Congestion Control Algorithm for Disseminating Uni-Priority Safety

Transcription

1 Congestion Control Algorithm for Disseminating Uni-Priority Safety Messages in VANETs 1 Mohamad Yusof Bin Darus, 2 Kamalrulnizam Abu Bakar 1, Universiti Teknologi Malaysia (UTM), yusof@tmsk.uitm.edu.my *2, Universiti Teknologi Malaysia (UTM), knizam@utm.my Abstract VANETs (vehicular ad hoc networks) perform crucial functions in road safety, detection of traffic accidents and reduction of traffic congestions. Many of safety applications built for VANETs require real-time communication with high reliability. In dense network, a large number of vehicles transmit the beacon messages at a high frequency and event-driven safety messages are broadcast multiple times. When the traffic density is above a certain value, the problem is the choking of the shared medium by an excessive number of the same safety broadcast messages. This problem is called channel congestion, which is caused by the heavy traffic load on CCH communication channel. The CCH communication channel must free from congestion in order to ensure timely and reliable delivery of event-driven safety messages. Many of studies suggest that congestion control algorithm is one of the solutions to control the load and congestion in VANETs. The main objective of congestion control algorithm is to prevent sustained overloads of network nodes and links. However, most of congestion control algorithms are not really applicable to event-driven safety messages. The event-driven safety applications require real-time communication with high reliability and must be delivered before certain time deadline. Although certain congestion control algorithms have been investigated, there is still a lack of understanding on the concepts congestion for uni-priority of event-driven safety messages. The uni-priority congestion is caused by the traffic of the same priority, typically the warning messages of safety applications from different transmitters. In this research, we propose congestion control algorithm for disseminating of uni-priority of event-driven safety messages. In this context, we explore and design congestion control algorithm for uni-priority of event-driven safety messages. The effectiveness of the proposed congestion control algorithm is evaluated through the simulations. Keywords: VANETs, IEEE p, CSMA/CA, Congestion Control, Safety Messages. 1. Introduction VANETs are composed of vehicles equipped with advanced wireless communication devices without any base stations. Each vehicle equipped with VANETs device will be a node in the adhoc network and can receive and relay others messages through the wireless network. This type of networks can provide wide variety of services such as ITS such as safety applications [5] [6]. The safety applications play a crucial role in the transportation system, which enhances the safety of traffic systems [14]. Basically, we can divide safety applications in two (2) categories; periodic (beacon) and event-driven safety messages. The periodic safety message exchange is preventive in nature, and its objective is to avoid the occurrence of dangerous situations. The periodic safety message may contain information regarding the position, direction, and speed of vehicles. The event-driven safety message may be generated as a result of a dangerous situation or when an abnormal condition is detected such as road accident [5] [6]. The event-driven safety messages disseminated within a certain area with high priority. The event-driven safety messages require low latency and reliable deliver. Both of these safety messages will send through one single channel kwon as Control Channel (CCH). The Federal Communications Commissions (FCC) has allocated the frequency spectrum between and GHz for Dedicated Short Range Communication (DSRC) in VANETs. The DSRC spectrum is divided into seven (7) 10MHz channels range from 3 to 27 Mbps. The central channel (channel 178) is the control channel (CCH)[10], which is restricted to safety communications only, as shown in Figure 1. International Journal of Digital Content Technology and its Applications(JDCTA) Volume7,Number7,April 2013 doi: /jdcta.vol7.issue7.5 41

