異質行動計算環境下基於需求之換手策略

Abstract

行動計算的目標是無論何時何地都能提供使用者無所不在的資訊服務。在異質環境中，目前依然沒有一種有效的方法可以提供行動主機無接縫式地換手到一個最好的網路上。在本論文中，我們一開始先回顧一些現存於無線網路的換手及資料重新遞送的方法。在異質行動計算環境下，我們提出一個基於使用者需求的換手策略，以期行動主機透過一個最合適的平台來存取網路。這種跨平台的換手策略需要在各種系統的網路承載率、傳輸速率、移動率及電力耗損之間作取捨。我們的方法整合幾個曾經應用於公眾地面行動網路中的策略: 分割、區域主播及二段式換手。本設計的主要構想是依使用者對網路的需求，如頻寬、移動率及延遲，來選取適當的網路平台。當一個行動主機向一個遠端代理者註冊時，該遠端代理者會依使用者的需求層次，決定是否需要向上層主要遠端代理者註冊或是直接服務該行動主機。藉由主要遠端代理者在近端的協助，我們可以預先傳遞使用者資料到下一個遠端代理者，以減少換手時所造成的傳輸延遲。我們的方法提供行動主機一個更有彈性的方式去選取一個適合的行動計算環境，而且比其他方法使用更少的網路頻寬。模擬結果顯示，我們的方法可以有效地降低垂直換手的次數，及在連線時間不長的情況下，可以減少連線費用。

Information anywhere is the vision of mobile computing. There is still no feasible method to seamlessly handoff a mobile host (MH) to the best network at anytime and anywhere in heterogeneous environments. In this thesis, we first review existing handoff and data rerouting schemes used in wireless networks. We propose a requirement-enable handoffs scheme in heterogeneous mobile computing environments to fit a connection with a right platform, and to make tradeoff among network characteristics, such as network capacity, transmission speed, mobility, and power consumption. We incorporate the partitioning, local anchoring, and two-step handoff schemes which have been used in public land mobile network (PLMN), in our approach. The basic idea of our design is to choose a right plateform by user's requirements, such as bandwidth, mobility, and latency. When an MH registers to a foreign agent (FA), the FA would decide to register to an anchor FA at the next higher level, or to serve the MH directly. In addition, with the help of anchor FA, we can pre-route user data to next FA to reduce handoff latency. Our approach provides a more flexible way for an MH to select a suitable mobile computing environment, and uses bandwidth more effective than other approaches. Simulation results support that our approach can reduce the number of vertical handoffs, and also can reduce the connection fee when user connection time is not too long.