Proceedings of the 8th International IEEE Conference on Intelligent Transportation Systems Vienna, Austria, September 13-16, 2005
WA5.5
CSMA based Inter-Vehicle Communication Using Distributed and Polling Coordination Su Yang, Student Member, IEEE, Hazem H. Refai, and Xiaomin Ma, Member, IEEE Abstract-- Intelligent transportation systems rely heavily on inter-vehicle communication (IVC) systems to support, among other things, safe driving by relaying essential information in real time. It is very important to guarantee the delivery of an imminent collision warning in real time in order to make appropriate actions to avoid it. Several existing MAC layer protocols are modified to address real-time traffic in ad hoc vehicle environment; carrier sense multiple access (CSMA) is one such protocol. This paper implements a priority in CSMA using different backoff time spacing (BTS) to allow higher priority traffic to access the medium faster than those with medium-to-low priority. The backoff time spacing is inversely proportional to the priority; the higher the priority, the lower the backoff time. The paper also proposes a new media access control (MAC) protocol implementation that combines distributed and polling coordination functions to further enhance the BTS-priority-based CSMA and to ensure the absolute delivery of traffic with the highest priority. A computer simulation was carried out to evaluate the performance of the proposed protocol. When compared with the BTS-based CSMA implementation, the simulation results indicate significant improvements in the system throughput and successful packet rate for the traffic with highest priority.
Traffic transmitted by vehicles is prioritized based on the closeness of the transmitting vehicle to a receiving vehicle and on the message type: emergency message that rquires immediate access to the medium or general message that doesn’t require it. If the message type is emergency message such as collision warning and the transmitting vehicle is close to the receiving one, the traffic is assigned the highest priority. The protocol guarantees that the transmitting vehicle has access to the medium. The developed protocol is called carrier sense multiple access with priority and polling (PP-CSMA). This paper is arranged as follows. Section II describes the previous work of contention-based CSMA protocol developed for IVC systems. Section III presents the proposed PP-CSMA protocol. Section IV shows the performance evaluation and comparison of the proposed PPCSMA to the CSMA protocol with only a priority scheme (P-CSMA) using computer simulation. Finally, Section V presents the conclusion. II. PREVIOUS WORK
I.
INTRODUCTION
I
nter-vehicle communication is an integral part of any intelligent transportation system (ITS). It facilitates communication among vehicles to promote efficient and safe driving. Hence MAC protocols implemented in an IVC environment are required to promptly handle timecritical traffic such as collision warnings and also to absolutely guarantee the delivery of such traffic. The currently existing IVC MAC protocols [1-11] can be classified into two categories: contention-based (distributed coordination) schemes [1-5] and contention-free (polling) schemes [6-10]. In this paper, we propose a new protocol that combines both schemes into a new one. The proposed protocol incorporates priority and polling schemes based on a vehicles’ positions with respect to each other. Su Yang is a Ph.D. student with the Department of Electrical and computer Engineering, University of Oklahoma-Tulsa, Tulsa, OK 74135 USA, (e-mail:
[email protected]). Hazem H. Refai is an Assistant Professor with the Department of Electrical and Computer Engineering, University of Oklahoma-Tulsa, OK 74133, USA (phone: 918-660-3243; fax: 918-660-3238; e-mail:
[email protected]). Xiaomin Ma is an Assistant Professor with the Department of Engineering & Physics, Oral Roberts University, Tulsa, OK 74171, USA (email:
[email protected]).
0-7803-9215-9/05/$20.00 ©2005 IEEE.
In a contention-based IVC system, all vehicles share the same wireless transmission media. A vehicle uses a certain access methodology to compete with other vehicles in order to get access to the media. This scheme provides superior average delay performance while it suffers a packet collision problem, particularly at high traffic loads which adversely affect the performance of the system throughput and packet success rate. To solve for this problem, many protocols implement collision detection and collision avoidance schemes. In addition, priority implementation is an important factor in designing a real-time IVC system. Several studies depicting priority implementations for real-time data transmission using different MAC protocols are discussed in [12-20], some of which are contention-based [12-17] while others are contention-free schemes [18-20]. The following two subsections briefly describe the basic CSMA protocol implementation and BTS-based priority implementation, respectively. Both form the foundation of this paper’s information. A. CSMA Scheme Carrier sense multiple access (CSMA) is the most frequently implemented media access protocol in IVC [1-4]. It is also the foundation of the later 802.11 WLAN MAC protocol development.
