A scalable data dissemination protocol for both highway and urban vehicular environments

Vehicular ad hoc networks (VANETs) enable the timely broadcast dissemination of event-driven messages to interested vehicles. Especially when dealing with broadcast communication, data dissemination protocols must achieve a high degree of scalability due to frequent deviations in the network density. In dense networks, suppression techniques are designed to prevent the so-called broadcast storm problem. In sparse networks, protocols incorporate store-carry-forwardmechanisms to take advantage of the mobility of vehicles to store and relay messages until a new opportunity for dissemination emerges. Despite numerous efforts, most related works focus on either highway or urban scenarios, but not both. Highways are mostly addressed with a single directional dissemination. For urban scenarios, protocols mostly concentrate on either using infrastructure or developing methods for selecting vehicles to perform the store-carry-forward task. In both cases, dense networks are dealt with suppression techniques that are not optimal for multi-directional dissemination. To fill this gap, we present an infrastructure-less protocol that combines a generalized time slot scheme based on directional sectors and a store-carry-forward algorithm to support multi-directional data dissemination. By means of simulations, we show that our protocol scales properly in various network densities in both realistic highway and urban scenarios. Most importantly, it outperforms state-of-the-art protocols in terms of delivery ratio, end-to-end delay, and number of transmissions. Compared to these solutions, our protocol presents up to seven times lower number of transmissions in dense highway scenarios. Introduction Vehicular ad hoc networks (VANETs) are expected to serve as support to the development of a wide range of applications related to safety, transport efficiency, and even infotainment [1]. Such applications are built upon internal sensor data that is continuously gathered, processed, and disseminated to other vehicles in the neighborhood. Since the acquired data is usually of interest to a number of vehicles in the region, e.g., data about accidents, broadcasting becomes the predominant communication paradigm. However, several challenges arise when relying on broadcast communication. Broadcasting is particularly unreliable due to the lack of acknowledgments in the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism present in the 802.11p standard. Also, vehicular networks are very dynamic in nature with *Correspondence: [email protected] Pervasive Systems Group, Department of Computer Science, University of Twente, Enschede 7522 NB, The Netherlands large deviations in density depending on the current road traffic. Scalability becomes then a paramount factor to be taken into account when designing data dissemination protocols for VANETs. In dense networks, a pure flooding scheme results in excessive redundancy, contention, and collision rates [2], which is referred to as the broadcast storm problem. Such a problem is tackled with broadcast suppression techniques. Most of these techniques aim to assign vehicles to different delay values before attempting to rebroadcast that are inversely proportional to their distance to the sender. In this way, only the farthest vehicles would rebroadcast, thereby allowing for quick data dissemination [3]. Vehicles assigned to delay values sufficiently higher to hear a rebroadcast echo can suppress their transmissions. This separation in time is accomplished by means of time slots, where each time slot is equivalent to a message’s transmission time. Conversely, in sparse networks, vehicles may face network disconnections when the transmission range © 2013 Schwartz et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Schwartz et al. EURASIP Journal onWireless Communications and Networking 2013, 2013:257 Page 2 of 19 http://jwcn.eurasipjournals.com/content/2013/1/257 employed cannot reach other vehicles farther in the direction of interest. In such scenarios, protocols should also incorporate a store-carry-forward mechanism to take advantage of the mobility of vehicles to store and relay messages until a new opportunity for dissemination emerges. Despite numerous efforts, most related works focus on either highway or urban scenarios, but not both. On one hand, highways are most commonly addressed with a single directional dissemination, as the data generated is assumed to only affect vehicles in one road direction, e.g., upon the event of an accident. However, such an assumption is not valid in urban scenarios, where a complex road grid with multiple road directions must be considered when relaying data messages. On the other hand, protocols designed specifically for urban scenarios usually concentrate on methods for selecting vehicles to perform the store-carry-forward task or rely on infrastructure to support the data dissemination. Nevertheless, in both types of scenarios, protocols still rely on suppression techniques that are not optimal for multi-directional dissemination. In this work, we fill this gap by proposing the infrastructure-less