802.11無線網路PCF協定即時傳輸服務之研究

Abstract

在未來的無線網路，不同的即時傳輸服務特性將有不一樣的傳輸要求；如傳輸效率，最大容忍錯誤，延遲時間等等。然而在目前的無線網路像是IEEE 802.11無線區域網路(Wireless LAN)中，並沒有提供一個標準的方法來滿足這樣的即時傳輸服務特性。

IEEE 802.11的媒體存取控制(Medium Access Control)提供非同步及有時間限制的方式在基礎架構(Infrastructure)及隨意架構(Ad Hoc)模式下來傳輸無線網路資料。其中基本的IEEE 802.11無線區域網路的媒體存取控制方式就是使用分散協調式(Distributed Coordinator Function)；它是利用載波感測多重存取及碰撞避免(Carrie Sense Multiple Access / Collision avoidance)的技術，及利用backoff的模式來傳輸無線網路資料。另一個選擇性的IEEE 802.11無線區域網路的媒體存取控制是使用集中協調式(Point Coordinator Function)；它是使用輪詢的方法為基礎。在集中協調式模式下，無線工作站的使用者會週期性被詢問並給予傳輸的機會來傳送資料並且避免無線網路下碰撞的發生。因此，不同IEEE 802.11的媒體存取控制方法有不同的特性，不論是在傳輸效率或延遲需求也都需要更進一步來討論及研究。

我們在這篇論文中提出了改善PCF輪詢表(polling list)的方法，來滿足多媒體在無線網路上的即時傳輸特性。首先在Extended PCF Polling List Method (EPCF)中，它無條件的重新使用剩餘的免競爭週期時間(Contention Free Period)，來避免原有PCF中，浪費在等待最大免競爭週期(CFPMaxDuration)時間到達。在Adaptive PCF Polling List Method (APCF)中，它以EPCF為基礎並且考慮到多媒體在無線網路上的即時傳輸特性。在APCF中，我們利用Association Request中的Capability information field欄位，讓無線工作站透過Beacon Frame通知Point Coordinator提供即時傳輸服務。它重新安排更適合的輪詢表(polling list)並且減少原本PCF中Point Coordinator可能輪詢(poll)到沒有資料要傳送的無線工作站的機率。最後在Advanced PCF polling list method (ADPCF)中，它以APCF為基礎並且考慮到影音資料在傳輸的特性。在ADPCF中，我們利用影音資料在傳輸時會被切割成多個Frame的特性,讓Point Coordinator對同一個無線工作站採取連續性的輪詢(poll)方式，縮短影音資料撥放(playback)的延遲時間。這個方法更進一步的改善原有在PCF上支援即時服務的傳輸能力，並且給予使用多媒體的無線工作站使用者，滿足即時性服務的傳輸要求。為了評估這個方法，我們將它設計在無線網路存取器(Access Point)中,並透過網路模擬器(Network Simulator 2)來分析它的傳輸效率，最大容忍錯誤及延遲時間。在我們模擬結果，使用Advanced PCF polling list method (ADPCF)的無線工作站在即時性服務的傳輸，錯誤率及延遲時間上比原來DCF及PCF無線工作站較有效率。

In future wireless networks, different traffic classes will exhibit a large variety of characteristics and real-time transmission service requirements, such as transmission rate, maximum tolerable error rate and timeout specifications. However, currently there is no standard way of guaranteeing real-time transmission service in wireless access networks like Wireless LAN which is based on IEEE 802.11.

IEEE 802.11 Medium Access Control (MAC) is proposed to support asynchronous and time bounded delivery of radio data packets in infrastructure and ad hoc networks. The basis of the IEEE 802.11 WLAN MAC protocol is Distributed Coordination Function (DCF), which is a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) with binary slotted exponential back-off scheme. Another optional IEEE 802.11 WLAN MAC protocol is Point Coordination Function (PCF), which based on a "polling" access method. PCF periodically polls stations and gives them the opportunity to transmit and thus avoids contention for the channel. Since IEEE 802.11 MAC has its own characteristics that are different from other wireless MAC protocols, the performance of reliable transport protocol over 802.11 needs further study.

In order to provide the real-time services in wireless networks, we propose three methods (1) an extended PCF polling list method. The extended PCF polling list method is used to solve the original PCF which wastes time on waiting for CFPMaxDuration. In this method, we resume the remaining of the contention free period and extend the polling list gratuitously. (2) The adaptive PCF polling list method (APCF) which is based on extended PCF polling list method that consider about a more satisfactory scheme for real-time transmission requirements. In this method, we use a subfield in Association Request management frame of Capability information field to carry the information of transmission requirements on the Beacon frame. (3) An advanced PCF polling list method (ADPCF) which uses a sequence of polling to transmit all of frames which constitutes a picture consecutively. This method is suitable for communications of multi-media streams.

These proposed methods use PCF polling list in an AP that according to transmission rate, maximum tolerable error rate and delay requirements. In order to evaluate the performance, we also introduce an analytical model to compute the throughput, delay, jitter and loss ratio based on NS-2 network simulator.