Design and simulation of a collision notification application with geocast routing for car-to-car communications
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Car-to-X Communications are envisioned to improve road safety, traffic efficiency, and information services through short-range and real-time systems. The enabling applications have varied requirements such as low latency, specific forwarding patterns, and reliable data exchange between vehicles and infrastructure. Hence, a cross-layer architecture for vehicular applications should be defined according to the goals of the implementation, so as to consider proper routing and dissemination mechanisms, communication protocols, and the application’s design and operation.
In this context, we propose the characterization of the upper layers of a safety-oriented Car-to-Car application. We propose a cross-layer application/network layer design for a Post Collision Notification (PCN) application, and by means of a coupled simulation model that combines the communication network with vehicular traffic flow, we assess the performance of the application design.
Three main results can be derived from our work. First, the selected geocast protocol (DRG) shows to be an accurate protocol for safety applications and it may be a scalable routing mechanism for other applications. Second, that DRG is effective even for larger urban areas. And third, there is a reduction of acceleration and speed of the closest vehicles to the incident zone, which is a positive impact of the PCN application on the traffic flow.
Consequently, the proposed cross-layer architecture and the implementation of geocast routing has shown a suitable support and good performance for safety applications, and could be extended to other application on the Car-to-X domain.
KeywordsCar-to-car communications Geocast routing Collision warning application Vehicular Ad Hoc Networks
Vehicular Ad-Hoc Networks (VANETs) enable the wireless communications platform for Intelligent Transportation Systems (ITS) and its applications. VANET communication scheme allows vehicles and infrastructure to transmit real-time data in order to forewarn drivers about traffic situations, as well as mobility and environmental conditions. In this context, the safety sphere aims to reduce fatalities and mitigate damages caused by traffic accidents through Vehicle-to-Vehicle (V2V or C2C) applications and other ITS measures .
We propose a cross-layer architecture for the Post Collision Notification (PCN) application.
We show the impact of the application on the mobility patterns of vehicles after the notification messages are disseminated by means of the Distributed Robust Geocast protocol.
We conduct a set of simulations and obtain experimental results that demonstrate the effectiveness of the proposed design for the PCN application.
The reminder of this paper is organized as follows. Section 2 presents the related work. Section 3 introduces the characterization and design requirements of our Post Collision Notification application. Section 4 reviews the Distributed Robust Geocast routing protocol. Section 5 describes the simulation results and discussion. Finally, Section 6 concludes this paper.
2 Related work
A wide range of routing protocols and a diverse categorization for the vehicular communications has been considered in previous literature. Nevertheless, the decision-making process for deciding which criteria and routing schemas should be considered according to the VANET application is not a trivial task. A close examination of dissemination mechanisms leads to categorize routing protocols based on the destination target: unicast, multicast and broadcast . Moreover, the variety of protocols offers a number of benefits for the implementation of vehicular applications. However, due to the nature of the collision notification applications, i.e., a strict low latency and a highly reliable communication scheme for a delimited area of interest, we focus this work on the revision of multicast routing, specifically on geo-based protocols or geocast; in this context, multicast can be considered an efficient data dissemination method for vehicular communications . Correspondingly, according to Michoud et al. , the network layer of the VANET requires either adaptation of MANET protocols or new routing protocols design.
Furthermore, many location-based services demand the support for specific routing patterns of the information, namely spatial (n-dimensional geo-data) and temporal specifications . In this context, position based routing is essential for vehicular applications, in which the geographical destination is a relevant decision criterion, as mentioned by Maihofer et al. . In the safety sphere, Kihl et al.  proposed two redesigned geocast protocols: DRG and ROVER. Both protocols are based on the geographical location of the vehicle. Our PCN application implements a variation of the original Distributed Robust Geocast, which will be explained in section 4.
Different from previous works, we propose the design and simulation of the Post Collision Notification application within a cross-layer architecture, by the implementation of Distributed Robust Geocast over the DSRC/WAVE standards. We also propose on this work a set of messages for upper layers called WaveAppMsg.
3 Characterization of the post collision notification application
The integration of the functionality and the effectiveness of the routing protocol in the VANET leads to model and couple the components of the logic (upper layer) and the network layer within a modular architecture. Our goal was to design a collision warning application and to adopt the most accurate dissemination method; to this end, we have developed a cross-layer architecture based on the requirements from the network and logical layers. This chapter describes the conceptual and modeling phases for the implementation of the PCN application.
