車載行動網路是將無線通訊整合至車輛的新興智慧型運輸系統科技，智慧型運輸系統的應用包含著行車安全性、運輸效率、資訊及娛樂等應用。在車載的環境下主要的終端設備為汽車，與一般人攜行走動的終端設備來比較，其移動性較高，換手的行為將會經常發生，若換手造成的延遲過長時將會影響網路傳輸的品質，尤其對於一些即時性的網路服務將有重大的影響，故傳統的行動管理機制已經不能符合車載行動網路的需求，許多快速換手的機制相繼提出。然而一般選擇換手存取點的方式為選取最佳訊號強度的存取點當做換手的存取點，並沒有考慮到不同存取點的負載狀況，故不能使網路的頻寬有效的利用，當某些存取點過度忙碌時將會使服務的品質下降。
在車載的環境中，存取點可事先建置完成，並依照不同路段的車流量可以調整區域內存取點的數量以達到網路流量分散提高服務品質的目的，例如在容易塞車的路段相對地網路使用量較高，若是只有一個存取點做服務，勢必造成網路壅塞的狀況。我們提出一適用於車載環境的快速換手機制，除了減少換手的延遲並考慮到區域存取點的負載平衡，透過建置好的存取點控制器可以定期收集各存取點忙碌的狀況，並預測下一時刻的網路流量，經由計算過後的參數透過廣播訊標訊框時廣播給鄰近的行動車輛節點，行動車輛節點可依據此參數選取對本身服務品質最佳的存取點。如此一來，可以達到區域存取點的負載平衡並提高服務的品質，提升車載行動網路中的換手效率。

Vehicular ad hoc network (VANET) is an emerging intelligent transportation system (ITS) that integrates wireless communications with vehicles. ITS applications currently include driving safety, transportation efficiency, information and entertainment-related applications. Compared with handheld devices, the mobility of vehicles, the main terminals in VANETs, is obviously higher and the handoff procedure consequently occurs frequently. Too long handoff latency works upon the transmission quality, especially on some real-time network services. Since traditional mobility management mechanisms no longer meet the needs of VANETs, many fast handoff schemes were proposed. However, without considering the loading status of different APs, most handoff mechanisms choose the AP with the optimal signal strength as the handoff location. In such a manner, the network bandwidth cannot be utilized efficiently and the overload of some APs may degrade service quality.
In VANETs, APs can be established in advance. According to different traffic flow, the number of APs can be adjusted in order to distribute network traffic and improve service quality. For example, the network traffic is undoubtedly high in a crowded road section. If only one AP provides service, the network traffic is bound to be congested. Therefore, we proposed QualityScan, a fast handoff scheme for VANETs to reduce the handoff latency and keep the APs load-balanced by deploying AP controller (APC) that periodically collect the status of the APs in the subnet and predict the network traffic at the next moment. Based on the result parameter broadcasted in beacon frames, the neighboring MNs can determine the best AP for the service. Thus, the APs in the area can be load-balanced, the service quality can be improved, and the handoff efficiency over VANETs can be further enhanced.

第一章  緒論	- 1 -
1.1	前言	- 1 -
1.2	動機與目的	- 1 -
1.3	論文章節架構	- 2 -
第二章  相關背景研究	- 4 -
2.1	車載行動網路（Vehicular Ad Hoc Network, VANET）	- 4 -
2.1.1	交通資訊系統（Traffic Information System, TIS）	- 5 -
2.2	基於IEEE 802.11無線網路下的換手	- 7 -
2.2.1	被動掃描	- 7 -
2.2.2	主動掃描	- 8 -
2.2.3	Layer2換手延遲	- 9 -
2.3	現有的快速換手機制	- 11 -
2.3.1	Neighbor Graphs方法介紹	- 12 -
2.3.2	SyncScan方法介紹	- 12 -
2.3.3	DeuceScan方法介紹	- 13 -
2.4	802.11無線網路的負載平衡	- 13 -
2.4.1	負載平衡指標（Load Balance Index）	- 16 -
2.5	IEEE 802.11p	- 17 -
2.6	IEEE 802.11e	- 18 -
2.6.1	EDCA（Enhanced Distributed Channel Access）	- 19 -
