be reliably delivered to the intended recipients within a short time. The non-safety-related applications of VANETs are also referred to as comfort or commercial applications. Typically, these applications aim to improve traffic efficiency, passenger comfort and commercial platforms in terms of advertisements and electronic toll collection (ETC). These applications include provision of weather information, current traffic and the ability to locate various Points of Interest (PoI) such as nearest parking lots, gas stations, shopping malls, hotels, fast food restaurants, etc. The owners of these aforementioned businesses can install some stationary gateways to transmit marketing adverts for the mobile customers travelling via the VANET enabled vehicles. The compelling argument in allowing comfort and commercial VANET applications is that of distraction and interference with safety-related applications thereby defeating the aim of improving safety and traffic efficiency in our motorways. Consequently, a possible solution would be achieved by using separate physical network channels for safety and non-safety applications or by applying traffic prioritization where safety-related messages are accorded higher priority than non-safety-related messages. The current key research challenge of VANETs is the lack of central communication co-coordinator associated with all the existing wireless access technologies earmarked for VANET set-up, implementation and deployment. Deploying wireless communication in vehicular environment effectively requires that some intrinsic issues ranging from technical application development and deployment up to economic concerns must be resolved. Though VANET is a form of MANET, its behaviour and characteristics are fundamentally different. Some of the basic VANET research challenges that must be addressed to achieve effective vehicular communication are briefly discussed below. Many high-speed wireless access technologies and standards have been suggested, recommended and considered for use in VANET connectivity by many researchers [17] [19] (see Table III ). Some of the technologies and air interface protocols capable of supporting high-speed communication in vehicular environment which are currently being considered for VANETs include: The 2G and 2.5G technologies provide reliable security and wide communication coverage while 3G and 4G technologies which are swiftly taking over offer highly improved communication capacity and bandwidth. In USA, Europe and Japan, many fleet and telematics projects are already using different generations of cellular technology [17] . However, the apparent high cost coupled with its high latency rate and limited bandwidth discourages its possible use as future communication base for VANETs.
For instance, safety-related messages require a minimum transmission frequency of 10Hz with a maximum latency of 100ms REF .
Advances in vehicular ad-hoc networks (VANETs): Challenges and road-map for future development
International Journal of Automation and Computing
Mathematics