SUV: Related Work and Performance Evaluation

I. RELATED WORK The problem of medium access control and traffic dissemination in vehicular networks has attracted a great deal of interest, however most of the proposals that have appeared in the literature deal with either delay-tolerant or safety/emergency applications. An example of MAC scheme is in [1], where vehicles are synchronized and transmit into time slots according to their position: spatial cell resolution and temporal slot schedule are centralized and notified through an out-ofband control channel. In [2], [3], the use of rate-less codes and nodes cooperation are exploited to ensure fast and efficient dissemination of delay-tolerant traffic or single messages, in highly mobile networks. The work in [4], instead, focuses on driving an information piece from a source to one or multiple home zones, where the information is then maintained and provided to interested users. Such tasks are performed by exploiting periodic messages, broadcasted by each vehicle and announcing its future route: the knowledge of future movements, both its own and those of its neighbors, allows a vehicle carrying a content to decide whether to keep the information or to hand it to a neighbor with a lower estimated time-to-home zone. A broadcast protocol, specifically addressing the problem of intermittent connectivity, has been presented in [5]. There, using the solution in [6], each node determines to which connected dominating set (CDS) it belongs, based on the local position information received from its neighbors through periodic beacon messages. Beacons also include acknowledgments of broadcast messages. Upon receiving a broadcast message, a node decides whether to forward it or not, depending on the information acquired through its neighbors’ beacons, so as to reduce the number of retransmissions and ensure reliability. A survey on routing and data dissemination in vehicular networks can be found in [7]. Focusing on the support of streaming media in VANETs, few works have addressed the problem of channel access and traffic forwarding [8], [9], [10], [11] and the majority of them consider a highway scenario. In particular, the network architecture in [8] aims at propagating video streaming through forwarding nodes; however, unlike our case, the solution is tailored to traffic delivery in a highway scenario from multiple sources to a single receiver. The study in [9] proposes to improve video quality, while reducing bandwidth consumption, by applying frame skipping and frame rate reduction in 802.11-based VANETs. The work in [10] proposes an application-layer approach to deliver live video streaming, by exploiting a cluster-based network topology that however requires high cluster-formation overhead. We consider a similar network architecture as in [11] (i.e., synchronized broadcasting performed through forwarders), but we deal with an urban environment, which poses several additional challenges with respect to the highway case. Unlike our solution, [11] does not leverage the application characteristics. Finally in the context of sensor networks, the solution in [12] builds a virtual grid of forwarders and implements a TDMA schedule through a vertex coloring algorithm. In particular, [12] assumes a static, dense network and presents a distance-2 coloring heuristic with performance O(1) from the optimum. As for channel access, traffic recovery is achieved trough an RTS-CTS-DATA-ACK mechanism. Although related to our study, we remark that the work in [12] differs from ours in many aspects. In particular, we address a highly dynamic scenario and we do not require high node density. We define a distance-k coloring, where k can be any positive integer, which is optimal and has the same asymptotic complexity as the algorithm in [12]. Furthermore, we design a channel access scheme by leveraging the specific characteristics of video sources, such as variable bit rate and video encoding.