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In CSMA, each vehicle senses the condition of the medium (wireless channel) before transmission. If the channel is busy, the vehicle waits a random backoff time and senses the channel again. The vehicle repeats the procedure until the channel is free of data transmissions. When multiple vehicles simultaneously transmit their data packets, collision occurs at the receiver. The performance of the CSMA degrades as packet collisions increases. B. Priority Scheme Using Random Backoff Time Recently, several research studies have been conducted to design an effective priority scheme for the 802.11 WLAN MAC protocol [15-18]. As a result, the inter-frame spacing (IFS) and the backoff time window were developed [15-17]. The basic concept of both approaches is to use shorter IFS or backoff time spacing (BTS) for high priority traffic, thus allowing this traffic to access the medium faster than low priority terminals. However, neither approach guarantees that high priority traffic will always have access before traffic with low priority. III.
THE PP-CSMA PROTOCOL
This section describes assigning priority using backoff time spacing. It also describes the polling methodology to guarantee that high priority traffic will always have access to the medium faster than low priority traffic. A. CSMA with Backoff-based Priority (P-CSMA) This protocol was derived from the basic non-persistent CSMA protocol. When a packet collision occurred, this protocol computed different backoff time spacing to allow for different priority levels. Table 1 shows four different priority levels that were determined based on the proximity of the transmitting vehicle to the receiving one and the type of message transmitted. Figure 1 shows the corresponding backoff time spacing. The high priority traffic backs off the least amount of time as depicted in Figure 1. TABLE I PRIORITY SCHEME WITH 4 LEVELS Priority Level Level 0 Level 1 Level 2 Level 3
Comments Vehicle range: far , message type: general Vehicle range: medium, message type: general Vehicle range: low, message type: general Vehicle range: close, message type: emergency
Figure 1. Priority scheme using backoff time spacing
The backoff time spacing (BTS) was computed using (1). ª rand (3 i ) º Backoff _ Time _ Spacing « » u T Backoff (1) 4 ¬ ¼ where rand is a uniformly distributed random number between [0,1) , i [0,1,2,3] is the priority level of the data packet with i 3 being the highest priority, and T Backoff is the total size of the backoff time window. B. CSMA with Priority and Polling (PP-CSMA) In addition to the BTS-priority-based implementation (PCSMA), PP-CSMA implements a polling scheme in which the receiving vehicle polls only vehicles with emergency message and within a close proximity. If a polled vehicle’s data packet with a message type of collision warning is ready for transmission, the vehicle generates a tone indicating that state; otherwise, it generates no tone. The generated tone is out of the frequency band used for data transmission. Upon receiving the tone, the receiving vehicle clears it to transmit the data packet. Each vehicle maintains a polling table that holds other vehicles’ positional information relative to it. The table is populated based on only the proximity of the transmitting vehicle relative to the receiving one. A vehicle is assigned a high priority if and only if it exists in the polling table of the intended receiving vehicle and the message type it desires to transmit is emergency message such as collision warning. The table is updated periodically. Vehicles periodically broadcast their respective GPS coordinates. If none of the polled vehicles generate a tone, the medium is released for vehicles with lower priority levels. The developed PP-CSMA guarantees vehicles with the highest priority level to access the medium faster than its counterparts with lower priority levels. To illustrate the PP-CSMA protocol, Figure 2 shows an example scenario. It is assumed that eight transmitting vehicles (TXn) are within coverage and that they all have communicated at least once with the receiving vehicle (RX). The priority level for each vehicle is displayed next to the transmitting vehicle identifier. For example, vehicles TX7 and TX8 have a priority level of 3. First, vehicle TX1 senses the channel and finds it free, so TX1 successfully transmits its data packet. After having successfully received the data packet, vehicle RX examines its polling table, which contains close proximity vehicles TX7 and TX8, and finds that there is no packet ready for transmission. Hence, vehicle RX continues working in contention-based mode. Second, after sensing the wireless channel, vehicles TX3 and TX5 transmit their packets. A packet collision occurs, and the two vehicles delay their data packets using a backoff time according to (1). Since TX5 has a higher priority level (Level 2) than TX3 (Level 1) has, it has a shorter backoff time. After its backoff time expires, TX5 senses the channel again and finds it free. So TX5 successfully transmits its data packet. This description illustrates the basic P-CSMA
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protocol. During the transmission of TX5, TX7 has a data packet ready for transmission. It senses the channel and finds that TX5 is using the channel; therefore, it delays its transmission. After TX5 successfully transmits its packet, vehicle RX examines its polling table, listens to the BBs, and finds that vehicle TX7 has a data packet ready. Vehicle RX then clears TX7 for transmission; TX7 successfully transmits its packet. The transmission of TX3’s data packet is further delayed by another random backoff time. After another successful transmission, RX polls for possible high priority data transmissions.
TX8, TX7 is cleared to transmit. The functional flow chart of the proposed PP-CSMA protocol is shown in Figure 3. IV. PERFORMANCE EVALUATION Computer simulation was carried out to analyze the performance of a BTS-priority-based IVC system and BTS with a polling-based system. The simulation implements modified non-persistent CSMA. The overall system throughput and packet success rate are the two performance parameters that were analyzed by the simulation. The following two subsections depict the results of the simulation.
