This paper focuses on the routing algorithm for the communications between vehicles and places in urban VANET. As one of the basic transportation facilities in an urban setting, buses periodically run along their fixed routes and widely cover city streets. The trajectory of bus lines can be seen as a sub map of a city. Based on the characters of bus networks, we propose a bus trajectory-based street-centric routing algorithm (BTSC), which uses bus as main relay to deliver message. In BTSC, we build a routing graph based on the trajectories of bus lines by analyzing the probability of bus appearing on every street. We propose two novel concepts, i.e. the probability of street consistency (PSC) and the probability of path consistency (PPC) which is used as metrics to determine routing paths for message delivery. This aims to choose the best path with higher density of busses and lower probability of transmission direction deviating from the routing path. In order to improve the bus forwarding opportunity, we design a bus-based forwarding strategy with ant colony optimization (FACO) to find a reliable and steady multi-hop link between two relay buses in order to decrease end-to-end delay. BTSC makes the improvements in the selection of routing path and the strategy of message forwarding. Simulation results show that our proposed routing algorithm has a better performance in transmission ratio, transmission delay and adaptability to different networks.
S the development of the intelligent transportation system (ITS), vehicular ad hoc network (VANET) is excepted to support more and more new applications to improve the qualities of people's life and traffic, such as location-based services, travel planning services, traffic accident reporting services, intelligent parking services and social vehicular platforms [1][2][3][4]. Acting as mobile intelligent communication equipment, vehicles are expected to access the network wherever and whenever possible. There have been several papers focused on security [5][6][7][8] and mobility topics [9,10], but efficiency also plays an important role in VANET. Thus it's significant to design efficient routing algorithm in VANET. In this paper, we will focus on the design of routing algorithms for the communication between vehicles and some places.
In VANET, vehicles equipped with on-board units (OBUs) can spontaneously form a self-organization network without depending on other infrastructures [11][12][13][14]. Vehicles can directly communicate with other vehicles or infrastructures within their communication range by using dedicated short range communication (DSRC) technology or indirectly communicate with others by multi-hop links [11,[15][16][17][18]. Intermediate vehicles between sender and destination play the role of router and deliver packets by using the carry-andforward mechanism [17,[19][20][21][22][23]. On one hand, the movement of vehicles is affected by the driver’s subjective awareness, causing the uneven distribution and unpredictable trajectories of vehicles [5,21]. Dense network is beneficial for multi-hop link and has a good routing performance. However, in sparse network, it’s difficult to find the relay to deliver packets. Consequently, routing performance becomes poor [23][24]. On the other hand, the movement of vehicles makes rapid and frequent change of the network’s topology [25][26]. Therefore, the conventional routing based on the technology of ad hoc network has poor performance [26][27][28][29][30][31][32][33][34].
As one of the infrastructures in VANET, road side units (RSUs) can work as the router to store and forward packets [26,28,35]. However, the performance of RSU-based routing is limited by the number and also by the location of RSUs [23]. Only the intensive deployment of RSU can maximize the availability of RSU and provide a stable service for message transmission in VANET. Otherwise the link between vehicles may disconnect in the blind area of communication [36]. However, the deployment and management of RSU cause a lot of overhead, and the limited communication range and fixed position cause short connection between RSU and moving vehicles [23,[36][37]. Therefore, the RSU-based routing is inapplicable and uneconomical.
As one of the basic transportation facilities, buses pervade the main roads in city, and have specific driving trajectories and departure intervals [11,37]. Compared with the common vehicular network, the bus network has the characteristics of wide coverage, relatively uniform distribution of nodes, fixed trajectory and regular service [38][39]. Due to the unique advantage of buses, many researchers have paid attention to the superiority of bus network, and proposed some bus-based routing algorithms to solve the aforementioned routing problem [15,23,38,40,43]. Literature [23] and [40] built the A bus-based backbone graph for routing. Literature [41] used taxis and buses as the communication backbone. Literatures [15] and [42] proposed the bus-based two-tier VANET architecture, where buses are the only relay to forward packets. On one hand, the existing bus-based routing algorithms are all node-centric: the routing path is a multi-hop link which consists of some buses between sender and destination, and once the routing plan is made, only specified buses are used to forward packets in sequence. The carrier of packet can forward packets to the next only when it encounters with the specified next relay bus. And buses in different bus lines can encounter only in the overlap of the trajectories of their bus lines. When carriers miss the specified next relay bus in their meeting place, it will take a long time before the next encounter, which happens quite often in practice. On the other hand, in those existing bus-based routing strategy, bus is the only relay to deliver packets and ordinary vehicle only serve as a sender or receiver. Therefore, bus undertakes all the routing tasks in the network, and the number of buses is less than the ordinary vehicle’s, which may cause network congestion.
To solve the above problems, in this paper, we proposed the bus-based street-centric routing strategy (BTSC) with considering that the movement of vehicles is restricted by the topology of streets. Firstly, we build the bus lines-based routing graph used to select routing trajectory by analyzing the relationship between the trajectory of bus lines and streets. Secondly, based on our
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