2 Figure 1. DSRC Channel Arrangement for IEEE p In VANETs, vehicles are interested in the same kind of information for example information about any accident, road block, parking, and fuel station or weather situation of particular route. So the broadcast is frequently used for information sharing [13]. In high traffic, a large number of vehicles broadcast beacon safety messages at a high frequency or event-driven messages are broadcast multiple times; the communication channel will easily get congested. This situation will decrease a throughput while delay is increasing significantly. On the other hand, the simplest way of broadcasting beacon safety messages to all nodes in VANETs is by using blind flooding. Each node that receives the packets will rebroadcast this packet and will lead the broadcast storm. The IEEE p MAC protocol has been as the standard of protocol for the VANETs. It is also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol [3] [6] [7] [11]. Because of the shared wireless medium, blindly broadcasting the packet may lead to frequent contention and collisions in transmission among neighboring nodes. Moreover, a node can experience very long channel access delays due to the risk of the channel being busy during its listening period messages [2]. This will affect the reliability and performance of safety applications in VANETs. This paper aims to develop congestion control algorithm to provide reliability for disseminating event-driven safety messages. This paper also focuses on uni-priority of event-driven safety messages. Finally, the performance of congestion control algorithm will evaluate based on warning packet delay and average waiting times. The propose congestion control algorithm will improve the network performance such as the reliable delivery and delay for safety applications and maintain performances of comfort applications. 2. Related Works There have been several works addressing the congestion problem in VANET. In research [1], they developed a congestion control approach based on the concept of dynamic priorities-based scheduling. They evaluated dynamic priority factor based on: node speed consideration, message utility consideration and message validity consideration. This approach requires context exchange between neighbour nodes, which generates a communication overhead. On other hand, the congestion control algorithm for event-driven safety messages is proposed in [9]. This congestion control evaluated the performance of the Safety Electronic Brake Light with Forwarding (EEBL-F). Research in [9] set the predefined threshold in their congestion control algorithm based on channel usage level. Each device periodically senses the channel usage level, and detects the congestion whenever the measured channel usage level exceeds the predefined threshold. Measuring the channel usage level is too difficult to analyse under realistic environment due of the different traffic load. In addition, this research fails to notice important issue which is uni-priority messages. In a similar study in [4], they proposed congestion control algorithm for DSRC based on safety applications. However, they just assumed the CCH channel is successfully reserved for event-driven applications without testing the successfully rate for event-driven safety messages. In research [4], they set the channel occupancy time as threshold. If channel occupancy time measured at a node in CCH interval is longer than a given threshold, all beacon safety messages will be blocked immediately in the remainder of that CCH interval and the CCH interval followed to reduce channel load and reserve space for event-driven safety messages. Measuring the channel with the channel occupancy time is too difficult to analyse needs the proper rate control design. 42

3 3. The Congestion Control Algorithm We propose congestion control algorithm to provide reliability of disseminating event-driven safety messages. The propose congestion control algorithm focusing on uni-priority issues of event-driven safety messages. The propose congestion control algorithm can be divided into two main parts: A. Congestion Detection and Congestion Control. The purpose of the congestion detection is to monitor CCH channel and detect congestions. Two kinds of congestion detection methods in our congestion control algorithm are measurement-based detection and event-driven detection. The measurement-based congestion detection will monitor CCH channel based on packets channel queue. The CCH channel is congested if the number of messages in the packet queue exceeds a defined threshold. Based on research [5] is concluded that a packet queue with a length of five beacon messages is sufficient to be used for p beaconing. The proposed congestion control algorithm will discard further a beacon safety messages whenever the length of packet queue more than five beacon safety messages. The flowchart steps for proposed congestion control algorithm are demonstrated in Figure. 2. Figure 2. Flowchart steps of the proposed congestion control algorithm The event-driven detection method monitors the event-driven safety message and decides to start the congestion control algorithm whenever event-driven safety message is detected or generated. The congestion control will launch immediately the queue freezing method for all MAC transmission queues except for the event-driven safety message. In order to send event-driven safety message with the minimum delay, the lower priority messages such as beacon messages emission is freeze. Currently, the event-driven detection method has been used in the existing of congestion control algorithm [9]. 43

