動態VoWLAN傳送速率的估計與調整

Abstract

當在具有移動性的無線網路傳輸環境之下，一直維持相同的傳輸速率不是隨時都適合的。太快的傳輸速率會導致封包遺失率攀升，太慢的傳輸速率則會讓單一個客戶端裝置占據過久的媒體使用時間導致其他用戶端無法傳輸資料。大部分的傳輸速率調整演算法都是考量以傳送的頻寬當作主要的衡量標準，但是像VoIP這類的應用軟體的需求和特質卻沒有被納入考量。因此在本篇論文中我們將會找出一個能配合VoWLAN的需求在有移動性的環境之下的傳輸速率調整演算法。我們除了使用mean opinion score (MOS)來當作衡量聲音傳輸品質的標準，我們還用medium consumption (MC)來衡量介質被占據的時間。MC是計算一個用戶端在完成其VoIP傳輸時的總時間中占據的百分比。所以我們提出的演算法Voice Quality based AutoRate (VQAR) 來針對每個封包選擇可以有最小的MC且MOS在一定的品質(>3.5)的傳輸速率。從我們模擬的結果，ARF/AARF、 VQAR和RBAR都可以達到所需要的MOS值，但是VQAR的MC分別比起ARF/AARF 和RBAR低11%和27%。

When transmitting VoIP traffic over wireless medium under mobility conditions, using constant rate for transmission would not be suitable. Rates too fast would result in massive packet loss; rates too slow would result in high channel occupation by a single station. Most research on rate selection only considers throughput, but the characteristics of applications, such as VoIP, is not considered. In this paper, we estimate an adapt transmission rate for VoWLAN with mobility. Besides mean opinion score (MOS) for voice quality evaluation, we introduce a measure, medium consumption (MC), for channel occupation. MC is the percentage of channel time occupied by a station during its VoIP connection. We propose Voice Quality based AutoRate (VQAR) to select per packet transmission rate that minimizes MC while achieving the required MOS (> 3.5). From our simulation results, all compared methods ARF/AARF, VQAR and RBAR meet the required MOS, but VQAR has lower MC than ARF/AARF and RBAR by 11% and 27%, respectively.