近幾十年來，各大主要城市的交通狀況持續惡化，行車安全問題也越來越被重視。在城市中道路容量有限的情況下，對於妥善利用城市中有限的空間，並增進運輸系統效能的需求越來越迫切。所幸，由於電子技術與無線通訊技術不斷地進步，車用隨意網路(Vehicular Ad Hoc Networks, VANETs)的研究立刻被各國政府重視。包括美國的聯邦通訊委員會(Federal Communications Commission, FCC)就在5.9GHz的頻段上分配75MHz(5.850GHz-5.92GHz)的頻寬給車用系統使用，這個技術被稱為專用短距離通信(Dedicated Showt Range Communication, DSRC)。其他的機構如德國的Network on Wheels、歐洲的C2CCC、日本的InternetITS、以及台灣的ITS Taiwan均為類似的目標而設立。

車用無線隨意網路(VANET)是行動式無線隨意網路(Mobility Wireless Ad Hoc Networks, MONETs)的一個分支，具有高速機動性、網路規模大、受限制的網路拓樸、以及通訊設備較沒有電力限制等等特殊的特性，這些特性也深深地影響到在VANET上各種網路演算法的設計。

VANET的節點是裝備有無線網路通訊設備、全球定位系統(Global Positioning System;GPS)、電子地圖及其它用以回報相關訊息之感測器，行駛於道路上的交通工具。這些行動節點透過節點對節點(Vehicle-to-vehicle)或是節點對路旁基地台(Vehicle-to-Roadside)的方式來交換資料。由於其特殊環境，路由機制就成為了VANET環境裡的一大挑戰。
在MANET上有許多用來傳送資料的路由機制，但多半不是特別針對VANET的環境設計。在VANET的環境裡，節點的快速移動使得路由協定所建立的路徑很快的失效，來源節點只傳送了一小部份資料，就必須再重新尋找路徑，造成路由請求封包充斥於網路環境之中，形成頻寬資源的浪費和極大的路由負擔。

本研究主要是參考(Diagonal-Intersection-Based Routing Protocol, DIR)的對角線路由的作法，將其原本是Unicast的特性擴展為支援(群播)Multicast的路由協定。DIR利用設下對角線錨點，使資料在每一段的傳輸過程中，隨時都至少有兩條以上的路線以供選擇，在其中一條路線失效或是傳輸延遲過高時，可以動態的調整傳輸路徑，減少資料在VANET的環境中的傳輸延遲並增加傳輸成功率。最後再利用程式模擬與其他舊有的群播路由協定套用在VANET的情況作比較。

In the first decade of 21st century, traffic conditions of modern cities around the world are getting worse. Since traffic capacity in cities is limited, taking advantage of limited capacity in cities properly and improving traffic system efficiency become very urgent. Fortunately, as the electronics and wireless communication technologies improve, "Vehicular Ad hoc Networks" (VANETs) has been taken seriously by many countries. The U.S. Federal Communications Commission (FCC) has allocated a 75MHz radio spectrum at 5.9GHz for wireless vehicular ad hoc networks (VANETs). IEEE standardizes this vehicular technology, known as Dedicated Short Range Communication (DSRC).

Wireless vehicular ad hoc networks is a form of Mobile ad hoc networks with some different characteristics such as highly mobility, bigger network scale, restricted network topology, and no energy concerned. The NODEs in VANETs usually means vehicles on roads, which equip wireless communication hardwards, Global Positioning System (GPS), digital maps, and several sensors. These mobile nodes can transmit data through vehicle-to-vehicle or vehicle-to-roadside.

In this research, we refer to the method of Diagonal-Intersection-Based Routing Protocol (DIR), and try to extend this unicast protocol to a multicast protocol. DIR protocol uses specific anchors to route data and provides auto-adjustability, which alleviates packet delivery delay and increases packet delivery ratio by specific diagonal-intersection based routing concept. We will take advantage of diagonal-intersection based routing concept to propose our DIM protocol. We expect DIM can efficiently alleviate packet delivery delay by the diagonal-intersection routing concept.

[1] M. Williams, "PROMETHEUS-the european research programme for optimising the road transport system in europe", In Proc. of the IEE Colloquium on `Driver Information' (Digest No.127), 1988, pp. 1–9.
[2] S. Shladover, C. Desoer, J. Hedrick, M. Tomizuka, J. Walrand, W.Zhang, D. McMahon, H. Peng, S. Sheikholeslam, and N. McKeown, "Automatic vehicle control developments in the PATH program", IEEE Transactions on Vehicular Technology, vol. 40, no. 1, pp. 114–130, 1991.
[3] W. Enkelmann, "Fleetnet - applications for inter-vehicle communication", In Proc. of the IEEE Intelligent Vehicles Symposium, 2003, pp. 162–167.
[4] F. Li, Y. Wang, "Routing in Vehicular Ad Hoc Networks: A Survey", IEEE Vehicular Technology Magazine Volume 2, Issue 2, 2007, Pages 12-22.
[5] Andrew S. Tanenbaum, Computer Networks, 4rd edition, Prentice Hall, 2002
[6] C.E. Perkins, E.M. Royer, "Ad hoc on demand Distance Vector routing", mobile computing systems and applications, 1999 Proceedings WMCSA '99 Second IEEE Workshop on, 1999, p90-p100.
[7] Y.W. Lin, Y.S. Chen, S.L. Lee, "Routing Protocols in Vehicular Ah Hoc Networks: A Survey and Future Perspectives", Journal of Information Science and Engineering, 2009.
[8] A. Bachir and A. Benslimane, "A Multicast Protocol in Ad hoc Networks Inter-Vehicle Geocast", IEEE Semiannual Vehicular Technology Conference, vol. 4, Apr. 2003, pp.2456-2460.
[9] J. Zhao, G. Cao, "VADD: Vehicle-Assisted Data Delivery in Vehicular Ad Hoc Networks", IEEE International Conference on Computer Communications, (INFOCOM 2006), Barcelona, Caralunya, Spain, pp.1-12, 23-29 April 2006.
[10] V. Naumov, T. Gross, "Connectivity-Aware Routing (CAR) in Vehicular Ad Hoc Networks", IEEE International Conference on Computer Communications (INFOCOM 2007), Anchorage, Alaska, USA, pp. 1919-1927,6-12 May 2007.
[11] Y.S. Chen, C.Y. Pan, "DIR: Diagonal-Intersection-Based Routing Protocol for Urban Vehicular Ad Hoc Networks", Telecommunication System, Vol. 46, Issue 4, May 2011.
[12] B. Karp, H. T. Kung, "GPSR: Greedy perimeter stateless routing for wireless networks", In Proc. of ACM MobiCom, Boston, MA, August, 2000, pp. 243-254.