Reliable congestion control mechanism for safety applications in urban VANETs
Introduction
Intelligent Transportation Systems (ITS), i.e., the integration of telecommunication and information technologies into transportation systems, were developed to address critical issues such as passenger safety and traffic congestion [1]. The vehicular ad hoc network (VANET) forms the cornerstone of modern ITS. It consists of mobile vehicles connected by wireless communication in an ad hoc manner without central control while moving along roads. Dedicated Short Range Communication (DSRC) is considered the most promising wireless access technology to support vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) information exchange [2]. Vehicles generally disseminate two types of information related to event-driven messages and beacons (also called BSM Basic Safety Messages in the US or CAM Cooperative Awareness Messages in Europe). Beacons are periodically broadcast by vehicles to inform their neighbors of information such as position, direction and speed. On the other hand, event-driven messages are multi-hop distributed in a geographic area with higher priority in the case of emergencies, such as car collisions, accidents and road surface collapse [3]. These messages assist vehicles in expanding their awareness range beyond the line of sight as well as avoiding potential dangers.
Depending on reliable and timely beacon exchange via V2V or V2I communication, many safety applications such as car collision avoidance and slow vehicle indication have been designed for VANET, revolutionizing the quality of experience for drivers and passengers. Research [4] shows that 60% of car collisions can be avoided if safety applications were provide a warning to drivers on time before car collisions. But safety applications timely warning relies on reliable and timely disseminating of beacons between vehicles. Therefore, disseminating beacons reliably and in a timely manner is crucial for proper operation of safety applications.
However, reliably disseminating beacons to all surrounding vehicles in a timely manner in VANET remains a challenge [5]. First, IEEE 802.11p protocol, which is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), is employed as the PHY and MAC layer protocol of DSRC. The broadcast procedure of the IEEE 802.11p MAC protocol follows the basic medium access protocol of the distributed coordination function (DCF) without three functions (acknowledgment, request to send (RTS)/clear to send (CTS), and retransmission). All vehicles employ the shared channel to broadcast their beacons through a contention manner referred to as the back-off mechanism. The broadcasting mechanism in dense networks may lead to frequent contention and packet collisions in transmission among neighboring vehicles [6].
In addition, the DSRC standard provides a control channel (CCH) as a shared channel to transmit beacons. Its limited bandwidth (10 MHz) brings limited transmit capacity. In dense traffic conditions as the V2V deployment scales up, the resultant channel load on CCH increases and leads to channel congestion [7]. Meanwhile, dense vehicles will inevitably increase the number of hidden terminals, which aggravates channel congestion and contention. Such congestion might have devastating consequences for network performance; in particular, data transmission reliability will become poorer in congested networks. Safety application reliability may be degraded, possibly endangering the safety of road users. Therefore, reducing channel congestion and providing access to the harmonized and fairness channel among vehicles is necessary.
Traditionally, reliability in the context of VANET broadcast services is defined as the ability for all intended mobile nodes in an area of interest of a network to receive broadcast messages [4], [5]. However, this reliability does not reflect the reliability of the safety application due to the memory-less property of the safety application [8]. In addition, the quality of service (QoS) requirements for each safety application are different. Although some QoS metric is adopted in [9], [10], [11] to evaluate the reliability of safety applications, it is difficult to evaluate whether all safety applications’ QoS requirements are satisfied.
To address the aforementioned channel congestion issues, an adaptive beacon generating rate (ABGR) congestion control mechanism based on vehicle density is proposed in this paper. The main idea of the ABGR mechanism is that the beacon generation rate of the vehicle is adaptively adjusted according to the vehicle density and congestion threshold to reduce the probability of packet collision, thereby effectively controlling channel congestion and improving the reliability of the VANET safety application. The congestion threshold is derived from the analysis result of the effect of vehicle density on QoS requirements for the safety application. Then, the QoS requirements for the three typical safety applications that are believed to have the most stringent QoS requirements are specified and discussed. Based on the correlation between vehicle density and speed in traffic theory, a dynamic application-level reliability assessment scheme (T-Pro) is proposed. Metrics for QoS requirements of safety applications need to change not only for different safety applications but also for the same application as the density changes. The T-Pro scheme will dynamically adjust parameters in metric with changing density. The main contributions of this paper are detailed below:
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An adaptive beacon generation rate (ABGR) congestion control mechanism based on vehicle density is proposed in this paper. The ABGR mechanism can automatically adjust the vehicle beacon generation rate according to the change in vehicle density. It can effectively reduce channel congestion and packet collisions in dense networks, improving system reliability.
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Based on the correlation between road traffic and vehicle speed, a dynamic application-level reliability assessment scheme(T-Pro) is proposed to evaluate application-level reliability of various safety applications with different traffic densities.
