IEEE 802.11s無線網狀網路的位置管理與流暢換手機制

Abstract

本論文針對以Proxy Table為路由基礎的802.11s無線網狀網路 (Wireless Mesh Network)提出一套位置管理與流暢換手機制。802.11s WMNs採取以Proxy為基礎的layer-2路由機制，並藉由Proxy Table作為位置追蹤的依據。本機制僅需使用少量的控制訊息來維護Proxy Table以達到位置追蹤 (Location Tracking)，並且能以更有效率的封包轉送方式來實現流暢換手 (Smooth Handovers)。 在802.11s WMNs中，Mesh節點需要參照Proxy Table才能對行動裝置作定位，也因為如此，從WMNs外部進入的網路封包才得以正確從傳送端到達目的地。但是Proxy Table的維護不易，對目的地Proxy的查找可能會產生誤失或是過期的問題，再加上沒有明確的平順換手機制，所以802.11s WMNs對於行動性的支援更顯的困難，雖然802.11s WMNs可以透過AODV (Ad-hoc On Demand Protocol)的程序來解決對目的地Proxy發生查找誤失的問題，不過AODV可能會產生成本相當高的廣播訊息；而透過封包轉送方式雖然可以達到平順換手，但是卻有可能衍生出三角路由 (Triangular Routing)或是轉送路徑過長等問題。 目前各研究所提的位置管理與流暢換手機制，都是為了達到位置追蹤或是完成封包轉送，其中位置追蹤的方法又可分成集中式及分散式兩種。集中式的位置追蹤方法通常會有一個特定機器 (位置伺服器)紀錄所有行動裝置的位置，行動裝置可以直接或是間接的向位置伺服器執行位置註冊的動作，這種方式的缺點是缺乏彈性，在建立連線時可能都必須向位在他處的位置伺服器進行詢問的動作。相對的，分散式的位置追蹤方法則是把紀錄行動裝置的表格分散在各個網路節點上，雖然建立連線時可能可以免去詢問的動作，但是維護這些分散在各處的表格通常會採用廣播的方式進行位置的更新，需要花費更大的成本。此外，為了解決封包轉送產生的三角路由問題，大多數的研究方法都會選擇直接對來源端進行位置更新，但是無線的傳輸過程卻可能導致在更新過程的延遲或是更新失敗。 本論文提出一套有效率的解決方案包含三種機制: (1) 兩層式Proxy Table架構、(2) Proxy的解析及更新機制、(3) 逐步最佳化路徑轉送機制。兩層式架構的Proxy Table: 利用Location Registrar (LR)集中處理行動裝置的位置註冊並搭配各Mesh節點的Proxy Table達到位置追蹤的目的，LR具備Proxy的解析及更新機制，除了能正確解析出目的地Proxy還可以對來源Proxy進行更新，而逐步最佳化路徑轉送機制能提供更好的封包轉送方式，此外，由於Wi-Fi僅支援在相同ESS (Extended Service Set)的換手行為，為了能支援跨越不同ESS的換手行為，我們還設計了動態位置更新機制來輔助。 本論文對於所提出的方法，透過目前學術界廣為人知的網路模擬器NS2進行模擬實驗，並分別以AODV還有行動IP (MIP)、Ant做為比較對象。在位置追蹤的實驗中，我們發現無論是建立連線的延遲時間，或是所需要的控制訊息量，AODV都明顯比我們的方法多上許多；在封包轉送的實驗中，若以換手過程中封包經過的路徑長度來看，MIP與Ant皆固定且高於我們的方法，此外，我們的方法在更新過程的延遲時間及封包的遺失率的表現都優於其他兩者。

In this thesis, we proposed a Location Management and Smooth Handover Scheme with Proxy Table-based Routing Mechanisms for the 802.11s wireless mesh networks (WMNs). Unlike other WMNs, 802.11s WMNs adopts proxy-based layer-2 routing mechanism, and can track the locations of mobile stations based on Proxy Tables situated at Mesh Access Points. Although several location management or smooth handover techniques for WMNs have been proposed in the literature, none of them have tailored their proposal for 802.11s WMNs. They either adopt layer-3 routing or layer-2 routing without considering proxy tables. In 802.11s WMNs, a Mesh node needs to lookup its Proxy Table to locate the proxy (MAP) that are serving a designated mobile node (MN) before it routes the packets to the MN. However, it is difficult to maintain up to date proxy table entries, and the destination serving Proxy lookup may fail or find an out of date serving proxy. Although the Proxy Table miss problem can be resolved by Ad-hoc On Demand Protocol (AODV) procedure, AODV may introduce excessive broadcast messages in searching the MN. In addition, 802.11s WMNs does not specify how to accomplish smooth handovers when MNs change their locations. Smooth handovers could be accomplished by packet forwarding in 802.11s WMNs to achieve smooth handovers. However, packet forwarding may result in a triangular route or a lengthy forwarding path. Even if the triangle routing problem may be resolved by sending location updates to the source proxy directly, location updates via wireless links may take too long or even fail. Therefore, in this thesis, we proposed a location management and smooth handover scheme that utilize the proxy tables and layer-2 routing mechanism to reduce the overhead of location management, and achieve better packet forwarding for smooth handovers. Location tracking methods can be classified into two categories: Centralized and Distributed. A centralized location tracking method usually employs a centralized Location Server that can record the location of each MN. An MN itself or its serving proxy can register the MN’s location with the Location Server. The drawback of the centralized location tracking method is that an MN must query the location server and wait for response before it can setup a connection with the corresponding nodes. Distributed location tracking methods, on the other hand, distributes the location tables to each mesh node. Although this approach may avoid the query procedure, it normally needs a high cost broadcasting mechanism to maintain the distributed location tables. The proposed scheme consists of three mechanisms: (1) Location Tracking with Two-level Proxy Tables, (2) Proxy Resolution and Update Mechanism, and (3) Progressive Shortcut Forwarding. We use a centralized Location Registrar (LR) to keep track of the locations of all MNs. The centralized LR and the Proxy Tables of Mesh nodes form a two-level hierarchy of proxy tables. LR and can cooperatively perform Proxy Resolution to find out the destination proxy of the designated station. A Mesh node needs to query LR for Proxy Resolution only when it cannot find from its local proxy table the destination proxy. When LR receives a query, it will perform a Proxy Resolution to find out the destination Proxy and sends a proxy update (PU) to inform the source Proxy of the new destination proxy of the designated station. Furthermore, we adopt a Progressive Shortcut Forwarding scheme to provide better packet forwarding. In addition, because 802.11 Wireless LAN Adaptors behave differently in inter-ESS (Extended Service Set) handovers than in intra-ESS handovers, we also propose a Adaptive Location Update Mechanism to support inter-ESS handovers. Finally, we conduct a simulation using a well known network simulator NS2 to compare the performance of our proposed mechanism with AODV for location tracking, and with Mobile IP (MIP), and Ant for packet forwarding. The simulation result shows that show that our proposal introduces less control messages than AODV for location tracking, and has a shorter connection setup delay. As for packet forwarding, our proposal has a much lower hop counts than MIP and Ant, which both have stably high hop counts. In addition, our solution surpasses others in terms of location update latency and packet loss rate.