一個異質無線網路換手機制之研究

Abstract

近年來各種無線網路技術迅速發展，如WiFi、 WiMAX、LTE，讓使用者隨時隨地可以藉由行動裝置連上網際網路擷取多樣的網路服務。行動裝置的運算能力也變得越來越高並提供多模上網功能，使用者可以藉由多模上網同時連上不同網路，因此造成異質網路間的換手問題。在使用者移動到其中一個無線網路的涵蓋範圍之外時，會造成使用者在該網路的連線中斷。為了解決這樣的問題，研究人員提出許多種方法，如Mobile IP 針對Layer 3的換手提出一種解決方案，但是使用Mobile IP需要先擴充網路裝置的能力，成本較高。另一方面Multipath TCP被提出，它擴充原本Layer 4 TCP的能力，讓使用者的應用程式可以同時用兩種介面啟始TCP連線，當其中一個介面連線中斷時，應用程式可以利用依然存在之另一個介面連線繼續進行通訊，讓使用者感覺不出連線中斷，間接解決了異質網路換手的問題。但是使用者同時用兩個介面發起連線的同時也造成無線資源的浪費。因此Multipath TCP提供了一個選項，讓使用者可以在Multipath TCP連線建立後，只使用其中一個介面的連線，另一個連線當作備援。本論文在NS2的環境下實做Multipath TCP的該選項並提出一個跨階層的方案幫助Multipath TCP在WiFi與 WiMAX間進行換手，並評估其結果。結果證明在大部分的情況下，本方案可以幫助Multipath TCP在異質無線網路下快速並順利地換手。

In recent years, various wireless technologies such as Wi-Fi, WiMAX and LTE have advanced very rapidly. These technologies allow users to access a variety of services provided on the Internet anytime and anywhere using mobile devices. Computation power of mobile devices also becomes very powerful, and able to provide multihoming connection to heterogeneous wireless networks at the same time. Therefore, vertical handover becomes an essential issue. A mobile node moving out of the coverage area of a wireless network will cause disruption of the connection. And Mobile IP technique was proposed to provide IP layer mobility, however its deployment cost is high because Mobile IP needs support of network backend. On the other hand, Multipath TCP is proposed and it extends TCP to make applications able to initiate connections using different interfaces concurrently. If a connection from one of the interface becomes corrupt, a connection from the other interface can still be used to sustain the session of the application. This makes user not aware of the corruption of the connections, and solves vertical handover problem indirectly. But, Multipath TCP causes waste of radio resource because it keeps multiple connections from both interfaces concurrently. Multipath TCP also provides an option which allows a user to direct the traffic flow to one of the interface, while connection from the other interface is just for backup. In this thesis, we implement the priority option of Multipath TCP and propose a cross-layer scheme to support handover between Wi-Fi and WiMAX with Multipath TCP in NS-2, and evaluate the performance and save the radio resource. Results show that our scheme can support very fast handover under heterogeneous wireless network with Multipath TCP in a common scenario.