IEEE 802.11(e) 動態串流指派與頻寬保證設計

Abstract

近年來，IEEE 802.11 標準已成為最普遍的無線技術，但其本身並不適合多媒體應用，無法具備提供服務品質 (QoS) 支援與頻寬保證的能力。 於是，IEEE 另開發出 802.11e 標準來修正傳統 802.11 標準的種種缺點。在 IEEE 802.11e 內，定義了一個 poll-based的HCF 頻道控制管理 (HCCA)。在 HCCA 中，允入控制 (Admission Control) 單元需根據資料流的需求與目前頻道狀態，來決定是否允許資料流加入排程。

開始發展 802.11e 標準後，許多研究者提出了他們的方法來保證多媒體資料的處理量，提升 802.11e 的能力。對於 802.11e 的允入控制與排程上，不少新的觀念看法被提出。但是，只取決於無線存取點本身的資訊，來做出允入或拒絕的決定，並不足以為整個無線系統增進 QoS 保證。在一個公開場所，需要有充足的無線存取點來提供完整的訊號覆蓋和頻寬。以最強的信號為依據選擇存取點，可能導致無效率的頻道運用與 QoS 支援。

本文主要集中在討論 802.11e 的 HCCA 部分，提出動態串流指派系統，來提升 802.11e 無線網路的實用性，有效率的使用頻道，增進其表現。系統並考慮各條資料流的用戶優先權，根據優先權，調度排程者決定承認或拒絕請求。模擬結果顯示，比起原本的 802.11e 無線網路，我們的系統可支持更多 QoS 要求，因而增加系統的總處理量。此外，關於存取點間交換訊息與處理資料流造成的時間損耗，模擬實驗數據顯示，其值微小、不會大幅影響資料流表現。

The IEEE 802.11 standard, which is the most popular technology in recent years, does not satisfy the needs of multimedia applications and cannot provide the guarantees for quality of service (QoS). As a result, IEEE 802.11e was developed to enhance the shortcomings of the traditional IEEE 802.11 standard. For the poll-based HCF Controlled Channel Access (HCCA) defined in 802.11e, an admission-control algorithm is needed to make decisions on whether or not to admit a traffic stream (TS) based on the stream’s requirements, the utilization of currently-used channel.

In accordance with the current version of the IEEE 802.11e standard, many researchers have proposed their schemes to guarantee the throughputs of multimedia applications and to refine the policies of 802.11e. Some of them give a new perspective on the admission control and the scheduler of 802.11e. However, making decisions, which depends on the information of an access point itself, is not enough for the QoS guarantee for an entire wireless network. For example, in an open space a network operator is eager to deploy sufficient access points to provide users with adequate network coverage and bandwidth. In such a case, all users associate with the access point with the strongest signal may cause the unbalanced channel utilization and poor QoS support. As such, we proposed a new scheme to remedy this problem.

In this paper, a dynamic-assignment scheme, based on the HCCA parts of 802.11e, is proposed for enhancing the performance and channel utilization of a practical 802.11e-based wireless network. This scheme also considers the user priority of each traffic stream. Depending on the priorities of traffic streams and the messages from other access points, the scheduler makes a decision to admit, deny, or re-transmit the request from a mobile station. The simulation results show that our scheme can support more QoS demands of applications and thus increase the total throughputs of the whole network. Besides, the overheads of exchanging necessary control messages are also estimated via simulations. The results show that the overheads generated by our scheme are not significant.