隨著無線通訊技術與智慧行動裝置的快速發展，智慧行動裝置的網路流量日益劇增。近年來，由於VoIP之技術成熟，各行動平台紛紛推出創新之通訊軟體，運用通訊軟體達到資訊傳遞已成為當今主流。眾多通訊軟體中，以SIP協定之應用最為廣泛，而一般情境下，SIP伺服器與用戶端之連接需透過網際網路，導致用戶在無法連網之情境下VoIP將無法使用。

本研究提出一VoIP系統架構，整合了SIP伺服器與用戶端於同一智慧行動裝置，使用戶端具有擔任伺服器之能力，並於內網中建立一內部通話群組，達成在無法連網之情境下，VoIP系統仍能透過內網維持運作。此外，本研究亦提出一伺服系統選擇機制，能於內部通話群組中挑選擔任伺服器之最佳人選，透過伺服器選擇機制能有效提升群組之整體連通性，並提供群組更佳的服務品質。根據實驗結果顯示，相較於非定期選擇與隨機選擇機制，本研究所提出之伺服系統選擇機制能分別有效提升群組整體連通時間達63%及28%。

本研究基於擴大群組的通訊範圍，模擬了一Wireless Mesh Network環境，分別於內網下之區內通話、區間通話與連網VoIP通話進行實驗，比較其平均延遲時間與封包遺失率。根據實驗結果顯示，區內通話提供了最佳的通話品質，區間通話則隨著節點跳躍數增加，導致平均延遲時間與封包遺失率隨之增加，進而造成通話品質下降。相較於連網VoIP通話，內網下之區內與區間通話，仍能提供較好的通話品質。

With the rapid development of wireless communication technology and mobile devices, network traffic on smart mobile device has increased gradually. Since VoIP is a well-known and mature technology, many communication applications are follow the SIP standard. In a conventional VoIP system, the SIP server is usually implemented on a remote server and connected to an IP network. Therefore, VoIP is unavailable in specific area with a limited Internet connection.

This work proposes a VoIP system architecture that involves the implementation of SIP server and client on a smart mobile device. Therefore, every user can play the roles of both SIP server and client, and users can form a self-organized group in a WLAN to solve the problem of the unavailability of VoIP in specific area with limited Internet connection. Moreover, this work also proposes a Server Selection Mechanism (SSM) that evaluates the capacity of each smart mobile device to select the most appropriate user to act as the SIP server. The Server Selection Mechanism enhanced the period of whole group connectivity and provided better service for group users. A comparison reveals that the Server Selection Mechanism increase the period of group connectivity to 63% and 28% over those achieved using the non-regular select and random select mechanisms respectively.

To scale up the coverage area of the group, Wireless Mesh Network environment for VoIP communication is simulated. Communication is categorized as intra-area communication in a WLAN and cross-area communication in a WLAN, and conventional VoIP communication. Intra-area communication has the lowest delay time and packet loss rate, and latter increases with the number of hops in cross-area communication. A comparison with conventional VoIP reveals that the intra-area and cross-area communications in the WLAN are of better quality.

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