3.1 PCN application layer
A Post Collision Notification application detects an incident based on the speed and deceleration of each vehicle or node. In this context, the term node refers to the vehicles on the ad-hoc network. After the identification of the incident, an accident-message is sent to the nodes in the region of interest. The PCN application is based on the communications standards defined for Wireless Access in Vehicular Environments (WAVE): IEEE 1609  and IEEE 802.11p . The design of the cross-layer architecture mostly depends on the intended target of the VANET application [14, 15]; for safety applications, the goal is to notify neighboring vehicles of hazard conditions (e.g. collision ahead); thus, the drivers can take actions previous, during, and before the emergency situations. For these types of applications the region of interest is commonly of medium-size (up to 1500 m), and the messages require a high priority and a strict low latency communication (typically less than 100 ms).
Parameters initialization: The application layer of the PCN is responsible of the initial phase, including parameters definition, such as: operational range of the application (in miles or kilometers), beacon intervals (in seconds) and the connection establishment with the Global Positioning System (GPS) or urban mobility simulator.
Data encapsulation: We propose a set of messages, so called the WaveAppMsg, following the modular design. A WaveAppMsg packet contains an identification number (ID), the header field, the data and an optional field. The data correspond to the geographical coordinates of the emergency event; for PCN, an additional field is employed to indicate the accident status, showing that either the accident is ongoing or it has been successfully resolved.
Accidents detection: To avoid collisions with new arrival vehicles in the scene of a previous accident, PCN detects and notifies when there is a stopped vehicle on the road due to an accident. According to the speed and deceleration, PCN calculates an abruptly halt and triggers the notification event called accident signal.
Triggering of the sending mechanism: PCN starts the data dissemination process once the event trigger -accident signal- is launched right after the accident’s detection. This layer also defines the periodicity and the amount of packets to send through the VANET.
Vehicle position retrieval: PCN requires the up-to-date geographical location of the vehicle, so as to deliver reliable information about an occurred accident within actual coordinates. The application layer employs geo-data from a GPS or a road network simulator.
Data storage: Once data are delivered to the destination, each recipient node stores the information related to the accident. The reason is to update nearby drivers about road events and to maintain traffic data for statistical and mobility purposes.
3.2 Network requirements for PCN
Participants: it specifies if it is a Vehicle-to-Vehicle (V2V) or a Vehicle-to-Infrastructure (V2I) communication.
Latency: it defines the total delay experienced from the time the packet is sent by the source to the moment it is delivered to the recipient: low (< 100 ms) and medium (> 100 ms). This parameter provides message priority information.
Region of interest: it describes the geographical range of the application: long (> 1500 m), medium (~1000 m) and short (< 500 m).
Geo-location: it describes if the vehicle’s position in the geographical coordinate system is necessary to perform any data process.
Recipient pattern: it describes the message receiver’s pattern. In other words, it says to whom the messages emitted have to be transmitted. The pattern can be one-to-many, one-to-a-zone, one-to-one, and many-to-one.
Trigger: it states how the application is triggered. There are three different modes; Beacon mode (periodic), event-triggered mode (event driven), and user-initiated-on-demand mode (user-initiated).
Routing protocols: this network attribute is highly related to the Recipient pattern criterion. There are four different categories: broadcast (one-to-many) and geocast (one-to-a-zone) for safety applications, and unicast (one-to-one) and aggregation (many-to-one), which are more suitable for commercial applications. As we want to cover the direct neighbor of the crashed vehicle with PCN messages, we will favor the geocast routing scheme.
Message packet format: it specifies the network packets format used to encapsulate application messages. There are two standards from which the application’s designer can choose, the Internet Protocolv6 (IPv6) or the WAVE Short Message Protocol (WSMP) .
PCN Application requirements
Max. End-to-End delay
Low (< 100 ms)
Region of interest
Medium (< 1500 m)
Recipient pattern trigger
WAVE Short Message (WSM)
Control Channel (CCH)
4 Geocast routing scheme for the PCN application
A common problem to overcome in most VANET applications is the low reliability of the network, caused in many cases, by the highly variable topology of the mobile ad hoc network.
Improvements of the reliability of the vehicular networks can be handled by the proper routing mechanism on the VANET. In other words, the reliability relies on an effective routing method and the forwarding techniques. For this sake, multi-hop and position-based multicast are implemented to cover geographic regions while assuring packet delivery to all nodes in the area of interest. Distributed Robust Geocast (DRG), a geo-based routing protocol, is a completely distributed and stateless protocol for a reliable location-based dissemination with no control overhead.
