具訊框調整及功率控制能力之可適性RR-ALOHA無線隨意網路存取協定

Abstract

在無線通訊網路中，如何減少傳輸的延遲是一項很重要的議題。尤其對於智慧型運輸系統(Intelligent Transportation System, ITS)中的安全性應用更為重要，稍微差了些微的時間，可能都會造成重大的危害。 現有的無線網路媒體存取控制(Medium Access Control, MAC)主要可分為兩大類: (1)基於競爭模式的媒體存取控制(Contention-based MAC protocol) 以及(2)基於排程模式的媒體存取控制(Schedule-based MAC protocol)。在高密度的網路之中，基於競爭模式的媒體存取控制會造成大量的競爭碰撞，因此無法保證訊息傳輸的可靠性。而基於排程模式的媒體存取控制透過將時間分割，讓節點在屬於自己的時間內傳輸資料，其他節點無法在他人的時間內傳輸資料因而達到無碰撞的資料傳輸。其中，屬於排程模式媒體存取控制類型的RR-ALOHA還能避免隱藏節點問題。然而，當節點的數量超出訊框長度時，會造成部分節點無法獲得時槽(time slot)傳遞資料。解決此問題的方法大致可分為兩種，一種是增加訊框長度，另一種是減少節點的傳輸功率。但這兩種方法都會造成較大的傳輸延遲。 在此論文中，我們提出一種可適性RR-ALOHA，藉由將RR-ALOHA結合訊框調整技術以及功率控制技術以避免時槽壅塞(slot congestion)問題，並同時保持較低的傳輸延遲。

Reducing transmission delay is an important issue in wireless networks. For example, the delay is particular critical to safety applications of Intelligent Transportation System (ITS). Existing MAC protocols for wireless ad hoc networks typically be classified into two categories: (1) Contention-based and (2) Schedule-based MAC protocols. In dense networks, the contention-based MAC cannot guarantee the reliability because of the collisions. The schedule-based MAC can achieve collision free by assigning unique time slot for each node to send data. Moreover, RR-ALOHA can avoid hidden terminal problem. However, if the number of nodes exceeds the frame size, some nodes cannot reserve time slot to transmit data. Two methods can solve the slot congestion problem, one is increasing the frame size, and the other is reducing the transmission power. But, these two methods may increase the end-to-end delay. In this thesis, we present an adaptive RR-ALOHA for wireless ad hoc networks which combines RR-ALOHA with an adaptive mechanism with frame size adjustment and power control to solve slot congestion problem and retain a lower end-to-end delay.