The MAC of newly emerged vehicular communication systems is based on the IEEE 802.11 DCF. While DCF has been extensively studied in the stationary indoor environment (e.g., WLAN), the performance of DCF in the highly mobile vehicular environment is still unclear. As is well known, the SNR decreases as the distance to BS increases. Thus, the data rate should decrease as the distance to BS increases. As an example, IEEE802.11b defines 4 different date rates, i.e., 1, 2, 5.5, and 11 Mbps, for different SNRs. With multi-rate transmissions, the system throughput is throttled by the lowest data rate such that the performance is seriously degraded. On addressing this issue, we study the following two models: the constant-window-size model and the varying-window-size model. Furthermore, the vehicular communication system is expected to provide real-time services, e.g., VoIP, as well as non-real-time services, e.g., web surfing. Therefore, we study the mixed model with the prioritized MAC, where the high-priority real-time nodes use a constant window size, and the low-priority non-real-time nodes use varying window sizes. Based on the embedded Markov chain, we derive the analytical method to calculate the performance measures of interest, e.g., nodal throughput, collision probability, and system throughput. It is shown that the constant-window-size model can achieve a constant throughput but usually a lower one, whereas the varying-window-size model cannot achieve a constant throughput but usually a larger one. It is also shown that with the proposed prioritized MAC, the high-priority node can maintain a constant throughput regardless of its location, whereas the low-priority node can achieve a better system throughput. Last but not least, the analytical results are validated by the simulation results, where the simulation programs are written in C. The analytical results are reasonably close to the simulation results.

最近新興的車輛通訊系統網路之MAC層是以IEEE 802.11的DCF機制為基礎。雖然DCF在靜態的室內環境中已廣泛地被研究(如WLAN)，其效能在高度移動性的車輛環境下仍然不甚明朗。眾所皆知，隨著車輛與基地台的距離上升，SNR比值會下降，因此資料傳輸率也會隨之降低。舉例來說，IEEE802.11b為不同的SNR情況定義了四種資料傳輸率，分別為1，2，5.5，11Mbps。在多重速率的傳輸下，系統的成功送達率會為最低的資料傳輸率所抑制，進而造成系統效能嚴重下降。為了探討此議題，我們研究以下兩種模型: 固定視窗大小模型以及變動視窗大小模型。此外，車輛通訊系統也被預期能提供即時性服務如，VoIP，以及非即時性服務，如網頁瀏覽。因此，我們具有優先權化MAC之混合模型，其中具有高優先權的即時性節點使用固定視窗大小，而低優先權的非即時性節點使用變動視窗大小。在以嵌入式馬可夫鏈為基礎之下，我們推導數學解析模型來計算感興趣的效能指標，如節點成功送達率、碰撞機率以及系統成功送達率。我們顯示了固定視窗大小模型能達成固定的成功送達率，但值通常較小；而變動視窗大小模型雖不能達到固定的成功送達率，但值通常較大。我們也顯示了在使用我們所提出的優先權化MAC情況下，不論節點位置為何，具有高優先權的節點皆能維持穩定的成功送達率，而低優先權的節點能達成較好的系統成功送達率。最後也很重要的是，我們的數學解析模型會由模擬結果驗證，其中模擬程式是以C撰寫，且數學解析結果與模擬結果合理接近。

[1] J. Ott and D. Kutscher, “Drive-thru internet: IEEE 802.1 1b for ‘Automobile’ Users,” IEEE INFOCOM, 2004, pp. 362-373.
[2] J. Ott and D. Kutscher, “A Disconnection-Tolerant Transport for Drive-thru Internet Environments,” IEEE INFOCOM, 2005, pp. 1849-1862.
[3] V. Bychkovsky, B. Hull, A. Miu, H. Balakrishnan, and S. Madden, “A Measurement Study of Vehicular Internet Access Using in Situ Wi-Fi Networks Categories and Subject Descriptors.” ACM MobiCom, 2006,
[4] IEEE P802.11 Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. 2007, pp. 51-60.
[5] M. Heusse, F. Rousseau, G. Berger-Sabbatel, and A. Duda, “Performance Anomaly of 802.11b,” IEEE INFOCOM 2003, pp. 836-843.
[6] D. Y. Yang, T. J. Lee, and K. Jang, “Performance Enhancement of Multirate IEEE 802.11 WLANs with Geographically Scattered Stations.” IEEE Trans. on Mobile Computing, 2006, pp. 906-919.
[7] D. Hadaller and T. Brecht, “MV-MAX : Improving Wireless Infrastructure Access for Multi-Vehicular Communication,” ACM CHANTS, 2006.
[8] B. Yu and C.-Z. Xu. “Admission Control in Roadside Unit Access. ” IEEE IWQoS, 2009, pp. 1-9.
[9] J.W. Mark, “Voice Capacity Analysis of WLANS with Channel Access Prioritizing Mechanisms,” IEEE Communications Magazine, vol. 46, Jan. 2008, pp. 82-89.
[10] L. Cai, X. Shen, J.W. Mark, L. Cai, and Y. Xiao, “Voice Capacity Analysis of WLAN With Unbalanced Traffic,” IEEE Transactions on Vehicular Technology, vol. 55, May. 2006, pp. 752-761.
[11] P. Shankar, T. Nadeem, J. Rosca, and L. Iftode, “CARS: Context-Aware Rate Selection for Vehicular Networks,” 2008 IEEE International Conference on Network Protocols, Oct. 2008, pp. 1-12.
[12] F. Calı`, M. Conti, and E. Gregori. “Dynamic Tuning of the IEEE 802.11 Protocol to Achieve a Theoretical Throughput Limit,” IEEE/ACM Trans. on Networking, 2000, pp.785–799.
[13] T.H. Luan, X. Ling, and X.S. Shen, “MAC Performance Analysis for Vehicle-to-Infrastructure Communication,” IEEE Communications Society, 2010, pp. 1-6.
[14] G. Anastasi, E. Borgia, M. Conti, and E. Gregori, “Wi-fi in Ad Hoc Mode: a Measurement Study,” IEEE PerCom, 2004, pp. 145-154.