基於無線區域網路之語音服務的快速換手機制

Abstract

隨著無線區域網路〈IEEE 802.11-based wireless local area network〉的普及，吸引著人們在其環境上部建許多創新的應用。而其中又以無線語音服務〈VoIP over wireless LANs〉最是受到注目，我們也想信此服務將有可能成為在無線區域網路上的一個非常重要的殺手級應用。然而在無線語音服務獲得廣泛使用前，我們必須仔細的檢視無線區域網路對於即時通訊應用的支援上，提供了那些不同的特性。無線區域網路的主要目的在於提供一個無線寬頻的資料存取途徑，而行動換手的議題並非其主要的考量，所以它並沒有提供相關的機制，來達成像個人通訊系統〈PCS〉一樣的無接縫換手效能。 隨然有許多關於如何在無線區域網路上減少換手延遲的研究，但它們很少考慮到如何在進行換手期間，確保在網路層或是應用層的連線不至於中斷。本篇論文檢視無線語音服務在換手時所遭遇的問題，提出一個有效率的換手策略並整合各個層面對於換手機制的支援。此篇論文主要目的在於針對無線區域網路發展一個快速換手機制，最後並將在一個即時的網路語音運作環境裡，對此設計執行實際的驗證。

The increasing popularity of IEEE 802.11-based wireless local area networks (LANs) attracts people to deploy innovative applications over wireless LANs. Among those applications, the most exciting is voice over IP (VoIP), and we think it may become an important killer application for the wireless LANs. However, to make VoIP over wireless LANs become widely used, we must thoroughly investigate the characteristics of wireless LANs in the aspect of real-time communication support. Wireless LANs aim at providing a wide bandwidth wireless link to access data networks, and the handoff issues are not the primary concern; so, wireless LANs can't support seamless handoff as the PCS does. There has been much research on how to reduce the handoff delay in wireless LANs, but very little consider the topic about the network and application layer connections during the handoff. In this thesis, we investigate the VoIP handoff problem in wireless LANs, and propose an efficient handoff scheme which integrated handoff technologies of various layers, including selective active channel scanning, system channel map, IP pre-assignment, transport layer fast handoff, and SIP mobility support. Our goal is to develop a fast handoff mechanism between access points (APs) of IEEE 802.11-based wireless LANs. In addition, the handoff design has been evaluated on a real-time VoIP test-bed.