Figure 3. Functional flowchart of PP-CSMA TABLE II SIMULATION PARAMETERS Parameter
Figure 2. Typical scenario of the PP-CSMA protocol
Third, after sensing the channel, TX7 and TX8 transmit their packets at the same time. A packet collision occurs, and the two vehicles delay their packet transmissions using a backoff time determined by their priorities. While TX7 and TX8 are waiting for their backoff time to expire, TX3 successfully transmits its packet after sensing the channel. After TX3’s successful transmission, RX examines its polling table and finds that two vehicles (TX7 and TX8) with the same priority levels have their packets ready for transmission. RX clears the closer vehicle for transmission. In this example, the closer vehicle is TX8. However, if the two vehicles are at an equal distance to the receiving vehicle, the vehicle that cleared for transmission will be selected randomly. Following the successful transmission of
Number of Priority Levels Normalized Propagation Delay (d) Total Number of Vehicles Number of Level 0 Vehicles Number of Level 1 Vehicles Number of Level 2 Vehicles Number of Level 3 Vehicles
Value 4 0.01 100 25 25 25 25
A. Analysis of Throughput The simulation results of system throughput for the PCSMA and PP-CSMA are shown in Figures 4 and 5, respectively. In Figure 4, vehicles with different priority levels acquired different throughput capacities. The highest priority (Level 3) obtained the largest portion of the total available throughput, and the lowest priority (Level 0) acquired the smallest portion. Since non-persistent CSMA
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is the foundation of the developed protocol, the total system throughput is represented by the following theoretical expression [21]. Ge dG (2) Throughput CSMA G (1 2d ) e dG where d is the normalized propagation delay which was set to be 0.01 in the simulation and G was the offered traffic. Figure 4 shows that the implementation of a priority scheme based on backoff time spacing with no polling was not enough to guarantee the delivery of high priority traffic. As a matter of fact, the high priority traffic was adversely affected the most when more traffic was offered to the network. Figure 5, on the other hand, shows that as more traffic was offered to the network, the high priority traffic dominated the network, and the lower priority traffic ceased to exist. Hence the wireless network transformed from being contention-based (distributed coordination) to contentionfree (polling-based coordination). As a result, the system throughput capacity improved over the theoretical one that was calculated for contention-based protocol.
B. Analysis of Packet Success Rate The simulation results of a packet success rate for the PCSMA and PP-CSMA are shown in Figures 6 and 7, respectively. Generally, the packet successful rate decreased when the offered traffic G increased for both P-CSMA and PPCSMA. However, Figure 7 shows that the high priority traffic successful data rate was affected the least among the different traffic priority classes, thus increasing the probability of high priority packet delivery. The reason for the general decrease of rate was that the system capacity was limited by the physical layer implementation. To increase the system capacity, improvements in the physical layer implementation were required.
Figure 6. Packet successful rate of P-CSMA system
Figure 4. Throughput of P-CSMA system
Figure 7. Packet successful rate of PP-CSMA
Figure 5. Throughput of PP-CSMA system
It is clear that the PP-CSMA implementation favored the traffic with the highest priority and penalized the traffic with lower priority. Hence, a vehicle with information on 170
imminent collision had immediate access to the medium to transmit the information, regardless of the offered traffic.
[11]
V. CONCLUSION This paper proposed a new MAC layer protocol based on the implementation of backoff time spacing and polling schemes to guarantee the delivery of high priority traffic which, is in our case, was reserved for collision waning messages. Vehicles periodically broadcast their respective positions in order to update their polling tables. The priority level of any traffic transmission was determined by the transmitting vehicle position relative to the receiving one and the message type. An out-of-band tone was used to inform the polling receiver of an emergency message ready for transmission. The new MAC implementation combined the distributed coordination function with point coordination function to provide the traffic with high priority faster access to the medium. Computer simulation was carried out to analyze the network throughput and data packet success rate. The results of the simulation showed improvements of the traffic with high priority levels while degrading the performance of the lower priority traffic.