The rest of this paper is organized as follows. Section 2 presents related work within the field. Section 3 illustrates the features of the traffic scenario, and an application-level evaluation model is built up in this section. Section 4 introduces the proposed ABGR channel congestion control mechanism and T-Pro application-level reliability assessment scheme. The numerical results and corresponding performance evaluations are discussed in Section 5. Finally, Section 6 provides the conclusion of this paper.
Section snippets
Related work
The development of an optimal congestion control scheme faces many challenges. Congestion control mechanisms for the VANET have been studied in many studies. They usually dynamically adjust one or more transmission parameters, i.e., beacon generating rate, transmission power, and parameters in the CSMA/CA protocol.
Tonguz et al. [12] proposed an adaptive protocol, called Distributed Vehicular Broadcast (DV-CAST), in which a neighbor detection mechanism estimates the local topology using a
Scenario analysis
The performance and reliability of real-world radio networks are influenced by many factors such as concurrent transmission collision, interference from hidden terminals, channel fading and pass loss. To generate a simplified yet reasonable analytical model, several scenario assumptions are made as follows.
Various types of roads, such as highways, expressways and elevated roads, are used in urban and rural environments to create a public road network. A typical multi-lane straight highway
Proposed congestion control approach
In this section, an adaptive beacon generation rate (ABGR) channel congestion control mechanism is first introduced. Based on the vehicle density, the ABGR mechanism adjusts the beacon generation rate to reduce the interference of channel conflicts and improve system reliability. Then, a dynamic reliability evaluation approach(T-Pro) is proposed based on re-consideration of the safety application’s operating characteristics and the relationship between vehicle speed and density.
Simulation description
In this section, we set the most stringent test conditions for simulations, including the highest density of vehicles, farthest intended distance of region of interest (ROI) for the applications, and the maximum length of a beacon. The highway scenario is set as Fig. 1 with three lanes in each direction. Each vehicle in the network is equipped with DSRC capability with the following communication parameters as listed in Table 3. Communication range (transmission/carrier sensing) is .
Conclusion
Designing a congestion control mechanism to make the network safer and more reliable is one of the challenges in VANET. In this paper, we first analyze the influence of the change of communication environment parameters on DSRC communication reliability. The parameters include data transmission rate, beacon generation rate, contention window size, and beacon length. Based on the analysis result, the ABGR mechanism is proposed to improve the reliability of the VANETs safety application. ABGR can
Declaration of Competing Interest
We declare that we have no conflict of interest.
Acknowledgment
This Work was supported by the Project of Shandong Province Higher Educational Science and Technology Program, P. R. China (Grant no. J18KA351), the Natural Science Foundation of Shandong Province-pl2X-sim-(Grant nos. ZR2016AL04, ZR2016FL05, ZR2017MF039 and ZR2012AM021) and the High-Level Training Project of Taishan Medical University (No. 2015GCC07 ).
Wenfeng Li has completed his B.S. degree in Computer Software from Shandong University of Science and Technology and M.E. degree in Computer Application from Jiangxi University of Science and Technology in 1997 and 2007, respectively. He got the Ph.D. degree at Tongji University, Shanghai, China in 2017. Now he is a lecture in the College of Medical Information Engineering, Shandong First Medical University & Shandong Academy of Medical Sciences, Taian, China. His research interests include
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Wenfeng Li has completed his B.S. degree in Computer Software from Shandong University of Science and Technology and M.E. degree in Computer Application from Jiangxi University of Science and Technology in 1997 and 2007, respectively. He got the Ph.D. degree at Tongji University, Shanghai, China in 2017. Now he is a lecture in the College of Medical Information Engineering, Shandong First Medical University & Shandong Academy of Medical Sciences, Taian, China. His research interests include VANET and LTE.
Wuli Song was born in Wenshang, Shandong, P.R. China, in 1978. He received his masters degree from Shandong University of Science and Technology, P.R. China. Now he is studying at China University of Mining and Technology; he is an associate professor of Shandong First Medical University & Shandong Academy of Medical Sciences. His research interest include computational intelligence, information security and big data analysis.
Qiang Lu received the B.S. degree from Qingdao University of Science and Technology, Qingdao, China, Master degree from Shandong University of Science and Technology, Qingdao, China, the Ph.D. degree in control theory and control engineering from Tongji University, Shanghai, China. He is currently a association professor in College of Medical Information Engineering, Shandong First Medical University & Shandong Academy of Medical Sciences, Taian, China (e-mail: [email protected]). His current research interests include intelligent robot control and neural networks.
Chao Yue was born in Shandong, China, in 1971. He is a reviewer for Mathematical Reviews on AMS. He received the Ph.D. degree in mathematics from the Shanghai University, Shanghai, China, in 2015. From September 2015 to July 2016, he was a Visiting Scholar in the Department of mathematics, Tongji University, Shanghai, China. He is now an Associate Professor with the College of Medical Information Engineering, Shandong First Medical University & Shandong Academy of Medical Sciences.