現有的VoIP（Voice over Internet Protocol）技術主要包含 VoLTE、VoWiFi以及OTT 等三種透過網路傳輸語音的技術。在 VoLTE、VoWiFi 及 OTT 中，由於 OTT 語音服務的免通話費特性，使得 OTT 語音服務相較另外兩者更為受歡迎。此外，手機用戶在使用 OTT 服務時，偏好選擇 Wi-Fi 來使用服務。這是因為大部分的 Wi-Fi 可以免費使用，若使用 LTE 網路來使用 OTT 服務的話，則會被電信商收取封包傳輸量的費用。當手機用戶在一個 Wi-Fi 佈建密集的區域使用 OTT 服務時，手機很有可能會頻繁地在 Wi-Fi 及 LTE 間交遞。若手機從 Wi-Fi 交遞到 LTE 後，在很短的時候內又交遞到另一個 Wi-Fi 底下，在這樣短暫的時間內很可能沒辦法傳輸資料。當在這極短的時間內沒辦法傳輸資料時，從 Wi-Fi 交遞到 LTE 的程序就會顯得不必要及耗電。因此，這篇論文提出延遲交遞演算法，藉由延遲交遞的時間點來避免不必要的交遞程序這篇論文也提出數學分析和模擬來彼此驗證。透過模擬實驗，這篇論文分析延遲交遞演算法的效能並提供延遲交遞演算法的參數設定指南。整體來說，延遲交遞演算法能避免不必要的交遞程序並減少在 LTE 中使用 OTT 服務的時間，進而減少使用 OTT 時的花費。

Voice over Internet Protocol (VoIP) mainly includes Voice over LTE (VoLTE), Voice over Wi-Fi (VoWiFi) and Over-The-Top (OTT). Among VoLTE, VoWiFi and OTT, OTT voice service is more popular than the others because of its free calling fee. Besides, people prefer to use OTT service with Wi-Fi connection since most Wi-Fi is free to use, otherwise, using OTT with LTE connection is charged by the data transmission volume by the operator. When people use OTT in a Wi-Fi intensive area, it is possible that the UE may handover frequently between Wi-Fi and LTE. However, if the time period between when the UE handovers from Wi-Fi to LTE and when the UE handovers from LTE back to Wi-Fi is too short to transmit data, the handover procedure from Wi-Fi to LTE can be considered unnecessary and power-consuming. As a result, this thesis proposes Delay-Handover-Algorithm (DHA) to avoid the unnecessary handover procedures by postponing the timing to handover from Wi-Fi to LTE. This thesis also proposes an analytical model which is validated against the simulation model. With the simulation experiments, this thesis analyzes the performance evaluation of DHA and offers a guideline to configure the settings of DHA. As a whole, DHA can avoid the unnecessary handover procedures and cut down the using time within LTE.

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