Adaptive Multi-directional data Dissemination (AMD) protocol that works seamlessly in both highway and urban scenarios. The key contributions of this work can be summarized as follows: • A generalized time slot scheme based on directional sectors to support multi-directional data dissemination. In each sector, the density of time slots is precisely controlled based on our method for single directional dissemination presented in [4]. • A store-carry-forward algorithm to support multi-directional data dissemination. To this end, we borrow concepts first introduced in our method for a single directional dissemination presented in [5]. • A comprehensive simulation campaign with a direct comparison against three state-of-the-art protocols, namely, distributed vehicular broadcast (DV-CAST) [6], S imple and Robust Dissemination (SRD) [5], and urban vehicular broadcast (UV-CAST) [7], under both realistic highway and urban scenarios. In particular, we take a real map fragment from the Manhattan area in New York City, NY, USA, including the shape of buildings that are used to model radio obstacles. The remainder of this paper is organized as follows. First, we review the literature and outline problems with current data dissemination protocols. Next, we describe the AMD protocol in detail. The results of our performance evaluation is then detailed and discussed. Finally, this work is concluded with a discussion and outline for future directions. Related work Various solutions for VANETs have been proposed to cope with message dissemination under different traffic conditions. In dense networks, various broadcast suppression techniques have been proposed to prevent the so-called Broadcast Storm Problem. The ultimate goal is to select only the set with the minimum number of vehicles to rebroadcast and disseminate a message toward the region of interest. In the context of mobile ad hoc networks (MANETs), several solutions to address this problem were proposed and outlined in [2,8]. In [8], the authors present a comprehensive comparison study of various broadcasting techniques in MANETs organized into four categories: (1) simple flooding methods, without any form of suppression; (2) probability-based methods, that rely on network topology information to assign a probability for each rebroadcast; (3) area-based methods, which use distance information to decide which nodes should rebroadcast; and (4) neighbor knowledge methods, which maintain state on the neighborhood via periodic hellomessages to decide on the next forwarding node. However, these solutions are mostly concerned with providing means for route discovery with minimum extra network load and, therefore, do not take into account the highly dynamic environment present on roads, neither exploit specific characteristics of vehicular networks such as the predictable mobility pattern of vehicles’ movements. In VANETs, it is generally assumed that each broadcast data message relates to a certain event of a specific geographical region, and, thus, it is targetedmostly to vehicles traveling through that region. With this goal, protocols that rely on positioning information falling into categories 3 and 4 are most suitable. In category 3, nodes in the location-based scheme [2] rebroadcast whenever the additional coverage is higher than a pre-defined threshold. In category 4, most protocols require nodes to share onehop or two-hop neighborhood information with other nodes [9-11]. This is particularly not suitable in vehicular environments, since such information can quickly become outdated due to the high speed of vehicles. In addition, adding neighborhood information to periodic messages results in high network overhead. As pointed out in [12], decreasing message overhead is crucial for leaving sufficient bandwidth for even-critical messages. In view of these drawbacks, several protocols have been proposed specifically for VANET applications. Such protocols present lightweight solutions in terms of overhead and elaborate on previous solutions in category 3 such as in [2] in order to control, based on distance, the thresholds determining when vehicles should rebroadcast. In the following, we select and describe a few of these efforts. For a complete survey of solutions, we refer the reader to [13]. Schwartz et al. EURASIP Journal onWireless Communications and Networking 2013, 2013:257 Page 3 of 19 http://jwcn.eurasipjournals.com/content/2013/1/257 The common approach to reduce broadcast redundancy and end-to-end delay in dense vehicular networks is to give the highest priority to the most distant vehicles toward the message direction. In [3], three ways of assigning this priority are presented: Weighted p-Persistence, Slotted 1-Persistence, and Slotted p-Persistence. In the first scheme, the farthest vehicles rebroadcast with the highest probability. In the second approach, vehicles are assigned to different time slots depending on their distance to the sender, where vehicles with the highest priority are given the shortest delay before rebroadcasting. Finally, the third approach mixes probability and delay by giving vehicles with the highest priority the shortest delay and highest probability to