在現今車載傳輸安全資訊中最重要的兩個評估效能參數 -- Latency和reliability，要兼顧到這兩個的效能在現今不可靠的無線連接中是很困難的工作。這主要的挑戰是來自於在網路連接層的廣播，使用了不可靠的傳輸，例如:當某點接受到一個訊息時並不會傳輸一個反饋的信息給來源端通知它已接受到此信息。在現今有許多的方法是利用多於的點去增加接受的可靠性，但這將會增加網路頻寬的負載。
在我們的論文中我們提出了一個新的傳輸安全資訊的方法，使用少數relay去完成快速的安全資訊傳輸，並且在相同時間內去保持高效能傳輸的可靠性。在這我們介紹了兩個relay的型式同時地去減少end-to-end 傳輸延遲時間和增加傳輸範圍可靠性。地理樹狀演算法是由樹演算法而來的，它可以減少不必要的relay和傳輸資訊碰撞的發生，而802.11p則是用在優先權比較高的點擁有比較小的重新傳輸延遲時間。

Vehicular environments impose a set of new requirements on today’s wireless communication systems. Latency and reliability are very important means to disseminate safety information like time-sensitive emergency messages (EMs) in Vehicular Ad hoc Networks (VANETs). Providing low-latency, high-coverage and scalable multi-hop EM broadcast is a hard task in VANET with unreliable links. The major challenge comes from the fact that the link-layer broadcast uses unreliable transmissions, i.e., no positive feedback to acknowledge the reception of the message. Many existing works have used redundant relay nodes to enhance the reliability of broadcast packet reception. However they often involve more relays than it is necessary, which increases the network load and undermines the scalability of the protocol. Moreover, large latency is often incurred due to coarse protocol design.
In this thesis, we propose a new EM broadcast scheme that uses a small number of relays to achieve fast multi-hop EM propagation, at the same time to maintain a high level of transmission reliability. Two types of relays are introduced to reduce latency and to enhance reliability simultaneously, so that low-latency, the desired reliability level and small overhead can be achieved at the same time. The inverse tree algorithm (ITA) is based on tree algorithm, the mechanism to select single relay distributive, which features an effective redundant relay suppressing mechanism and very small rebroadcast delay for high priority nodes. Simulation study shows that ITA and multi-channel achieves close to 100% reliability, while using a small number of relays with very low broadcast latency under a wide range of road traffic conditions.

[1] DSRC and IEEE 1609 standard family,
http://www.standards.its.dot.gov/Documents/advisories/dsrc_advisory.htm.
[2] M. Torrent-Moreno, M. Killat, and H. Hartenstein. The challenges of robust
inter-vehicle communications. IEEE VTC 2005, 1:319–323, 28-25 Sept., 2005.
[3] M. Torrent-Moreno, F. Schmidt-Eisenlohr, H. Fussler, and H. Hartenstein. Effects of
a realistic channel model on packet forwarding in vehicular ad hoc networks. IEEE
WCNC, 1:385–391, 2006.
[4] M. Torrent-Moreno, D. Jiang, and H. Hartenstein. Broadcast reception rates and
effects of priority access in 802.11-based vehicular ad-hoc networks. In Proceedings
of VANET ’04, pages 10–18. ACM, 2004.
[5] S.-Y. Ni, Y.-C. Tseng, Y.-S. Chen, and J.-P. Sheu. The broadcast storm problem in a
mobile ad hoc network. In IEEE/ACM MobiCom, pages 151–162, 1999.
[6] N. Wisitpongphan, O. Tonguz, J. Parikh, P. Mudalige, F. Bai, and V. Sadekar.
Broadcast storm mitigation techniques in vehicular ad hoc networks. IEEE Wireless
Communications, 14(6):84–94, December 2007.
[7] S. Oh, J. Kang, and M. Gruteser. Location-based flooding techniques for vehicular
emergency messaging. In IEEE MobiQuitous, pages 1–9, July 2006.
[8] O. Tonguz, N. Wisitpongphan, F. Bait, P. Mudaliget, and V. Sadekart. Broadcasting in
vanet. In MoVE, pages 7–12, May 2007.
[9] O. Tonguz, N. Wisitpongphan, J. Parikh, F. Bai, P. Mudalige, and V. Sadekar. On the
broadcast storm problem in ad hoc wireless networks. In BROADNETS, pages 1–11,
Oct. 2006.
[10] R. Mangharam, R. Rajkumar, M. Hamilton, P. Mudalige, and F. Bai.
Bounded-latency alerts in vehicular networks. In MoVE, pages55–60, May 2007.
[11] M. T-Moreno. Inter-vehicle communications: Assessing information dissemination
under safety constraints. In WONS, pages 59–64. IEEE, January 2007.
[12] H. Alshaer and E. Horlait. Optimized adaptive broadcast scheme for inter-vehicle
communication. In IEEE VTC, pages 2840–2844, May 2005.
[13] IEEE_Std_1609[1].4_2006.
[14] Tree algorithm.
[15] Ns2. http://www.isi.edu/nsnam/ns.
[16] Daniel Jiang, Luca Delgrossi Mercedes-Benz Research & Development North
America, Inc. IEEE 802.11p: Towards an International Standard for Wireless Access
in Vehicular Environments. Pages2036-2040, IEEE, 2008
[17] Ming Li and Wenjing Lou. Opportunistic broadcast of emergency messages in
vehicular ad hoc networks with unreliable links. July 2008.
[18] Shun-Ren Yang and Wei-Hsiang Hung. Efficient Selection of Relay Vehicles for
Broadcasting on Vehicular Ad-hoc NETworks. The International Conference on
Mobile Technology, Applications & Systems 2008 (Mobility Conference), 10-12
September, 2008.
[19] Q. Chen, F. Schmidt-Eisenlohr, D. Jiang, M. Torrent-Moreno, L. Delgrossi, and H.
Hartenstein. Overhaul of ieee 802.11 modeling and simulation in ns-2. In
MSWiM ’07, pages 159–168. ACM, 2007.
[20] 新通訊 2009 年 7 月號 101 期《 技術前瞻 》