IEEE 802.11無線區域網路中VoIP協定之允入控制設計與實作

Abstract

無線網路的應用隨著技術的成熟以及市場的需求越來越普及，透過網路撥打電話不是有線網路專屬，我們也可透過無線網路撥打網路電話。我們雖然可以在WLAN裡透過存取點撥打網路電話，但此並非一項有品質保證的服務。在有限的頻寬下，若一存取點負荷過量，則透過此存取點的無線網路使用者將遭受到嚴重的封包延遲，封包遺失率增加，進而降低網路的產能。由於IEEE 802.11原先是針對傳輸無線的資料封包而設計，因此面對有即時性要求的語音服務時有了重大的考驗。不能在無線網路上適時地傳輸即時性封包不但沒有達到使用者即時性的要求，同時增加了無線網路環境中的不必要的負荷。如何能夠在眾多不同種類的無線網路封包傳輸中保障即時性語音服務一向是產業界及學術界重要的研究方向。本論文中，提出了一套允入機制，即時地監控並動態設定允許進入此無線網路中使用者的個數。新語音服務使用者在啟用服務前能夠知道網路品質，同時透過點對點模擬語音傳送測試準確地預知網路狀況是否能夠負荷新的通。這個機制不但可以保障既有使用者語音服務的品質，也確認新使用者啟用語音服務後網路也能提供品質服務。

Voice over Internet Protocol (VoIP) is no longer only feasible in wired networks, but also applicable through wireless networks as prevalence of wireless networks grows with the technology maturity and rising demand. While making a VoIP call over WiFi wireless networks is applicable, this is not a service with guaranteed quality. With limited bandwidth, an overloaded AP may cause wireless users to experience delayed packets, high packet loss rate, and even low throughput. IEEE 802.11 wireless Local Area Network (WLAN) was originally designed only for data traffic transmission, and therefore deficiency in real-time traffic transmission, such as VoIP applications, is inevitable. Not being able to timely transmit real-time packets not only fails to meet the demand of users, but also increases unnecessary loading in the wireless network. Industries and academies have made lots of efforts on providing QoS for real-time voice traffic which usually coexists with other classes of traffic. In this thesis, a call admission control mechanism is proposed which not only timely monitors the wireless networks, but also dynamically adjusts the number of admissible clients to wireless networks. New VoIP users not only know the network condition before entering it, but also accurately predict transmission quality through sending probing packets. With the mechanism, existing VoIP users in the wireless networks have their service protected, and potential VoIP users can expect good quality of service once admitted.