私有電信網路的回電機制

Abstract

私有電信網路系統如企業電話系統和網路電話已經廣泛被建置，因為私有電信網路系統有許多優點如：省錢、加值服務等。近來因為網路電話相對於傳統電話的一些優點，很多企業將其內部的傳統交換機更新為網路電話交換機(IP PBX)。私有電信網路系統有一個相當大的限制即缺乏回電機制。無論是否使用DID服務，當PTN話機經由VPG或PBX打電話到PSTN，只能使用VPG或PBX之代表號當來電顯示號碼，所以PSTN受話端無法用來電顯示號碼回電找到PTN發話端，可能造成回電失敗。於論文中我們提出一個回電表格(callback table)的方法解決這個問題，這是一個不影響現有通話建立訊息流的方法。我們並將回電機制實作於系統中。我們設計幾個基本呼叫狀態模型(BCSMs)並實作於整合性呼叫代理程式(ICA)中來整合SIP和MGCP兩種協定。ICA是一個網路電話閘道控制器用來控制網路電話閘道器。所以此系統可使網路電話與傳統電話或手機能互通。此系統是softswitch的架構，因此擁有許多優點。此系統亦可當網路電話交換機使用。在整合性呼叫代理程式中我們亦加入SIP註冊機制來支援移動性。所以此論文除了提供網路交換機的設計外並加入回電機制與支援移動性。

Private Telecommunications Networks (PTNs), such as enterprise telephony and VoIP network, have been widely deployed in the recent years because they provide many advantages, such as saving money, and enhanced call services. One limitation of PTNs is lack of support for callback service. When a PTN caller makes a call to a PSTN user, it is not possible to use the PTN caller’s identity as the caller ID; instead that of the PBX or VoIP gateway (VPG) is used. Therefore, the PSTN callee cannot retrieve the caller ID information and call back the PTN caller later. This thesis presents a callback table approach to resolve this problem. We describe how the caller and callee information are stored into and retrieved from the callback table to provide callback service. Our approach is a plug-in solution that doesn’t affect the existing message flow. The design has been implemented in a MGCP-based system. We have developed BCSMs (Basic Call State Models) and implemented them in an integrated Call Agent (ICA) to interwork SIP and MGCP. The ICA controls VPGs to enable communications between SIP phones and POTS phones. This solution also can be used in IP PBX. Moreover, to support SIP phone user mobility, the ICA is also a SIP Registrar.