一個在無線網路上之電子郵件傳輸系統

Abstract

隨著無線上網越來越普及，網際網路上最多人使用的電子郵件，也開始擴展到無線網路的應用上。目前市面上已經有很多具有無線上網能力的機器，其作業系統裡皆附有電子郵件軟體，讓使用者可以直接收發網際網路上的電子郵件。本文將針對目前最為流行的無線上網方式：GPRS(General Packet Radio Service)與PHS(Personal Handyphone System)，分析其特性與有線網路的差別，然後探討現有的電子郵件傳輸協定，指出其在無線環境下運作的缺點。接著，我們將提出一個全新的無線電子郵件傳輸協定 (Wireless Mail Transfer Protocol, WMTP)，來解決這些問題，它不止是適合在無線網路下運作，並且適用於任何的上網機器，如手機、PDA(Personal Digital Assistant)、個人電腦等。但是為了要與網際網路上的電子郵件系統相連，我們將以WMTP為基礎，設計一個郵件閘道器，讓WMTP與既有的郵件協定相容，而不需要重新改寫以既有協定為基礎的郵件伺服器，如此可以就地解決這些通訊協定在無線環境下的缺點。最後我們將實作出整個系統，並且利用GPRS無線上網來實際測試整個架構的運作，並與舊協定比較相關的測試數據，來驗證我們的設計是否正確，最後結果正如我們所料，WMTP的確是可以大幅改善電子郵件在無線環境下的傳輸效率。

It's been widely accepted that email is one of the killer applications on internet. The same phenomenon seems will happen again in wireless world. There are presently a lot of wireless communication devices which have built-in email client software on the market. People can directly receive emails from the Internet via these devices. In this paper, we examine the characteristic of GPRS(General Packet Radio Service) and PHS(Personal Handyphone System) which are popular recently, and point out the difference between the wireless communications network and the wired network. We also analyze the present email protocols, point out the drawback of them on the wireless communications network. To overcome the drawback, we propose a all new email transfer protocol, WMTP(Wireless Mail Transfer Protocol). It is suitable for the wireless operating environment. Additionally, it supports all sorts of devices, e.g. cellular phone, personal digital assistant, notebook, etc. In order to interconnect with the existing email systems on the Internet, we also design a mail gateway base on the WMTP. The mail gateway make the WMTP compatible with the existing email protocol which is adopted by most email server in the world. Finally, we have implemented a wireless email transfer system base on mail gateway. We also test the operation of the system via GPRS communications network, and collect the statistics about efficiency. Through experiments, we observe that our system provides significant improvement over existing system for real-life user accesses for email transfer on the wireless communications network.