The cellular network coverage in sparsely populated and mountainous areas is often patchy. That can be a significant impediment for services based on connections between vehicles and their environment. In this paper, we present a method to reduce the waiting time occurring when a vehicle intends to send a message via a cellular network but is currently in a dead spot without sufficient coverage. We use a hybrid network approach combining cellular network access with ad-hoc networks between vehicles that are nearby. In particular, we introduce a data dissemination protocol that allows the vehicles connected through an ad-hoc network to find out which one will most likely leave the dead spot first. Messages can then be sent to this vehicle that forwards them as soon as it regains cellular network access. Further, we developed an initial implementation of this protocol using the technology WiFi Direct that is realized on many mobile phones. Implementation details of the prototype as well as analysis results regarding data transmission time limits of fast driving vehicles are discussed in the article as well.

No abstract

An often overlooked practical problem in Intelligent Transportation Systems (ITS) is the presence of areas without cellular network connectivity, so-called dead spots, which aggravates the communication between vehicles and the external infrastructure. In our previous work, we suggested to mitigate this problem by using a hybrid data dissemination protocol that combines cellular network communication with ad-hoc networks between vehicles. If the vehicles in such ad-hoc networks are in a dead spot but have a good estimation about the time they will leave it again, messages can be forwarded to the vehicle that is supposed to regain cellular network coverage first. Since this vehicle may transmit the stored messages immediately after having left the dead spot, the delivery time is improved. In this paper, we first analyze the behavior of the aforementioned data dissemination protocol in larger dead spots in which a message may be carried by several vehicles before being delivered via the cellular network. The analysis reveals that messages are not always delivered in the fastest possible time. To address this concern, a new protocol variant named context-aware message flooding protocol is introduced. This protocol, indeed, guarantees the fastest possible forwarding of messages to their recipients. This is achieved at the cost of delivering duplicates that, however, are only produced when the delivery of a message is sped up.

The cellular network coverage in sparsely populated and mountainous areas is often patchy. That can be a significant impediment for services based on connections between vehicles and their environment. This article presents a method to reduce the waiting time occurring when a vehicle intends to send a message via a cellular network but is currently in a dead spot, i.e., an area without sufficient coverage. The authors introduce a data dissemination protocol that allows vehicles to connect through an ad-hoc network. The ad-hoc network peers can then find out which one will most likely leave the dead spot first. The selected vehicle stores then the messages of all connected vehicles and forwards them to the remote infrastructure as soon as it regains cellular network access. This research also discusses message flows in larger dead spots in which a vehicle may consecutively form several ad-hoc connections. Further, the authors describe an initial implementation of the protocol using the technology Wi-Fi Direct that is realized on most modern mobile phones.

On many roads in rural and mountainous areas, the cellular network connectivity is intermittent and dead spots, i.e., zones without any coverage, are frequent. In previous work, we developed a data dissemination protocol to accelerate the transmission of messages in dead spots. It combines the cellular network with short-living ad-hoc networks between vehicles. A car in a dead spot can forward messages directed towards the environment, to the peer in its ad-hoc network that will leave the dead spot first, effectively reducing the delay. An issue, however, is to reliably identify the peer that is most likely the first one regaining cellular network coverage. This problem can be solved if the borders of the dead spot, the vehicles are in, are previously known. For that, we use a novel technology named dead spot prediction. Here, vehicles conduct local connectivity measurements that are aggregated to so-called connectivity maps describing the locations of dead spots on a road system. In this article, we introduce the combination of the data dissemination protocol with dead spot prediction. Particularly, our protocol is amended such that connectivity maps are considered when deciding which vehicle leaves a dead spot first. Since currently only few publicly available works about dead spot prediction exist, we further created a prototype of such a predictor ourselves that will be discussed as well.