無線區域數據網路上之限時服務訊務控制法則

Abstract

由於手提式電腦市場的快速成長，無線區域數據網路已成為今日通訊產業新興的技術。為了支援多媒體的傳輸，多工服務的封包交換網路必須針對每個連線都能提供服務品質保證。這些服務品質保證包括了傳輸率、網路延遲與網路延遲變異。其中最具研究與實用性的應用即是有延遲限制的限時服務。 在本論文中，我們考量了在提供服務品質保證的有線網路連接具有集中協調功能的IEEE 802.11無線數據網路的架構之下，有線網路端的使用者與無線網路端的使用者做雙向交談的情形，也就是說在任何的時間點，連線的兩個人一定會有一人產生語音資料的封包。在這個假設下，我們排除了兩個人同時說話和同時安靜的可能性。這樣的研究對於欲在如博物館裡佈置的無線區域網路上提供一個對封包延遲很敏感的電話形式服務應具有參考價值。 在提供服務品質保證的有線網路端，我們借用了非同步傳輸模式網路上的三種典型的來源參數保證模型：常速率、變速率與餘速率。根據每個已知的服務品質參數，我們推導出了在嚴格保證下行流量(由擷取點到無線工作站)的服務品質條件下，依序輪詢(RR_Polling)機制與我們新提出的最短期限優先輪詢(EDF_Polling)機制的允入控制法則。RR_Polling機制與EDF_Polling機制在可排程區域的比較詳述於後。 針對被允許建立的連線，我們研究其在RR_Polling機制與EDF_Polling機制下的平均上行流量延遲。需要注意的是，由於IEEE 802.11上並無任何可以交換服務品質參數的機制，因此上行流量無法有任何的嚴格服務品質保證。最後，不同的系統參數，例如，無競爭時段的長度與資料訊框的長度等，對系統性能的影響也將被提出。

The growing spread of portable computers has made the wireless local area networks (WLANs) an emerging technology for today's communication industries. To support the requirements for the transmission of continuous media, multiple-service packet-switching networks must provide service guarantees to connections, including guarantees on throughput, network delays, and network delay variations. One of the demanding applications that are of research and practical interest is the time-bounded service, which requires deterministically bounded delay guarantees. In this thesis, we study the problem of conveying two-way conversational sources over guaranteed Quality-of-Service (QoS) wired network access to an PCF-enabled IEEE 802.11 wireless LAN. The traffic patterns considered is assumed to fit the telephony-type two-way conversational model, i.e., at any time instance, exactly one of the two persons (caller and callee) can generate voice data. This excludes the unlikely situations (for a normal conversation) that both persons talk at the same time, or both keep silence. The study based on such source model shall be useful when one intends to provide delay-sensitive telephony-type services over WLANs, e.g. to deploy WLANs over an historical building to provide the telephony-type services in addition to the data-type services. For the QoS guaranteed wired network side, we borrow three typical classes of source parameter-guaranteed models from Asynchronous Transfer Mode (ATM) networks: constant bit rate (CBR), variable bit rate (VBR), and available bit rate (ABR). Base on these pre-given QoS parameters, the Admission Control Criteria for standard-suggested Round-Robin-based Polling scheme (RR_Polling) and our newly proposed Eerliest-Deadline-First Polling scheme (EDF_Polling) are both derived under the premise that the down-link traffic (from the Access Point to the Wireless Station) is hard-guaranteed to satisfy them. The comparison of the schedulable regions of the RR_Polling and EDF_Polling is then followed. For those admitted traffics, we investigate the average delays of the up-link traffics for both RR_Polling and EDF_Polling schemes. Note that the up-link traffic indeed provides no QoS parameter exchanging mechanism due to IEEE 802.11, and hence, can not be hard-guaranteed in any sense. Pro-and-cons for different system factors, such as the length of contention-free period, as well as the data frame size, are also addressed.