4 B. Scheduling In this research, we adopt priority-based Earliest Deadline First (EDF) scheduling algorithms to schedule uni-priority of event-driven safety messages. The EDF scheduling is one of the classic scheduling for real-time systems based on their deadline. The EDF based approach is one of the best approaches introduced for rule scheduling till now [12]. In VANETs, each packet has an assigned priority and deadline (maximum latency). The scheduler simply transmits packets in the order of increasing remaining deadlines and priority-based EDF scheduling algorithms serve the packets according to their priorities and deadline. The proposed priority-based EDF scheduling is defined in Eq. 1 and Eq. 2. where Pq is a packet queued to serve. Packet Queue (Pq)=Priority (P) + Deadline (D) (1) Deadline (D)=Packet Arrival (Pt) + Max Latency (Mx) (2) The proposed congestion control will be expressed with pseudo code below: /* Congestion Detection and Control */ If (event-driven safety message is locally generated) or (eventdriven safety message detected) Freeze all MAC queues except for the event-driven safety message Else If (Packet Queue > 5) Discard extra incoming beacon messages in CCH channel Else If (event-driven messages detected >1) Freeze all MAC queues except for the event-driven safety messages queue based on priority-based EDF scheduling 4. Simulation Results We examine the proposed congestion control algorithm through simulation experiments using Veins simulator. The Veins simulator make up of two distinct simulators, OMNeT++ for network simulation and SUMO for road traffic simulation. The simulation scenario is a 1500m x 1500m area of Los Angeles, extracted from the TIGER/Line database of the US Census Bureau. The simulation parameters are summarized in Table 1. 44

5 Table 1. Simulation parameters Parameter Value Simulation area 1500 m x 1500 m Number of vehicles 350 Packet type UDP Node Speed km per hour Transmission range 400 m Simulation time 300 s Data packet size 512 bytes MAC protocol IEEE p In this research, the experiments are conducted in dense networks (350 vehicles) to show the effect of high traffic density for effectiveness of transferring event-driven safety messages. The figure 3 illustrated the warning packets delay for event-driven safety messages in high density traffic. The blue line graph is showed the performance of event-driven safety messages for the proposed congestion control while the red line graph is for without congestion control algorithm. These experiments showed that the number of vehicles is crucial and this factor has an impact on overall network performance. The result simulation proved whenever the number of vehicles is increased; the packets delay consistently increased. In dense network, the result showed that the performance of proposed congestion control is excellent compared to without implementation of congestion control. The maximum warning packet delay only 37 ms in our proposed congestion control compared 80 ms without congestion control. The worst cases scenarios is 60 ms delays for safety applications. Figure 3. Warning Packet Delay in Dense Networks In this research, we also studying the uni-priority of event-driven safety messages in dense networks. The uni-priority congestion is caused by the traffic of the same priority, typically the warning messages of safety applications from different transmitters. In this study, we evaluate the performance of priority-based EDF scheduling algorithm in our congestion control algorithm. We assumed three event-driven safety messages are exploited which are lane change warning, forward collision and pre-crash sensing event-driven safety packages. In this experiments, the Veins simulator is used to validae our proposed priority-based EDF scheduling in our congestion control algorithm. We are used the same simulation scenario, map and parameters for evaluated congestion control in previous expriments. The figure 4 and figure 5 shows the performance of event-driven safety messages in the high density scenario with FIFO scheduling and with our priority-based EDF scheduling. 45

6 Figure 4. Average Waiting Time in FIFO Scheduling Figure 5. Average Waiting Time in Priority-Based EDF Scheduling The results show that the packets queues in scheduler with FIFO scheduling technique in scheduler are lane change warning, forward collision and pre-crash sensing. This result proved the average waiting time for pre-crash sensing using the proposed scheduling algorithm is about 24 ms shorter than that using FIFO scheduling. However, the average waiting times for lane change warning and forward collision packets is increased dramatically is about 12 ms for lane change warning and 10 ms for forward collision. Although, the proposed priority-based EDF scheduling reduces waiting time for the most critical event-driven safety packet which is pre-crash sensing. The average waiting time for lane change warning and forward collision packets still relevant for uni-priority packets in dense network. After extensive and fair simulation results show that, compared to the representatives of scheduling algorithm, the proposed scheduling algorithm performs the best in terms of average waiting time packet delivery ratio. These results confirmed that the priority-based EDF scheduling algorithm with adoption in the proposed congestion control algorithm is one of the best solutions for disseminating uni-priority of event-driven safety applications 5. Conclusion In this paper, we studied the congestion problem in VANET in favor of event-driven safety messages. We proposed algorithm for solving the congestion problem in VANET focused on unipriority of event-driven safety messages. The proposed congestion control and priority-based EDF 46

