一個無線區域網路上之高效率媒體串流輪詢機制

Abstract

IEEE 802.11e標準是為了增進無線區域網路上的語音和多媒體服務品質(QoS)而制定。它在HCF控制型通道存取(HCCA)模式中提出一個參考排程器來管理輪詢的順序。然而，這個排程器在大量多媒體資料傳輸負荷下，可能會大大的降低傳輸量及增加延遲時間，這是因為它所使用的輪詢機制可能造成一些不必要的傳輸額外負擔。為了要減少傳輸時間的延遲以及輪詢的額外負擔，在這篇論文中，我們提出了一個高效率輪詢機制來提升HCCA模式的服務品質。這個輪詢機制是基於每個工作站下一次被預估輪詢的時間的間隔，因此我們提出的策略是每次都以最早預估資料抵達時限的工作站為優先，以改善原先輪流輪詢機制可能遇到被輪詢的工作站無資料可送時，所造成頻寬的浪費。同時，由於一個無線存取點所支援工作站的數量是有限的，我們利用了最直接的線性搜尋方式以找出下一個被輪詢的目標，如此大大地降低了實作上的複雜度。此外，為了可以更正確地偵測出工作站的資料流間隔，這個新的機制利用工作站回傳的資料訊框中QoS欄位，讓混合式協調者可以更準確的調整下次輪詢的時間。而且新的機制亦會根據語音資料的特性，動態地調整傳輸資料流的服務間隔。從模擬的結果可以清楚的看到，我們所提出的方法與標準802.11e輪詢方式相比可以有效地減少傳輸延遲時間與抖動率，並提升整體傳輸量。

IEEE 802.11e specification was established for improving the quality of service (QoS) issues in voice and multimedia over wireless LANs (WLANs). In the HCF Controlled Channel Access (HCCA) mode, it offered a reference scheduler that maintains a polling list. However, under a high load of multimedia transmissions, this reference scheduler may encounter decreasing of throughput and increasing of latency. This is because the polling scheme it used usually causes some overhead in doing unnecessary polling. In order to reduce access delays and polling overhead, as well as to improve the throughput of the channel, in this thesis, we proposed a highly efficient polling scheme to provide better QoS in the HCCA mode. Our polling scheme is based on the intervals in estimated polling time of each traffic stream, for the reason, our strategy is to poll the station with earliest-due-date-first discipline, instead as in the round-robin polling of reference scheduler. Our scheme minimizes the waste of bandwidth when the polled stations have no data frame to send. In the mean time, due to the limited number of client stations under the coverage of an access point, we use the linear search to find the next target to be polled, so that the implementation complexity is greatly reduced. Besides, for precisely detecting the dataflow interval for traffic steams, we use the QoS control field of data frame sent from stations, Hybrid Coordinator (HC) could use this collected information to adjust the estimated polling time. Further, our scheme could dynamically adjust the service interval of voice transmission according the characteristics of voice flow. The simulation results show that our proposed scheme can reduce transmission latency and jitter as well as total throughput in comparing with the reference polling scheme in the IEEE 802.11e standard.