WAVE (Wireless Access in Vehicular Environments)標準提供車間通訊之IEEE 802.11擴充通訊協定以支援智慧型傳輸系統之應用程式。WAVE通訊協定主要應用在改善行車安全以及提昇頻寬的使用率與效能，並降低行車事故的發生機率。WAVE通訊協定之服務頻道(Service Channel, SCH)選擇研究主要探討兩個問題，亦即如何衡量頻道的壅塞程度以及如何選擇合適的頻道來傳送封包。WAVE標準建議服務提供者應選擇壅塞程度較低的服務頻道傳送封包，但並未說明如何衡量服務頻道的壅塞程度。因此，本篇論文提出MDSCH (Minimal Delay on Service Channel Selection)機制，利用所接收的WSA (WAVE Service Advertisement)封包數量以及考慮每一個節點的權重，並使用Markov Chain來計算此服務提供者在每一個服務頻道所產生的預估延遲時間，最後，選擇產生最低預估延遲時間的服務頻道做為該服務提供者實際使用的服務頻道，由數值分析結果可以看出，藉由MDSCH機制可以成功降低服務提供者傳輸封包的平均延遲時間，且同時可避免服務提供者在傳輸封包時會等待過長的延遲時間。

The Wireless Access in Vehicular Environments (WAVE) standard is proposed as an amendment to IEEE 802.11 to support Intelligent Transportation Systems (ITS) applications. It ensures emergency applications can be transmitted on time to prevent traffic accident and then enhance traffic safety and the bandwidth usage. The research for service channel (SCH) selection is mainly focus on two issues, how to evaluate the congested level of SCH and how to choose the optimal SCH to transmit the on-demand applications. The IEEE WAVE standard suggests that each provider chooses the least congested SCH, but it does not specify how to perform. Therefore, we propose a novel channel selection protocol, Minimal Delay on Service Channel Selection (MDSCH). The Markov chain model used in MDSCH takes two parameters into account, the number of WAVE Service Advertisements (WSAs) transmitted already on the control channel (CCH) and the priority of the on-demand data of the providers. By considering these two parameters into Markov chain model, the provider obtains the predicted value of mean delay on each SCH and then chooses the SCH with the lowest predicted value to transmit packets. According to numeric results, MDSCH can reduce the mean delay, and prevent providers from experiencing the long delay.

[1] J. Zhu and S. Roy, “MAC for dedicated short range communications in intelligent transport system,” IEEE Communications Magazine, vol. 41, no. 12, pp. 60–67, Dec. 2003.
[2] D. Rawat, D. Popescu, G. Yan, and S. Olariu, "Enhancing VANET Performance by Joint Adaptation of Transmission Power and Contention Window Size," IEEE Transactions on Parallel and Distributed Systems, no.99, pp.1, Jan. 2011.
[3] J.R. Gallardo, D. Makrakis, and H.T. Mouftah, “Mathematical analysis of EDCA’s performance on the control channel of an IEEE 802.11p WAVE vehicular network,” EURASIP Journal on Wireless Communications and Networking, Vol. 2010, Article ID 489527, 15 pages, Feb. 2010.
[4] “IEEE Standard for Information Technology-Telecommunications and Information Exchange Between Systems-Local and Metropolitan Area Networks-Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications," IEEE Std 802.11-2007 (Revision of IEEE Std 802.11-1999) , pp.C1-1184, Jun. 2007.
[5] G. Bianchi, “Performance analysis of the IEEE 802.11 distributed coordination function,” IEEE Journal on Selected Areas in Communications, vol. 18, no. 3, pp. 535–547, Mar. 2000.
[6] “Draft Standard for Information Technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications - Amendment 7: Wireless Access in Vehicular Environments,” IEEE P802.11pTM/D9.0, Sep. 2009.
[7] "IEEE Standard for Wireless Access in Vehicular Environments (WAVE)--Multi-channel Operation," IEEE Std 1609.4-2010 (Revision of IEEE Std 1609.4-2006), pp.1-89, Feb. 2011.
[8] T. Kim, S. Jung, and S. Lee, "CMMP: Clustering-Based Multi-channel MAC Protocol in VANET," Second International Conference on Computer and Electrical Engineering, vol.1, pp.380-383, Dec. 2009.
[9] C. Campolo, A. Cortese, and A. Molinaro, "CRaSCH: A cooperative scheme for service channel reservation in 802.11p/WAVE vehicular ad hoc networks," International Conference on Ultra Modern Telecommunications & Workshops, pp.1-8, Oct. 2009.
[10] Q. Wang, S. Leng, H. Fu, Y. Zhang, and H. Weerasinghe, "An Enhanced Multi-Channel MAC for the IEEE 1609.4 Based Vehicular Ad Hoc Networks," IEEE Conference on Computer Communications Workshops, pp.1-2, Mar. 2010.
[11] R. K. Lam and P. R. Kumar, "Dynamic Channel Reservation to Enhance Channel Access by Exploiting Structure of Vehicular Networks," IEEE 71st Vehicular Technology Conference, VTC 2010-Spring, pp.1-5, May 2010.
[12] M. R. Sheldon, Introduction to Probability Models, USA, Orlando, FL, 2006.
[13] P.E. Engelstad and O.N. Osterbo, "Delay and Throughput Analysis of IEEE 802.11e EDCA with Starvation Prediction," IEEE Conference on Local Computer Networks 30th Anniversary, pp.647-655, Nov. 2005.
[14] B. Xiang, Y.M. Mao, and J. Xie, "Performance Investigation of IEEE802.11e EDCA under Non-saturation Condition based on the M/G/1/K Model," 2nd IEEE Conference on Industrial Electronics and Applications, pp.298-304, May 2007.
[15] J.S. Vardakas and M.D. Logothetis, "End-to-end delay analysis of the IEEE 802.11e with MMPP input-traffic," International Symposium on Autonomous Decentralized Systems, pp.1-6, Mar. 2009.
[16] J. J. Blum, A. Eskandarian, and L. J. Hoffman, “Challenges of intervehicle ad hoc networks,” IEEE Transactions on Intelligent Transportation Systems, vol. 5, no. 4, pp. 347–351, Dec. 2004.
[17] S. Grafling, P. Mahonen, and J. Riihijarvi, "Performance evaluation of IEEE 1609 WAVE and IEEE 802.11p for vehicular communications," Second International Conference on Ubiquitous and Future Networks, pp.344-348, Jun. 2010.