802.11無線點對點網路傳輸率之改善及延遲時間之分析

Abstract

在本篇論文中，我們修改了原本無線網路802.11的實體層規範，使其具有單一頻道的跳頻功能。利用802.11的點對點模式並且搭配我們提出的channel allocation procedure (CAP)方法，以及ad-hoc on-demand distance vector routing (AODV)，建立一個適合多重智慧型代理人交通控制系統的網路通訊介面。這個方法解決了影響網路傳輸率的隱藏節點問題，成功地改善其因競爭通訊行為而造成的網路傳輸率下降問題，本研究所設計的方格狀點對點網路系統，可作為未來ITS的行動通訊平台。透過NCTUns網路模擬軟體，模擬結果顯示相較於原本的802.11點對點網路，不同網路節點的傳輸率有75%到2300%的改善量。 此外，由於網路控制系統（network control system; NCS）中，需要估測因通訊所造成的延遲時間作為史密斯估測器（Smith predictor）的設計參數，我們進而透過實驗的設計，分析了點對點網路中的延遲時間，藉由客戶端╱伺服端的程式，量測從應用層到應用層的延遲時間平均值，探討其特性。我們發現不同的網路傳輸速率，及不同的封包資料量，皆會影響網路延遲時間。實驗指出距離因素不影響延遲時間，但隨著封包負載的成長，網路延遲也會增加，若使用高速率傳輸則能得到較小的網路延遲。

In this thesis, a new method called the channel allocation procedure (CAP) is proposed and its physical protocol of IEEE802.11 ad-hoc network is further modified to sequentially apply the hopping procedure based on the single-channel communication. By combining both the proposed CAP method and the ad-hoc on-demand distance vector routing (AODV), an effective communication interface for the multi-agent traffic control system is thus established. By applying the CAP method, the hidden terminal problem which is the main cause of the degradation of network throughput is solved. By evaluating the CAP method using the NCTUns network simulator, results revealed that the throughput of the proposed CAP method increases by 75% to 2300% compared with the original 802.11 ad-hoc networks. Furthermore, since the value of the delay time caused by the wireless communication is required in the design procedures of the Smith predictor in the network control system (NCS), the average delay time of the ad-hoc network between the two application layers of the server and the client is measured and analyzed to suppress the delay effect. It is found that various transmit rates and payload sizes mainly affect the delay time. Experimental results further indicate that the delay time is independent of the transmission distance and it increases when the payload size increases. Moreover, applying the higher transmit rate leads to the lower time delay in NCS.