7 scheduling has been proved to an efficient congestion control scheme for the event-driven safety messages. As the next step of this work, we would like to integrate our proposal with the efficient transmit power control to maximize energy consumption and connectivity for point-to-point communications. 6. References [1] M.S Bouassida, and M. Shawky, On the congestion control within VANET, 1st IFIP Wireless Days. pp. 1-5, [2] K. Bilstrup, E. Uhlemann, E.G. Strom, and U. Bilstrup, Evaluation of the IEEE p MAC method for vehicle-to-vehicle communication, In Proc. IEEE 68th Vehicular Technology Conference.pp. 1-5, [3] C. Campolo, A. Cortese, and A. Molinaro, CRaSCH A cooperative scheme for service channel reservation in p/WAVE vehicular ad hoc networks, In Proc. International Conference on Ultra Modern Telecommunications & Workshops. pp. 1-8, [4] J. He, H.C. Chen, et al. Adaptive congestion control for DSRC vehicle networks, IEEE Communications Letters, Vol. 14, No. 2, [5] L. Hendriks, Effects of transmission queue size, buffer and scheduling mechanisms on the IEEE p beaconing performance, 15th Twente Student Conference, University of Twente, Faculty of Electrical Engineering, Mathematics and Computer Science, [6] M. Li, W. Lou and K. Zeng, OppCast: Opportunistic broadcast of warning messages in VANETs with unreliable links, In Proc. IEEE 6th International Conference (MASS '09). pp , [7] Y. Wang, A. Ahmed, B. Krishnamachari, and K. Psounis, IEEE p performance evaluation and protocol enhancement In Proc. of the IEEE International Conference on Vehicular Electronics and Safety Columbus, OH, USA. September 22-24, [8] L. Wischhof, and H. Rohling, Congestion control in vehicular ad hoc networks, IEEE 2005 [9] Y.P. Zang, L. Stibor, X. Cheng, H. J. Reumerman, A. Paruzel and A. Barroso, Congestion control in wireless networks for vehicular safety applications, In Proceedings of The 8th European Wireless Conference, [10] W. Zhang, A. Festag, R. Baldessari, and L. Le, Congestion control for safety messages in vanets: concepts and framework, In Proceedings of The 8th European Wireless Conference, p. 7, Paris, France, [11] S. Ullah, H. Higgins, Y.W. Cho, H.S. Lee and K.S. Kwak, Towards RF Communication and Multiple Access Protocols in a Body Sensor Network, International Journal of Digital Content Technology and its Application, vol. 2, no 3, pp. 9-16, [12] C.Y. Yang and S.C. Lo, Street Broadcast with Smart Relay for Emergency Messages in VANET, In Proceeding of 24th International Conference on Advanced Information Networking and Applications Workshops, pp , [13] G. Cui, L. Fan, G. Wang, Y. Li, J. Fan, Y. Zhang and W. Hu, A New Trusted Routing Scheme In Vanets International Journal of Advancements in Computing Technology(IJACT), Vol. 4, No.10, (pp ), 2012 [14] Z. Mi, L. Wang, and H. Wang, An information classification framework to support effective emergency data dissemination in VANETs with the aid of RSUs International Journal of Advancements in Computing Technology (IJACT), Vol. 4, No. 20, (pp ),