Abstract-Vehicular Ad Hoc Wireless Network (VANET) exhibits a bipolar behavior in terms of network topology: fully connected topology with high traffic volume or sparsely connected topology when traffic volume is low. In this work, we develop a statistical traffic model based on the data collected on I-80 freeway in California in order to study key performance metrics of interest in disconnected VANETs, such as average re-healing time (or the network restoration time). Our results show that, depending on the sparsity of vehicles, the network re-healing time can vary from a few seconds to several minutes. This suggests that, a new ad hoc routing protocol will be needed as the conventional ad hoc routing protocols such as Dynamic Source Routing (DSR) and Ad Hoc On-Demand Distance Vector Routing (AODV) will not work with such long re-healing times. For a typical Vehicular Active Safety (VAS) application, each vehicle periodically broadcasts its current GPS position, velocity, heading and other sensor information, e.g., breaking status, acceleration, etc. so that all the neighboring vehicles can receive and evaluate whether there is a dangerous driving situation that the driver should be aware of. Some VANET applications, however, aim at providing transport efficiency and commercial value to customers. Thus, they may require that certain traffic information be propagated to a larger group of vehicles in a multi-hop fashion. Therefore, the fundamental requirements for VANETs, from a networking perspective, are efficient information acquisition, exchange, dissemination, and processing mechanisms. Unlike conventional ad hoc wireless networks, a VANET not only experiences rapid changes in wireless link connection, but may also have to deal with different types of network topologies. For example, VANETs on freeways are more likely to form highly dense networks during bumper-to-bumper rush hour, while VANETs are expected to experience frequent network fragmentation in sparsely populated rural freeways or during late night hours. Until now, most of VANET research has focused on analyzing routing algorithms to handle the broadcast storm problem in a highly dense network topology [1, 2] , under the over-simplified assumption that VANET is a well-connected network in nature. In contrast, we believe that the disconnected network problem is also a crucial research challenge for developing a reliable and efficient routing protocol that can support highly diverse network topologies. In this study, we use vehicle traffic data measured on the I-80 freeway in California on June 27, 2006 to statistically characterize and model the traffic on typical highways in terms of key parameters, such as inter-arrival time and intervehicle spacing. Extensive Monte Carlo simulations based on these statistical models extracted from empirical traffic data show that although the average re-healing time for I-80 type of freeways is, on average, less than 30 seconds, such long network disconnection time could be a major problem for conventional ad hoc routing protocols such as AODV or DSR, which can only tolerate a network disconnection time of up to 2-3 seconds. In addition, some time-critical applications may not be able to function properly in disconnected VANETs as the end-to-end delay could be on the order of several minutes. The remainder of this paper is organized as follows. In Section II, we discuss the related work. The background on the lane-level traffic model is provided in Section III along with the proposed road-level traffic model. Simulation study along with the results and discussion are presented in Section V. Finally, we draw conclusions in Section VI. To the best of our knowledge, this paper is the first research effort in studying routing issues and the behavior of disconnected VANETs. However, there are some pioneering works on vehicular traffic modeling and Delay Tolerant Networks which address similar problems in MANETs. Random Waypoint (RWP) was proposed [3] as a generic mobility model for network simulation, where mobile nodes randomly select destinations with a randomly chosen velocity. Later, it was realized that such a simplified mobility model is inadequate in capturing many rich characteristics of personal mobility in the real world. Consequently, Reference Point Group Mobility (RPGM) [4] was introduced to emulate grouping behavior of battlefield scenarios; Freeway/Manhattan model [5] captured the impact of geographic restriction (i.e., road) on vehicular mobility; Simplified car-following model [6] is used to restrict vehicular movements on road defined by real map data. However, in most cases, the over-simplified assumptions necessary to make the analysis tractable often result in models that fail to adequately represent the extreme complexity of the real-world mobility patterns. In our study, our vehicular mobility model is directly obtained from empirical data collected in a real-world environment. Moreover, unlike most studies which concentrate on evaluating the impact of mobility models on MANET routing protocols in wellconnected scenarios, we point out the fact that VANET is 0743-166X/07/$25.00
Also, the frequent network partition and mergence due to the high mobility makes the MANET routing protocols based on endto-end connectivity ineffective in the VANET settings REF .
On the Routing Problem in Disconnected Vehicular Ad-hoc Networks
IEEE INFOCOM 2007 - 26th IEEE International Conference on Computer Communications
IEEE INFOCOM 2007 - 26th IEEE International Conference on Computer Communications
Computer Science