2.6.2	EDCA參數組合資訊元素（EDCA Parameter Set Element）	- 21 -
第三章  品質掃描機制	- 23 -
3.1	車輛換手時間點的預測	- 25 -
3.2	依照接收到的掃描資訊選取最佳的換手AP	- 27 -
3.3	區域存取點群組的流量預測	- 28 -
3.4	以服務取向暨流量預測的區域負載平衡	- 31 -
3.5	依照流量預測調整K值	- 33 -
3.6	換手條件	- 36 -
第四章  模擬環境及模擬結果分析	- 38 -
4.1	車行速度與換手延遲分析	- 38 -
4.2	負載分析	- 41 -
第五章  結論與未來展望	- 56 -
參考文獻	- 57 -
圖2.1 車載行動網路	- 5 -
圖2.2 國道高速公路交通資訊系統	- 6 -
圖2.3 訊標訊框格式	- 8 -
圖2.4 IEEE 802.11換手程序	- 10 -
圖2.5 IEEE 802.11e傳送佇列	- 19 -
圖2.6 EDCA 時序圖- 20 -
圖2.7 EDCA參數組合資訊元素	- 22 -
圖3.1 換手時間點示意圖- 25 -
圖3.2 系統架構圖- 27 -
圖3.3 區域存取點群組示意圖- 28 -
圖3.4 排隊理論參數示意圖- 29 -
圖3.5 選擇AP 示意圖 - 32 -
圖3.6 預測流量示意圖  - 34 -
圖4.1 車速與換手延遲關係圖	- 40 -
圖4.2 車速與換手延遲關係圖	- 41 -
圖4.3 模擬場景圖	- 42 -
圖4.4 車速與車輛密度關係圖	- 43 -
圖4.5 車道平均車量密度與車流量關係圖	- 44 -
圖4.6 平均車量密度與負載平衡關係圖（N=2）	- 46 -
圖4.7 平均車量密度與負載平衡關係圖（N=3）	- 46 -
圖4.8 平均車量密度與負載平衡關係圖（N=4）	- 47 -
圖4.9 各AP的忙碌程度（N=2, D=1）	- 48 -
圖4.10 各AP的忙碌程度（N=3, D=1）	- 49 -
圖4.11 各AP的忙碌程度（N=4, D=1）	- 49 -
圖4.12 各AP的忙碌程度（N=2, D=2）	- 50 -
圖4.13 各AP的忙碌程度（N=3, D=2）	- 50 -
圖4.14 各AP的忙碌程度（N=4, D=2）	- 51 -
圖4.15 各AP的忙碌程度（N=2, D=3）	- 51 -
圖4.16 各AP的忙碌程度（N=3, D=3）	- 52 -
圖4.17 各AP的忙碌程度（N=4, D=3）	- 52 -
圖4.18 各AP的忙碌程度（N=2, D=4）	- 53 -
圖4.19 各AP的忙碌程度（N=3, D=4）	- 53 -
圖4.20 各AP的忙碌程度（N=4, D=4）	- 54 -
圖4.21 各AP的忙碌程度（N=2, D=5）	- 54 -
圖4.22 各AP的忙碌程度（N=3, D=5）	- 55 -
圖4.23 各AP的忙碌程度（N=4, D=5）	- 55 -
表2.1  Layer2換手延遲統計表	- 11 -
表3.1 車輛速度對應換手時間點	- 26 -
表3.2 K值表	- 36 -
表4.1 模擬參數表	- 39 -
表4.2 模擬參數表	- 42 -
表4.3 車量密度與車流量關係表	- 44 -

[1]	H. Hartenstein and K.P. Laberteaux, “A Tutorial Survey on Vehicular Ad Hoc Networks,” IEEE Communications Magazine, vol. 46, June 2008, pp. 164-171.
[2]	K. Suriyapaibonwattana and C. Pomavalai, “An Effective Safety Alert Broadcast Algorithm for VANET,” International Symposium on Communications and Information Technologies, 2008, pp. 247-250.
[3]	C. K. Toh, “Future Application Scenarios for MANET-based Intelligent Transportation Systems,” Future Generation Communication and Networking (FGCN 2007), vol. 2, 2007, pp. 414-417.
[4]	D. Kwak, J. Mo and M. Kang, “Investigation of Handoffs for IEEE 802.11 Networks in Vehicular Environment,” International Conference on Ubiquitous and Future Networks (ICUFN 2009), 2009, pp. 89-94.
[5]	N. Choi, S. Choi, Y. Seokt, et al., "A Solicitation-based IEEE 802.11p MAC Protocol for Roadside to Vehicular Networks," Mobile Networking for Vehicular Environments, 2007, pp. 91-96.
[6]	Y. A. Powar and V. Apte, “Improving the IEEE 802.11 MAC Layer Handoff Latency to Support Multimedia Traffic,” Wireless Communications and Networking Conference (WCNC 2009), 2009, pp. 1-6.
[7]	K. Zhu, D. Niyato, P. Wang, E. Hossain and D. I. Kim, “Mobility and handoff management in vehicular networks: a survey,” Wireless Communications and Mobile Computing, Oct. 2009.
[8]	TANFB Traffic Information System, http://1968.freeway.gov.tw/
[9]	S. Pack, J. Choi, T. Kwon, and Y. Choi, “Fast-Handoff Support in IEEE 802.11 Wireless Networks,” IEEE Communications Surveys & Tutorials,  vol. 9, 2007, pp. 2-12.
