在IEEE 802.16 無線通訊標準中,802.16d是以提供固定式的無線城域連接服務。而在隨後所推出的IEEE 802.16e 是為支援可移動性的服務所產生的。在支援可移動性服務的狀況下,換手技術(handover)的開發乃為其中的關鍵。因為使用者是移動的狀態，所以跟基地台的訊號強度會有所改變。若訊號強度不足以維持目前通訊的要求，那麼使用端必須要向鄰近的基地台請求連線。但是此過程必須花費一些時間。而使用者若是有及時性質的連線，就有可能會造成不可避免的封包遺失，甚至是會中斷此連線。所以如何縮短換手所需的時間，與確保資料的及時送達，才是本論文的重點。在本文中，利用結合SIP (Session Initiation Protocol) ,IP層和IEEE 802.16e的MAC層作連結,採用事先取得IP位址(Pre _ DHCP)及資料傳送(data forwarding)跟限定MAC層的啟動時機等方法，來降低換手所需的時間，與確保資料的及時性。

IEEE 802.16 Wireless MAN aiming to broadband wireless access (BWA) through the standardization of IEEE 802.16e supporting
mobility on existing fixed WirelessMAN systems(IEEE 802.16d)。 Because the mobility, a mobile subscriber station (MSS) maybe proceed handover process when it moves to another base station (BS). Therefore, the MSS is not able to send or receive the data during handover process and these data should be delayed or dropped for real-time service。 In this paper, we use Pre_DHCP , Data Forwarding and the MAC Layer start condition to decrease the handover process time ,so we could make the data would not be dropped by over delayed。

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