在無線網路第二層及第三層之換手模式

Abstract

近年來由於無線網路的流行，許多城市建立綿密的無線網路環境，使得無線網路漸漸深入人們的生活。其中, Voice over WLAN正是其中一個應用，因為大部份的應用都是使用VoIP的機製，因此，如何處理在不同基地台及網段漫遊，通話仍然要存在，就是當今無線網路廠商最大的課題。 在本篇論文中，我們提出一種機製，使得廠商在使用無線網路模組時, 不需考慮漫遊的問題，無線網路模組會負責解決Layer 2及Layer 3的漫遊．首先，我們在無線網路模組中利用Semi-Soft的方法建立連線，使得裝置在進行漫遊之前，可以預先交換Layer 2及Layer 3的訊息，另外，我們還在無線網路模組中建立NAT的轉換，使得裝置上層取得的網際網路位址固定，應用程式不需要因為更改網際網路位址，而必須重新建立連線． 雖然我們採用Semi-Soft的模式可以預先交換Layer 2及Layer 3的資訊，不過，因為在實體上，仍然只有一組硬體，也就是說同一時間只可以連接一台AP，因此，當封包週期小於20ms時，對漫遊就會產生延遲的現象．不過，如果我們的應用專注在VoIP上，對於封包週期大於50ms的VoIP機製而言，這就足夠了．

As WLAN gets more and more popular, many cities have been set up the dense wireless network. Therefore, WLAN have gradually joined to people’s life. One of the application is Voice Over WLAN. However, most of the application are using the structure of VoIP. Hence, it is very important and indispensability for WLAN industry to overcome how to keep the network stable and how to roam between two base stations or different networks. In this paper, we will bring up a kind of structure. That will help the wireless LAN company to solve the roaming problem. The wireless LAN module will solve the roaming in Layer2 and Layer3. First of all, we build up the connection by Semi-Soft in wireless LAN module that could help the message exchange in advance in Layer2 and Layer3 when the module starts roaming. In addition, we also create the NAT transformation in WLAN module. It could fix the up layer’s network IP and the application function does not need to change network IP for rebuilding network. Although we can use Semi-Soft to exchange the information in Layer 2 and Layer 3 beforehand, we just have one real hardware module. It means we just can connect only one AP at the same time. For this reason, when the packet cycle is less than 20ms, the roaming will have delay problem. Nevertheless, if we focus our application on VoIP which packet cycle more than 50ms, the idea is sufficient.