rebroadcast. In delaybased schemes, vehicles assigned to later time slots have time to cancel their transmissions upon the receipt of an echo. This would be an indication that the information has already been disseminated and redundant rebroadcasts can be suppressed. Notably, to achieve the lowest possible end-to-end delay, deterministic approaches such as Slotted 1-Persistence should be preferred over probabilistic methods such as Weighted p-Persistence and Slotted pPersistence. The reason lies in always guaranteeing that the farthest vehicle is chosen, which is not the case with probabilistic-based methods. Delay-based schemes have been used in several other works with the goal of reducing rebroadcast redundancy, e.g., [14-16]. In [14], the contention-based forwarding scheme (CBF) is presented. Authors focus on a distributed delay-based scheme for mobile ad hoc networks that requires no beaconing information. In [15], the urban multi-hop broadcast (UMB) protocol is designed to cope with broadcast storm, hidden node, and reliability problems of multi-hop broadcast in urban areas. UMB has a special operation mode for scenarios with intersections. Nevertheless, it relies on the same time slot principle for directional data dissemination. Although efficient in tackling the broadcast storm problem, delay-based schemes still present scalability issues when not employed with optimal parameters. One clear limitation in most schemes proposed is the inability to dynamically choose the optimal value for the number and boundaries of the time slots used. Time slots are usually matched to geographical regions within the transmission range of the sender. However, this can lead to an uneven distribution of vehicles in each time slot. Since transmissions in a single time slot occur nearly simultaneously (see [17]) and cannot be canceled, the level of rebroadcast redundancy and collision is unnecessarily increased. To cope with collisions, the work in [18] introduces a means to control the number of time slots according to the network density. However, authors do not cope with the problem of nearly simultaneous transmissions in a single time slot. To the best of our knowledge, the DOT scheme presented in our previous work [4] pioneered in proposing a precise control of the time slots’ density by exploiting the presence of periodic beacons. Such beacons provide one-hop neighborhood information and are expected to be massively present to increase cooperative awareness in safety applications [19]. Authors in [20] had later a similar insight of time slots’ density control with the DAZL protocol. Another problem when relying on time slots schemes arises when the message must be disseminated to multidirections, as shown in Figure 1. In Figure 1a, vehicles follow a typical time slot scheme based on distance. Therefore, the most distance vehicle from the sender, i.e., vehicle v1, has the highest priority to rebroadcast in the neighborhood. However, such a naive solution clear prevents the dissemination of the message to both north and south directions, as vehicles v2, v3, and v4 would cancel their rebroadcasts upon hearing the early transmission from v1. The same problem occurs in a highway scenario as shown in Figure 1b, where the rebroadcast performed by v1 prevents the dissemination of the message to the other direction where vehicles v2 and v3 are located. This problem is addressed in [21], however, with no support for disconnected networks. All suppression schemes still depend on additional measures to cope with sparse disconnected networks when the transmission range does not reach farther vehicles in each possible road direction. The typical approach to cope with disconnected networks is to assign selected vehicles the task of storing, carrying, and forwarding messages when new opportunities emerge. The storecarry-forward paradigm is mostly present in works falling in the area of delay-tolerant networks (DTN) and opportunistic networks. In its simplest form, an epidemic routing is used [22], where flooding is used to disseminate messages throughout the network. In this approach, nodes exchange data as soon as new neighbors are discovered. The spray routing [23] generates only a small number of message copies in order to ensure that the number of transmissions are small and controlled. In the context of pocket switched networks (PSNs), where the nodes are devices carried by people, the BUBBLE algorithm is proposed [24]. It takes into account people’s social relationships to select the nodes that can best relay messages. However, these approaches were designed assuming a different mobility model from the one present in VANETs, as they usually consider a combination of the mobility of pedestrians, bicycles, and cars. In VANETs, the mobility of vehicles is constrained to single or multiple roads and by well-defined rules. Therefore, in order to achieve optimal results, more tailored solutions are needed. A few works apply the store-carry-forward mechanism specifically for VANETs [5-7,25,26]. In [6], the DV-CAST Schwartz et al. EURASIP Journal onWireless Communications and Networking 2013, 2013:257 Page 4 of 19 http://jwcn.eurasipjournals.com/content/2013/1/257