多網行動語音裝置之跨網換手機制的設計與實作

Abstract

隨著行動通訊的普及所帶來的便利性，行動通訊的語音與資料傳輸服務需求亦日益增長。許多新世代的無線通訊規格陸續被制訂出來，以期望能達到更好的語音通話品質及更快的資料傳輸速率。在多種行動通訊網路並存的情況下，行動裝置亦在此時蓬勃發展，提供更強大的通訊功能。

綜觀市面上之可攜式行動裝置，已有許多同時支援兩種或兩種以上的行動通訊規格。以支援GSM與PHS的雙模手機為例，雖然雙模手機標榜可以解決攜帶兩隻手機的問題、增加處於服務區的機會（GSM和PHS服務區的連集），但依然存在幾個重大的問題。首先，雙模手機因同時開啟GSM及PHS雙系統，因而造成雙模手機電池耗盡速度變快。其次，以現今文明社會人們更加注重科技進步對人類所帶來的負面影響，GSM手機平均通話中人體電磁波吸收比遠高於PHS的情況下，如何在雙模手機上有效的減少人體對電磁波的吸收更顯得重要。再者，雙模手機即使同時支援GSM及PHS雙系統，但還是無法避免GSM所建立的通話從進入PHS單網路環境（例如：醫院或高科技廠房）時通話的中斷，相同的情形也會發生在PHS通話建立後移至GSM單網路的環境。最後，在使用GSM與PHS雙模手機時，使用者必須分別付費給GSM及PHS系統業者，如何有效率的使用GSM/PHS雙模手機以減少使用者所需負擔的費用亦成為一項重要的議題。

為了解決上述問題，我們設計並且實做了一套「跨網換手機制」，透過GSM及PHS的「話中插接」、「來電顯示」及「電話簿」的功能達到GSM/PHS系統的交互換手。針對GSM/PHS雙模手機上電池耗電以及電磁波對於人體產生的影響，我們提供了低功率優先使用的換手策略。對於超出服務區造成斷話的問題，我們提供了自動偵測可用網路的策略，將連線快速換手到可用網路，以盡量縮減斷話時間。針對最後一個付費的問題，我們提供使用者優先使用網內互打，實做出低費率優先的換手策略。除此之外，在通話中我們還提供使用者以手動方式啟動GSM/PHS的交互換手功能，使用者可根據個人喜好將通話切換到個人偏好的系統。

加入跨網換手機制之行動裝置將能更有效率的支援不同的行動網路系統，並提供使用者更簡易、更健康與更方便的語音操作服務流程。

As mobile communication is getting more and more popular, the demand of voice communication and data transmission over cellular networks has grown up rapidly. In order to provide better voice quality and larger data transmission, many specifications of cellular networks have been defined in recent years. Different cellular network technologies offer different merits, such as in terms of coverage area, power consumption, and cost. As a result, a mobile phone may be equipped with multiple wireless network interfaces so that a user can use a preferred wireless network without carrying multiple handsets.

However, the selection of a preferred interface at different network environments is still a research challenge that involves many critical issues. In this thesis, we focus our discussion on GSM and PHS dual mode handsets. First, the standby time of a dual mode handset is shorter because it enables both GSM and PHS modules at the same time. Second, PHS has a Specific Absorption Rate (SAR) smaller than GSM; how to reduce the effects of SAR on humans is also an important issue. Third, each network may have its own applicable areas; for example, some institutes may inhibit the use of GSM but not PHS because GSM may interfere with electronic equipments. On the other hand, GSM has a larger coverage area than PHS. Finally, each network has its tariffs and how to reduce the cost for user is also an important issue.

With the above issues in mind, we designed and implemented an Inter-System Handover Platform for Multi-Mode Mobile Phones. In the platform, we use “Call Waiting”, “Caller ID” and “Phonebook” to achieve Inter-System Handover between GSM and PHS. We also provide a low power preferred handover strategy to increase the standby time of dual mode handset and reduce the effect of SAR on humans. Besides, we also implemented a network status detection scheme to setup a call automatically by the other network once the current network is not available. Furthermore, we also adopt intra-network first handover strategy to reduce the tariff of calls. Finally, we also provide hot-key for users to select a preferred network manually in the middle of calls.

We have implemented the Inter-System Handover Platform on an off-the-shelf GSM/PHS dual mode handset. The experimental results show that the Platform, indeed, can handover a call from one network to the other very effectively, and users is not perceivable of the network switching.