Reliable and Efficient Safety-Critical Message Dissemination in Vehicular Ad Hoc Networks

NCUIR > College of Electrical Engineering & Computer Science > Graduate Institute of Computer Science and Information Engineering > Electronic Thesis & Dissertation > Item 987654321/59229

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/59229

Title:	Reliable and Efficient Safety-Critical Message Dissemination in Vehicular Ad Hoc Networks
Authors:	蘇佳如;Sahoo,Jagruti
Contributors:	資訊工程學系
Keywords:	IEEE802.11p;信標;媒體接入控制（MAC）;廣播;緊急消息;車載隨意網絡;黑場;Active-Safety Application;Beacon;Black Burst;Broadcast;DSRC;Emergency Message;IEEE 802.11p;Medium Access Control(MAC);Vehicular ad hoc network
Date:	2013-01-22
Issue Date:	2013-03-25 16:16:30 (UTC+8)
Publisher:	國立中央大學
Abstract:	在陸上交通工具中嵌入一個進階的無線傳輸技術可以大幅提升車輛的安全性。整個網路結構是由裝上無線安全感測裝置的車輛組成，這些感測裝置會主動感知各種不同的安全狀況，而這個結構可以稱之為車載網路(Vehicular ad Hoc Network, VANET)。為了能建造出一個有效又安全的系統，我們必須要傳送兩種不同的安全性訊息：信標(Beacons) 和緊急訊息(Emergency Messages)。其中信標包含了車輛的一些基本狀態，像是位置和速度等，並且每台車會定期與周圍的車輛交換這個訊息。這個動作讓每台車輛可以互相提醒自己周圍的車輛，進而可以幫助車輛回避一些危險的狀況。另外，任何不正常的事件都會產生緊急訊息，而這個緊急訊息會在一個特定的地區內傳播。由此可見，所有的道路狀況和車輛資訊都包含在這兩種安全行訊息中，所以如何有效並確實的傳達這兩種安全系訊息對這個主動感知系統是非常重要的。在本篇論文中，我們設計了一個在車載網路安全性訊息傳播協定。二元分割輔助廣播BPAP(Binary Partition Assisted Broadcast) 是一個以減少緊急訊息延遲為目標的通訊協定，基本上我們採用二元分割法來選擇轉送節點，這種方法可以減少在每一個節點轉傳之前的延遲，最重要的是，協定的效能不受車輛密度變化的影響。模擬結果顯示，BPAP協定在延遲與訊息傳遞上比現今的車載網路所使用的多點傳輸協定有更好的表現。為了更準確的更新車輛狀態的資訊，安全的應用程序必須要求信標以100毫秒產生一次，由於在VANET中對安全性的要求，信標的大小可能會有很大的變化(500~800bytes，包括安全的有效載荷)。如果在媒體訪問控制(MAC)層中沒有適當的協調，如此高頻率會容易在車輛密度高的地區造成頻道壅擠。作為直接的結果，信標的可靠度、延遲效能以及緊急訊息會嚴重的受到影響。這種現象會構成挑戰於未來關於道路車輛的高市佔率。因此，我們設計一個基於時槽的媒體訪問控制協議，命名為CCC-MAC(Congestion Controlled Coordinator based MAC)，確保不碰撞的安全訊息傳輸存在於大量的車輛中。此協定核心的設定是將道路區分成多個小段，每小段道路上，透過由當時的協調車輛(local coordinator vehicle)所管理的時段排程機制(time slot scheduling mechanism)來取得傳輸媒介，事實上，此排程機制使用多種數據傳輸速率來降低信標(beacon)的傳輸時間，減輕了頻道的擁塞情況，而基於脈衝的保留機制(pulse-based reservation mechanism)則是快速傳送緊急訊息的重要機制，此外，CCC-MAC協定也有個分配使用未被佔用的傳輸時段的機制，可以增加頻寬的使用效率，從模擬的結果可以看出CCC-MAC也可以適用於不同的車輛密度的情境，更甚的是，在封包接收率(packet reception probability)和安全性訊息的延遲(latency of safety messages)方面，都比目前的媒體訪問控制層協定(MAC layer protocol)有更好的表現。Embedding advanced wireless communication technologies in road vehicles extends the horizon of vehicular safety to a greater level. The network formed by the equipped vehicles is referred to as Vehicular ad Hoc Network (VANET), which is used for realizing various active-safety applications. Building an efficient safety system on road requires dissemination of two safety-critical messages: beacons and emergency messages. Beacons carry vehicle status (position, velocity, etc.) and are exchanged periodically. They create mutual awareness in the immediate neighborhood and assist vehicles in preventing un-safe situations beforehand. On the other hand, an emergency message is generated in the wake of an abnormal event and is disseminated in a specific geographic area. Reliable and efficient transmission of safety-critical messages is a serious requirement of an active-safety system as the information embedded in those messages relate to critical road situations. In this thesis, we design protocols for safety-critical message dissemination in VANET scenarios. A multi-hop broadcast protocol BPAB (Binary Partition Assisted Broadcast) is designed with the objective to reduce the latency of emergency messages. Basically, we adopt a binary-partition approach to select a forwarding node. This approach reduces the delay incurred before rebroadcasting at each hop. Most importantly, the protocol performance is unaffected by variation in vehicle density. Simulation results demonstrate that BPAB protocol imparts greater performance in terms of latency when compared with the contemporary multi-hop broadcast protocols for Vehicular Ad Hoc Networks.For a more accurate update of vehicle status information, safety-applications require beacons to be generated at an interval of 100ms. Because of the security threats in VANETs, the size of a beacon may vary from 500-800 bytes including the security payload. Without proper coordination at the MAC layer, such a high frequency can easily result in channel congestion in regions of high vehicle densities. As a direct consequence, the reliability and latency performance of beacons as well as emergency messages are severely affected. This phenomenon will pose challenges in future with regard to high market penetration of road vehicles. We, therefore, design a time slot based medium access protocol named CCC-MAC (Congestion Controlled Coordinator based MAC) which ensures collision free transmissions of safety-critical messages in presence of significant number of vehicles. The core of the protocol is a configuration in which the road is virtually partitioned into a number of segments. Within a segment, medium access is accomplished by using a time slot scheduling mechanism supervised by a local coordinator vehicle. In fact, the proposed scheduling mitigates channel congestion by reducing the transmission duration of beacons through the use of multiple data rates. A pulse-based reservation mechanism plays the major role in achieving fast dissemination of emergency messages. The CCC-MAC protocol also provides scope for improving bandwidth utilization through a mechanism that controls the reuse of the unoccupied time slots. Simulation results show the ability of CCC-MAC to scale well in different vehicular density scenarios. Moreover, it outperforms existing MAC layer protocols with respect to packet reception probability and latency of safety messages.
Appears in Collections:	[Graduate Institute of Computer Science and Information Engineering] Electronic Thesis & Dissertation

Files in This Item:

File	Description	Size	Format
index.html		0Kb	HTML	769	View/Open

社群 sharing

Loading...