針對IEEE 802.11無線區域網路的二段式通話允諾控制演算法

Abstract

近年來，使用無線寬頻的人口與日俱增。因此，使各種資料傳輸(例：語音、數據或多媒體)在隨時隨地能維持一定的服務品質，變得更為重要。為因應這項需求，整合各種不同的無線通訊系統逐漸演變成現今科技發展的潮流。在眾多整合技術中，IEEE 802.11無線區域網路被廣泛地採用為整合的系統之一，它運用正交分頻多工的調變方法，因而提供了很高的傳輸速率（6Mbps~54Mbps）。然而，IEEE 802.11無線區域網路能涵蓋的範圍非常有限，所以只設置在一些使用者較眾多的地方，例如辦公室、校園或機場；除此之外，IEEE 802.11無線區域網路存在最另人詬病的一點－當使用者稍有一點移動速率，系統的傳輸效能往往也跟著大幅地降低。在本論文中，吾人對IEEE 802.11無線區域網路的實體層及媒介存取層進行分析，並檢視IEEE 802.11無線區域網的每一個模式，在不同的頻道下所能達到的最大傳輸流量。最後，吾人亦利用馬可夫鍊導出連線速率自動調節演算法的數學模型，並結合了所有分析而得的結果，提出了一套「針對IEEE 802.11無線區域網路的二段式通話允諾控制演算法」。此演算法不僅能提高IEEE 802.11無線區域網路系統的整體傳輸流量，更能使每一個在此系統中的使用者，均能得到一定的服務品質。

As the number of wireless subscribers rapidly increases, the demands for guaranteeing the quality of services for all types of traffic (e.g. voice, data, and multimedia) anytime and anywhere become more critical. Therefore, the integration of multiple communication systems has passed into the developing trend of today’s technology. IEEE 802.11a/WLAN is one of the major wireless networks being widely adopted to be part of the integration. It achieves high data rates ranging from 6Mbps to 54Mbps by using orthogonal frequency division multiplexing (OFDM). However, the coverage of 802.11a is basically from 30m to 50m only. Thus WLAN will be set up only at hotspots such as offices, campuses and airports. Besides, IEEE 802.11a supports poor mobility thus the performance declines while the movement arises. In this thesis, the first tier call admission control (CAC) analyzes the performance of the IEEE 802.11a physical layer (PHY) under various channel conditions and determines if the station may request for the association. Besides, analytical mathematical models of link adaptation techniques, Auto Rate Fallback (ARF) and its extension, Adaptive ARF (AARF), are derived from the proposed Markov chains; and it provides essential information when designing the Buffer Time (BT) based call admission control algorithm in the second tier. In the end, the first and the second tier CAC algorithms are combined and an idea called “A Two-tier Call Admission Control Algorithm” is given to improve the overall system throughput and guarantee the quality of service of every single user in the WLAN system.