隨著網際網路服務的進步以及智慧型裝置的普及，愈來愈多人開始使用網路通話軟體取代一般電話，線上語音通訊服務顯然成為了現今熱門的討論話題。然而透過網路的語音通訊服務為了維持良好的通話品質，需要使用大量的封包流量。雖然電信業者提供了無限流量的資費專案，但並非所有民眾皆會選擇此種專案。如何在不耗費大量流量的前提下，有效提升網路通話的品質及穩定性，成為了一項重要的議題。
為了解決語音通話所帶來的流量問題，我們設計了一套雲端橋接交換機(Cloud Bridging Switch, CBS)。此交換機以雲端架構為基礎，能依需求動態分配伺服器的負載平衡。更重要的是，CBS能將IP電話與公共交換電話網路結合，僅在初始通話的階段需要使用極少的網路流量。在通話階段時，所有語音封包皆透過公共交換機傳輸，故使用者不須支付昂貴的行動網路費用。除此之外，由於語音不經行動網路而係由伺服器直接傳遞至PSTN，相較傳統IP電話通話品質及穩定性將大幅上升。
在此篇論文中，我們建立了一套雲端伺服器以提供符合SIP協定的語音通訊服務。此套伺服器展現了雲端架構高擴展性的優勢。我們並且設計了一套可運行於Android移動平台上的Application，透過此App可存取雲端服務，以極低的流量進行一對一或是多對多的網路通訊。

With the advancement of the Internet services and widespread of smart devices, there are many people starting to replace public switch telephone with VoIP software. Nowadays, voice over the Internet apparently becomes a highly discussed topic. However, in order to maintain a good conversation quality, it must cost a lot of bandwidth to use VoIP. Although Internet service providers provide billing plan with unlimited network access, not all people choose this kind of payments. How to improve the quality and stability of Internet conversation without costing a large amount of bandwidth has become an important issue.
To deal with the bandwidth problem, we introduce Cloud Bridging Switch, CBS. Cloud Bridging Switch which bases on cloud architecture is loading balanced dynamically according to demands. Moreover, CBS can bridge VoIP and PSTN, and it only costs very little bandwidth when initializing the call. All the audio packets in the call are delivered through PSTN, therefore users need not to pay the extra fee to the service provider. In addition, since the packets do not rely on the mobile network, the quality and steadiness of call are much better than of traditional VoIP software.
In this thesis, we build a series of cloud servers to provide SIP based voice services. The servers feature the high flexibility of cloud architecture. We also developed three Android application programs which can access to the cloud service and make one-to-one or many-to-many calls at low cost.

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