[10]	A. Mishra, M. Shin, and W. Arbaugh, “An empirical analysis of the IEEE 802.11MAC layer handoff process,” ACM SIGCOMM Comput. Commun. Rev., vol. 33, Apr. 2004, pp. 93–102.
[11]	ITU-T Recommendation G.114, “One-way transmission time”, May 2005.
[12]	M. Shin, A. Mishra, and W. A. Arbaugh, “Improving the latency of 802.11 hand-offs using neighbor graphs,” in Proc. 2nd Int. Conf. MobiSys, Jun. 6–9, 2004, pp. 70–83.
[13]	I. Ramani and S. Savage, “SyncScan: Practical fast handoff for 802.11 infrastructure networks,” in Proc. 24th INFOCOM, Mar. 13–17, 2005, vol. 1, pp. 675–684.
[14]	Y.-S. Chen, M.-C. Chuang and C.-K. Chen, “DeuceScan: Deuce-Based Fast Handoff Scheme in IEEE 802.11 Wireless Networks,”  IEEE Transactions on Vehicular Technology, 2008, pp. 1126-1141.
[15]	L.-H. Yen, T.-T. Yeh and K.-H. Chi, “Load Balancing in IEEE 802.11 Networks,” IEEE Internet Computing, vol. 13, 2009, pp. 56-64.
[16]	Y. FUKUDA and Y. OIE, “Decentralized Access Point Selection Architecture for Wireless LANs Deployability and Robustness,” Vehicular Technology Conference, vol. 2, 2004, pp. 1103-1107.
[17]	O. Brickley, S. Rea, and D. Pesch, “Load Balancing for QoS Enhancement in IEEE 802.11e WLANs Using Cell Breathing Techniques,” Proc. IFIP Mobile and Wireless Communication Networks Conf., Int’l Federation for Information Processing, 2005, www.aws.cit.ie/personnel/ Papers/Paper268.pdf.
[18]	S. Vasudevan et al., “Facilitating Access Point Selection in IEEE 802.11 Wireless Networks,” Proc. Internet Measurement Conf., Usenix Assoc., 2005, pp. 293–298.
[19]	E. H. Ong and J.Y. Khan, “An Integrated Load Balancing Scheme for Future Wireless Networks,” International Symposium on Wireless Pervasive Computing ( ISWPC 2009), 2009, pp. 1-6.
[20]	S. Tartarelli and G. Nunzi, “QoS Management and Congestion Control in Wireless Hotspots,” Network Operations and Management Symposium (NOMS 2006), 2006, pp. 95-105.
[21]	H. Velayos, V. Aleo and G. Karlsson, “Load Balancing in Overlapping Wireless LAN Cells,” IEEE International Conference on  Communications, vol.7, 2004, pp. 3833-3836.
[22]	D.-M. CHIU and R. JAIN, “Analysis of the increase and decrease algorithms for congestion avoidance in computer networks,” Computer Networks and ISDN Systems 17, 1989, pp. 1-14.
[23]	E. Garcia, R. Vidal and J. Paradells, “Cooperative Load Balancing in IEEE 802.11 Networks with Cell Breathing,” IEEE Symposium on Computers and Communications (ISCC 2008), 2008, pp.1133-1140.
[24]	L. Stibor, Z. Yunpeng, and H. J. Reumerman, "Evaluation of Communication Distance of Broadcast Messages in a Vehicular Ad-Hoc Network Using IEEE 802.11p," Wireless Communications and Networking Conference, 2007 (WCNC 2007), 2007, pp. 254-257.
[25]	S. Eichler, “Performance Evaluation of the IEEE 802.11p WAVE Communication Standard,” Vehicular Technology Conference (VTC 2007), 2007, pp. 2199-2203.
[26]	K. Bilstrup, E. Uhlemann, E. G. Strom, and U. Bilstrup, “Evaluation of the IEEE 802.11p MAC method for Vehicle-to-Vehicle Communication,” Vehicular Technology Conference (VTC 2008), 2008, pp. 1-5.
[27]	G. M. Abdalla, M. A. Abu-Rgheff and S.-M. Senouci, “Space-Orthogonal Frequency-Time medium access control (SOFT MAC) for VANET,” Information Infrastructure Symposium (GIIS 2009), 2009, pp. 1-8.
[28]	D. Jiang, and L. Delgrossi, “IEEE 802.11p Towards an International Standard for Wireless Access in Vehicular Environments” Vehicular Technology Conference, 2008 (VTC Spring 2008), 2008, pp. 2036-2040.
[29]	Y. Xiao, “IEEE 802.11E QoS Provisioning At The MAC Layer,” IEEE Wireless Communications , 2004, pp. 72-79.
[30]	D. Gross and C. M. Harris, Fundamentals of Queueing Theory, WILEY-INTERSCIENCE, 3rd edition, 1998, pp.10-13.
[31]	John D. C. Little, Operations Research, INFORMS, Vol. 9, No. 3, May-Jun., 1961, pp. 383-387.