使用於協同式安全應用之分散式壅塞控制機制

Abstract

在車載網路 (Vehicular Ad Hoc Networks, VANET) 中，未了避免訊息的傳遞受到頻道壅塞所影響，如何在車載網路中實施壅塞控制一直是一項很重要的問題。尤其是對於智慧型運輸系統 (Intelligent Transportation System, ITS) 中的安全性應用更為重要，通道壅塞所造成安全訊息的遺失可能造成重大危害。 目前歐洲電信標準協會 (European Telecommunications Standards Institute，ETSI) 制定分散式壅塞控制 (Decentralized Congestion Control, DCC) 的標準，DCC是一個跨階層的協定分佈於存取層 (Access layer)、網路與傳輸層 (Network & Transport layer)、設備層 (Facilities layer) 及管理層 (Management layer)，目前分散式壅塞控制的方法主要位在存取層，利用控制車輛的調整傳輸頻率達到頻道的壅塞控制，但是對於在設備層的壅塞控制著墨不多。設備層的壅塞控制主要工作在於分配車輛可用的頻道資源，其中最基本的分配方式是均分可用的頻道資源給頻道中所有的車輛。然而，在安全性的應用下，並非每台車輛的所需的頻道資源皆相同。 在此論文中，我們提出在設備層的壅塞控制，此機制會依照前方碰撞警示 (Forward Collision Warning, FCW) 應用的特性與需求來分配車輛的可用的頻道資源，相較於均分的方法下能增進前方碰撞警示應用的效能。

Congestion control is an important issue in vehicular networks. The loss of messages due to congestion may cause seriously deteriorate to the reliability of Intelligent Transportation System (ITS) especially for safety applications. European Telecommunications Standards Institute (ETSI) has specified a set of Decentralized Congestion Control (DCC) standards. DCC is a cross-layer protocol across Access layer, Network &Transport layer, Facilities layer, and Management layer in ETSI TC-ITS. So far ETSI mainly focused on Access layer DCC and is lack of discussion on the Facilities layer issues. The task of Facilities layer DCC is to allocate available channel resource to vehicles with fairness assumption, dividing total channel resource equally amongst vehicles in the channel. However, this mechanism is not adaptive to safety applications where vehicles may have different priorities in terms of emergency. In this thesis, we proposed a channel resource allocation mechanism for the Facilities layer DCC. The mechanism takes account of needed and feature of Forward Collision Warning (FCW) application. Simulation results show that the proposed method can improve the performance of FCW compared to existing mechanism.