臺灣博碩士論文加值系統

IEEE 802.11e在MAC層以品質保證為目的，使用了許多的方法來達到這個目標，兩種主要的存取機制為HCCA（HCF Controlled Channel Access）和EDCA（Enhanced Distributed Channel Access）。一般而言，對於即時性的資料流，大都安排以HCCA存取機制下的輪詢方式傳送，此方式是採預約排程並妥善規劃每個站台使用通道的時間，使即時性資料傳送和延遲時間皆可得到確保。至於，非即時性資料則大都以EDCA存取機制的競爭方式去爭取通道使用權，由於競爭方式是以延後（Backoff）機制去避免站台間產生的碰撞問題，此機制必須犧牲時間以減少碰撞機會，這對於強調時效性的即時性資料是較不合適的。
具服務品質保證無線區域網路中，降低FER (Frame Error Rate) 是重要的課題之ㄧ，降低FER最常用的兩個方法就是MAC層的重傳機制以及PHY層的適應性調變。本論針對MAC 層的重傳機制做研究，在HCCA存取機制下，針對即時性訊務流的TSPEC (Traffic Specification) 中的SBA（Surplus Bandwidth Allowance）參數設計進行數學分析、模擬的驗證和效能的研究。SBA參數設定代表著給予訊務流超額頻寬的量，使得訊務流可以發起重傳以降低FER，本論文的數學分析結果可以找出滿足訊務流所需超額頻寬的量，據以算出相對的SBA值。
我們針對CBR (Constant Bit Rate) 的訊務流推導了計算SBA的公式，針對使用者的要求，以我們提供的公式可以求得滿足QoS (Quality of Service) 的SBA的參數值。另外對於CBR的訊務流，我們模擬讓數個SI (Service Interval) 共同使用重傳的機會，讓重傳的機會更有效率的被使用，如此一來在降低整體FER有相當不錯的表現，但是前段與後段間的FER會出現不小的差異，此現象可以適用於具UEP (Unequal Error Protection) 特殊的即時性訊務流上。接著我們以離散時間的馬可夫鏈架構出VBR (Variable Bit Rate) 的訊務流，利用我們在CBR訊務流推導的公式所計算出來的SBA值，套用到VBR的訊務流上，我們可以觀察到VBR訊務流使用重傳機制下的效能。

The goal of IEEE 802.11e MAC layer is to provide QoS-enabled WLANs and it adopts various methods to achieve the goal. Among which HCCA (HCF Controlled Channel Access) and EDCA (Enhanced Distributed Channel Access) are two main access mechanisms. Real-time streaming data are generally transmitted by polling through the HCCA mechanism. The HCCA mechanism guarantees the real-time transmission of data by scheduling the polling sequence of QoS-stations (QSTAs) in advance and allocating the polling time for each QSTA properly. As regards, the non real-time data are generally transmitted by contention through the EDCA mechanism. The EDCA mechanism uses time backoff to avoid collision problems between stations, which is time consuming and not suitable for real-time data delivery.
To reduce FER is an important task in QoS-enabled WLANs. Two frequently used methods are retransmissions in MAC layer and link adaptation in PHY layer. We investigate how to specify the SBA (Surplus Bandwidth Allowance) parameter in the TSPEC (Traffic Specification) of real-time traffic stream under HCCA mechanism. The SBA parameter can be specified by assigning additional number of retries for the application to reduce FER. We study on the relationship between the FER and the SBA for CBR (Constant Bit Rate) traffic stream. The calculated FER performance can be used to specify the required SBA for the traffic stream so that the QoS requirement of the application is satisfied. This thesis also investigates the performance improvement and fairness problem of how to distribute the allocated additional retries among the given transmitted frames if these frames are transmitted across multiple service intervals (SIs). This may cause unequal error performance of transmitted frames within different SIs and the result can be applied to video streaming over wireless with UEP (unequal error protection) data partitions. The analytical result obtained for CBR traffic can also be used to emulate the FER performance of VBR (variable bit rate) traffic over QoS-enabled WLANs. A discrete-time Markov chain model is proposed to generate the number of transmitted frames, not including the retries, within each SI for the VBR traffic. Stochastic simulation has justified the effectiveness of this approach.

中文摘要 --------------------------------------------------------------------------------- i
英文摘要 --------------------------------------------------------------------------------- iii
誌謝 --------------------------------------------------------------------------------- v
目錄 --------------------------------------------------------------------------------- vi
表目錄 --------------------------------------------------------------------------------- viii
圖目錄 --------------------------------------------------------------------------------- ix
第一章 緒論--------------------------------------------------------------------------- 1
1.1 研究動機--------------------------------------------------------------------- 1
1.2 研究目的-------------------------------------------------------------------- 2
1.3 研究方法與內容------------------------------------------------------------ 2
第二章 IEEE 802.11e與HCCA存取機制簡介--------------------------------- 4
2.1 無線網路簡介-------------------------------------------------------------- 4
2.2 IEEE 802.11e基本簡介---------------------------------------------------- 7
2.3 HCCA存取機制------------------------------------------------------------ 14
2.3.1 Traffic Specification （TSPEC） element ------------------------------- 15
2.3.2 HCCA存取機制說明------------------------------------------------------ 19
2.4 Block Ack機制說明-------------------------------------------------------- 21
第三章 數學分析--------------------------------------------------------------------- 25
3.1 前言--------------------------------------------------------------------------- 25
3.2 與SBA的關係---------------------------------------------------------
25
3.3 機率質量函數--------------------------------------------------------------- 27
3.4 與FER----------------------------------------------------------------
28
第四章 系統模擬與數據分析----------------------------------------------------- 37
4.1 前言--------------------------------------------------------------------------- 37
4.2 啟始系統設計--------------------------------------------------------------- 37
4.3 系統模型--------------------------------------------------------------------- 39
4.4 CBR訊務流重傳機制模擬----------------------------------------------- 39
4.4.1 CBR訊務流單SI重傳機制模擬與數據分析------------------------- 39
4.4.2 CBR訊務流多重SI重傳機制模擬與數據分析---------------------- 43
4.5 VBR訊務流重傳機制模擬----------------------------------------------- 45
4.5.1 產生VBR訊務流---------------------------------------------------------- 46
4.5.2 VBR訊務流重傳機制模擬與數據分析-------------------------------- 48
第五章 結論與未來研究方向------------------------------------------------------ 50
5.1 結論--------------------------------------------------------------------------- 50
5.2 未來研究方向--------------------------------------------------------------- 50
參考文獻 --------------------------------------------------------------------------------- 52

參考文獻
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[6] S. Mangold, “IEEE 802.11e Wireless LAN for Quality of Service,” in Proc. European Wireless ’02, Florence, Italy, Feb. 2002
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