LSGO: Link State aware Geographic Opportunistic routing protocol for VANETs

Xuelian Cai 0 Ying He 0 Chunchun Zhao 0 Lina Zhu 0 Changle Li 0 0 State Key Laboratory of Integrated Services Networks, Xidian University , Xi'an, Shaanxi 710071, China Robust and efficient data delivery in vehicular ad hoc networks (VANETs) with high mobility is a challenging issue due to dynamic topology changes and unstable wireless links. The opportunistic routing protocols can improve the reliability of routing by making full use of the broadcast characteristics and assist in data transmission through additional backup links. In this paper, we propose a Link State aware Geographic Opportunistic routing protocol (LSGO) which exploits a combination of geographic location and the link state information as the routing metric. The LSGO aims to improve the reliability of data transmission in a highly dynamic environment, which selects the forwarders and prioritizes them based on the vehicle's geographic location and the link's quality. We compare the performance of LSGO with GpsrJ + which removes the unnecessary stop at a junction and greedy traffic aware routing protocol (GyTAR) using network simulator ns-2. The simulation results show that it opens more nodes to participate in the opportunistic data forwarding and increases a connection's throughput while using no more network capacity than traditional routing. In the simulation, compared with other two protocols, when the number of vehicles and the average vehicle velocity increase, LSGO's packet dropping rate is reduced and the network throughput is improved. 1. Introduction Vehicular ad hoc networks (VANETs) [1] are first designed for safety applications; afterwards, a series of applications for increasing traffic efficiency and providing comfort to the vehicles passengers are proposed. The network layer has received the most attention when working on VANETs. As a result, abundant routing protocols in such a network with differing objectives and for various specific needs have been proposed [2]. Existing routing protocols of VANETs fall into two major categories: topology-based and geographic routing. Topology-based routing [3-5] uses the information about links that exist in the network to perform packet forwarding. Since link information changes in a regular basis, topology-based routing suffers from routing breaks, so this kind of routing protocols is not suitable for VANETs. Geographic routing [6-15] uses neighboring location information to perform packet forwarding. In this kind of routing protocols, nodes are unnecessary to maintain a topology map or exchange link state information or maintain established routes as they do in a conventional mobile ad hoc routing protocol. Therefore, geographic routing can better adapt to network size and topology changes. Greedy forwarding is the most widely used strategy in geographic routing. The fundamental principle is that a node forwards its packet to its neighbor that is closest to the destination. But the forwarding strategy can fail if no neighbor is closer to the destination than the node itself, and through this way, we can get the next hop which is nearly located beyond the transmission range of the forwarder. In this case, the established link is unstable and the signal strength may be reduced, which may cause an increase of the packet dropping rate. As the packet is forwarded using this kind of links, the probability of packet transmission failure is great. So, it will spend more resources on retransmissions. As a result, the network throughput is declined and the end-to-end delay is prolonged. To solve this problem, De Couto et al. proposed a new measure called the expected transmission count (ETX) [16], which is the predicted number of data transmissions required to send a packet over the source to the destination link, including retransmissions. The ETX is widely used in routing protocols for wireless multihop networks [17-20] since its goal is to find the paths with the higher throughput and the less expected total number of transmission [21]. The difficulty in using ETX in VANETs is that ETX does not consider the highly dynamic network environment, so we modified the ETX in this paper. Although greedy forwarding strategy makes the hop transmission to the greatest extent close to the destination, the link is very unstable, which is because the two nodes at both ends of the link are located at the boundary of each others communication range. So, another forwarding strategy opportunistic routing is proposed, which could improve the reliability of data transmission by making full use of the broadcast characteristics and assist in data transmission through additional backup links. It makes the packets have more opportunities to be received. In the existing opportunistic routing protocols, some take hop count as the routing metric, some pay more attention to the cost, some consider the distance to the destination to be the forwarding mechanism, and some care more about the energy. However, few of them take a combination of geographic location and the link state information into account. So, we proposed a Link State aware Geographic Opportunistic routing protocol (LSGO) which takes a combination of geographic location and the link state information as the forwarder selection mechanism. Firstly, we propose a candidate node set selection mechanism, which selects the forwarders based on the vehicles geographic location and the links quality. In our approach, the links quality is measured using the enhanced ETX metric. Secondly, we put forward a priority scheduling algorithm which prioritizes the forwarders by timer-based scheduling method. This routing protocol can greatly improve the packet delivery ratio, ensuring data transmission reliability under a highly dynamic environment. The rest of the paper is organized as follows: In Section 2, we will review the related work and introduce our motivation. Section 3 will present the details in the proposed LSGO scheme. The performance evaluations of the proposed scheme are presented in Section 4. Finally, Section 5 concludes the article. 2. Related work To design a routing protocol is to propose a method, by which the current node could select the appropriate intermediate node as its next hop. So far, geographic routing protocols are widely used in VANETs. However, there are some problems in conventional geographic routing protocols. For example, greedy perimeter stateless routings (GPSRs) [6] recovery mode has a problem called Baby Step Problem [22]. To solve this problem, greedy perimeter coordinator routing (GPCR) [7] is proposed. Packets are always greedily forwarded along the road from one junction to the other, which solves the Baby Step Problem in GPSR. However, even if a packet is forwarded along the street, it needs to stop at each junction node. GpsrJ + [8] removes the unnecessary stop at a junction while keeping the efficient planarity of topological maps. Recently, many researchers are concerned abo (...truncated)