The purpose of using “selective broadcast” is to limit the flooding area to a specific zone, based on geo-data namely geographical coordinates; this routing decision minimizes the network load in contrast to the broadcast storm caused by a generic broadcast scheme. As shown in Table 1, the PCN application requires a multi-hop delivery mechanism in order to cover a delimitated area. For these reasons, DRG was considered the suitable routing protocol for the network layer of our PCN application.
4.1 DRG protocol aspects
4.2 Forwarding mechanism of DRG
5 Simulation tools and ICT for vehicular communications research
5.1 Simulation setup of PCN application on the V2V domain
The simulation of vehicular communications is a valuable tool to assess the performance, requirements, and feasibility of the VANET applications. Since the implementation of many cooperative systems require high costs, a complex infrastructure and a high level of coordination and logistics, it is very common that these projects are first evaluated using a simulation approach. Recent literature of VANETs refers to hybrid simulation as the most accurate technique to analyze Inter-Vehicle Communications (IVC) ; for this sake, coupling a network and microscopic traffic simulator provides a representative model of the mobility and communication patterns in urban scenarios .
We have performed a cross-layer simulation, which includes the physical; media access control (MAC), and the routing and application layers to evaluate the performance of our design. Considering this approach, we want to validate three concepts: First, that DRG is an accurate protocol for safety applications with low latency demand, and that it is a scalable routing mechanism for other applications with similar dissemination requirements. Second, that DRG is effective even for larger urban areas. And third, what is the impact of the PCN application on road traffic, once an accident has been detected and the neighbor drivers are successfully notified. An accurate integration of the application functionality and the effectiveness of the routing protocol leads to the success of the application, hence, the applications can positively influence the traffic flow and the drivers' behavior and their interactions.
5.2 Simulation scenario
The simulation is built upon Veins , an open source vehicular network simulation framework. Veins makes use of the interface called Traffic Control Interface (TraCI) of traffic simulation suit SUMO Simulation of Urban MObility  and OMNeT++ . Our city-scenarios and geo-data come from the OpenStreetMap project, an open data repository for geographical information. The microscopic model employed is the car-following model proposed by Krauss , this model allows the simulation of platooning of vehicles and reproduces the drivers' behavior. This model provides the driver’s perfection parameter σ to adjust the accuracy of the drivers' behavior; in our scenario σ was set to 0.5 to add dawdle to the drivers' responsiveness; furthermore, the accidents have been manually set up.
Mobility Model Parameters
Max. Speed (m/s)
Min. Gap (m)
Driver’s Dawdle σ
Simulation Framework Parameters
Routing data length
Connection Mng daturation
Mac carrier frequency
Mac. 1609.4. Tx power
Phy 80211p sensitivity
Phy 80211p. Max Tx power
4500 m × 6000 m
End-to-End Delay: it is the time elapsed since a packet is sent by the application layer at the source node until the recipient node’s application layer receives the packet . The latency is a common metric used to show the effect of larger areas to cover and also the impact of nodes on the performance of the protocol.
Packet Reception Rate (PRR): it is defined as the percentage of nodes that successfully receive a packet from the tagged node given that all receivers are within transmission range of the sender at the moment the packet is sent out . It is defined by Khan et al.  as a percentage given by:
5.3 Simulation results
In this work, we have proposed a cross-layer design of a collision warning application coupled with a distributed geocast routing mechanism. We have characterized the application and network layer in order to achieve a high delivery ratio and low latency. Since the network layer is responsible of data dissemination, we have chosen and adapted the Distributed Robust Geocast scheme as the routing protocol. The implementation of geocast for our PCN application has shown a suitable support and good performance for safety application. The packet reception rate of DRG has shown to not be sensitive to the number of nodes and the zone of relevance.
The results also show that DRG can be employed for other applications, due to its scalability and stability, either for larger distances or for higher densities of vehicles within the zone of interest. Other applications with similar routing requirements such as the Stopped/Slow Vehicle Advisor (SVA), Emergency Electronic Brake Light (EEBL), Road Hazard Condition Notification (RHCN), Road Feature Notification (RFN) and Congested Road Notification (CRN) can be de- signed considering similar requirements in the network layer, and may implement DRG for data dissemination.
For our future work, we will test the cross-layer design with other safety-oriented applications that are also dependent on the geographic location. Furthermore, we would like to design other applications oriented to influence the traffic flow and to improve the vehicular traffic efficiency.
This work is partially funded by CONICYT Chile through Project FONDECYT No. 11140045. Authors would like also to thank Colciencias (Administrative Department of Science, Technology and Innovation, Colombia) for holding this project under the program ”Young Researchers and Innovators”.
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