[12] [13] [14]
[15] [16]
[17] [18]
[19]
REFERENCES [1]
K. Fujimura, T. Hasegawa, “Performance Comparison of Contention Schemes for Inter-Vehicle Communications”, The IEEE 5th International Conference on Intelligent Transportation Systems, pp. 342-346, Singapore, September 2002. [2] K. Tokuda, M. Akiyama, H. Fujii, “DOLPHIN for Inter-Vehicle Communications System”, Proceedings of the IEEE Intelligent Vehicles Symposium, pp. 504-509, Dearborn (MI), USA, October 2000. [3] T. Nagaosa, Y. Kobayashi, K. Mori, H. Kobayashi, “An Advanced CSMA Inter-vehicle Communication System Using Packet Transmission Time Decided by the Vehicle Position”, 2004 IEEE Intelligent Vehicles Symposium, pp. 111-114, Parma, Italy, June 2004. [4] J. Zhu, S. Roy, “MAC for Dedicated Short Range Communications in Intelligent Transport System”, IEEE Communications Magazine, vol. 41, issue 12, pp. 60-67, December 2003. [5] A. Pal, A. Dolgan, F. Ozguner, U. Ozguner, “A MAC Layer Protocol for Real-time Inter-vehicle Communication”, The IEEE 5th International Conference on Intelligent Transportation Systems, pp. 353-358, Singapore, September 2002. [6] Y. Kim, M. Nakagawa, “R-ALOHA Protocol for SS Inter-Vehicle Communication Network using Head Spacing Information”, Proceedings of the IEEE Intelligent Vehicles Symposium, pp. 278-283, September 1996. [7] Y. Inoue, M. Nakagawa, “MAC Protocol for Inter-Vehicle Communication Network Using Spread Spectrum Technique”, Proceedings of Vehicle Navigation and Information Systems Conference, pp. 149-152, September 1994. [8] M. Rudack, M. Meincke, K. Jobmann, M. Lott, “On Traffic Dynamical Aspects of Inter Vehicle Communications (IVC)”, IEEE 58th Vehicular Technology Conference (VTC 2003-Fall), vol. 5, pp. 3368-3372, October 2003. [9] F. Borgonovo, L. Campelli, M. Cesana, L. Coletti, “MAC for Ad-hoc Inter-vehicle Network: Services and Performance”, IEEE 58th Vehicular Technology Conference (VTC 2003-Fall), vol. 5, pp. 27892793, October 2003. [10] T. Nagaosa, T. Hasefawa, “A New Scheme of Nearby Vehicles’ Positions Recognition and Inter-vehicle Communication without Signal Collision – V-PEACE Scheme”, Proceedings of IEEE 51st
[20]
[21]
171
Vehicular Technology Conference (VTC 2000-Spring), vol. 2. pp. 1616-1620, Tokyo, Japan, May 2000. A. Ebner, H. Rohling, L. Wischhof, R. Halfmann, M. Lott, “Performance of UTRA TDD Ad Hoc and IEEE 802.11b in Vehicular Environments”, IEEE 57th Vehicular Technology Conference (VTC 2003-Spring), vol. 2, pp. 960-964, April 2003. J. Yin, Q. Zeng, D. Agrawal, “A Novel Priority Based Scheduling Scheme for Ad Hoc Networks”, IEEE 58th Vehicular Technology Conference (VTC 2003-Fall), vol. 3, pp. 1637-1641, October 2003. M. Markowski, A. Sethi, “Fully Distributed Wireless MAC Transmission of Real-Time Data”, Proceedings of IEEE 4thReal-Time Technology and Applications Symposium, pp. 49-57, June 1998. J. Sobrinbo, A. Krishnakumar, “Quality-of-Service in Ad Hoc Carrier Sense Multiple Access Wireless Networks”, IEEE Journal on Selected Areas in Communications, vol. 17, no. 8, pp. 1353-1368, August 1999. D. Deng, R. Chang, “A Priority Scheme for IEEE 802.11 DCF Access Method”, IEICE Transactions on Communications, vol. E82-B, no. 1, pp. 96-102, January 1999. C. Chou, K. Shin, “Inter-Frame Space (IFS) Based Service Differentiation for IEEE 802.11 Wireless LANs”, IEEE 58th Vehicular Technology Conference (VTC 2003-Fall), vol. 3, pp. 1412-1416, October 2003. Y. Xiao, “IEEE 802.11e: QoS Provisioning at the MAC Layer”, IEEE Wireless Communications, vol. 11, issue 3, pp. 72-79, June 2004. A. Kanjanavapastit, B. Landfeldt, “An Analysis of a Modified Point Coordination Function in IEEE 802.11”, Proceedings of IEEE 14th Personal, Indoor and Mobile Radio Communications (PIMRC 2003), vol. 2, pp. 1732-1736, September 2003. C. Ahn, C. Kang, Y. Cho, “Soft Reservation Multiple Access with Priority Assignment (SRMA/PA): A Novel MAC Protocol for QoSGuaranteed Integrated Services in Mobile Ad-Hoc Networks”, IEEE 52nd Vehicular Technology Conference (VTC 2000-Fall), vol. 2, pp. 942-947, September 2000. R. Davies, R. Watson, A. Munro, M. Barton, “Ad-Hoc Wireless Networking: Contention Free Multiple Access Using Token Passing”, IEEE 45th Vehicular Technology Conference (VTC 1995), vol. 1, pp. 361-365, October 1995. K. Pahlavan, Principles of Wireless Networks – A Unified Approach, Prentice Hall PTR, NJ 2002.
