在多通道無線區域網路中媒體接取控制協定之研究

MAC Protocols for Wireless LANs with Multi-Channels

劉仁筑(Jen-Chu Liu)

指導教授 ： 陳文村

國立清華大學/電機資訊學院/資訊工程學系/博士(2009年)

摘要

在本論文中，我們研究了在多通道(Multi-channel)無線區域網路中可能會遇到的一些問題，包括了裝備不同天線(Transceiver)數目的節點相容性問題、多通道無線網路中的隱藏節點(Hidden Node)問題、廣播封包遺失問題、通道選擇問題、通道使用率的問題、碰撞與壅塞控制等等的問題。
在本論文中首先提出了一個可以適合於裝備不同天線數目的節點的多通道媒體存取協定，名叫自我調整多通道媒體存取協定(Self Adjustable Multi-channel Medium Access Control, SAMMAC)。這協定修改了傳統IEEE 802.11訊框的檔頭部份，將原本的RTS/CTS改成可以支援多通道的MRTS/MCTS，另外為了保證通道保留的成功率，多增加了MCTS-R來實現三方握手通道保留機制。自我調整多通道媒體存取協定可以將許多裝備了不同數目天線的節點整合在同一個網路來傳輸資料，增加了多通道無線區域網路的可用度與彈性。我們更針對裝備多天線的節點設計了一個持續傳輸(Persistent Transmission)的機制，以提高網路整體生產量(Throughput)。
接下來，本論文則是探討在多通道及多天線的無線網路環境中，設計一個有效率的通道分配方法，讓網路節點能夠透過多通道來達到並行資料傳輸。我們提出了一個結合了階層(Hierarchy)與分群(Cluster)的通道分配方法，名叫階層式分群通道分配(Hierarchical Clustering Channel Assignment, HCCA)。通道的分配由最高階層依序向下分配，整個無線網路在通道分配後會被劃分數個子網路(不同的通道)，藉此減少網路節點在相同通道中的碰撞(collision)，同時也能夠解決端點對端點之間延遲(End-to-end Delay)的問題，提高整體網路效能。
本論文也針對多通道無線區域網路設計了兩種可以增進網路生產量的方法。首先是提出了一個名叫交通察覺多通道媒體存取控制(Traffic Aware Multi-channel MAC, TAMMAC)協定，這個方法的特色是可以根據現在網路交通的狀況，動態的調整傳輸信號區段(Beacon Interval)中的競爭時段(Contention Period)的大小，透過動態的調整，網路可以在生產量與要求傳送成功率間取得一個較佳的平衡。接下來是針對在多通道無線區域網路中壅塞(Congestion)問題的研究，在一個資料流傳輸頻繁的多跳點(Multihop)無線網路中，熱點(hop spot)可會因為內部資料流競爭(Intra-flow Contention)與外部資料流競爭(Inter-flow Contention)而能會遭受壅塞，導致封包遺失機率和傳送延遲都會增加，同時生產量也會減少。因此我們提出了一個適用於多通道無限區域網路的壅塞控制方法，名叫有效率的多通道資料流控制(Efficient Flow Control with Multi-channel, EFCM)方法，EFCM是採用一個逐跳模式(Hop-by-hop)的壅塞控制方法，利用修改過的MRTS/MCTS來控制封包協調，同時解決多通道環境下的隱藏節點的問題。另外，我們設定中繼節點比源頭有較高的優先權搶得通道的使用權，轉送節點有比原始節點較高的傳輸優先權，每個節點會監控流量的情況減少運輸擁擠的情況發生。
根據我們所做的實驗模擬結果，可以發現我們所提出的方法可以有效提昇在多通道無線區域網路中的效能，包括了降低網路延遲、增加網路生產量、擴充性與相容性。

關鍵字

無線區域網路 ； 多通道 ； 自我調整 ； 通道指派 ； 壅塞控制 ； 服務品質保證

並列摘要

The use of multiple channels in wireless networks may provide performance advantages in reducing collisions and enabling more concurrent transmissions. A mobile host that participates in multi-channel transmission may equip with one or more transceivers. A MAC protocol designed for multi-channel wireless networks should deal with the communications among the MHs no matter how many transceivers they use. Moreover, the multi-channel wireless networks should accommodate traditional MHs that support single channel transmission. Channel selection is also a key point to improving the performance of multi-channel wireless networks. A MH should choose proper channels which do not interfere with other MHs’ channels. Furthermore, the whole network throughput and performance should be guaranteed that higher than single channel environment.
In this dissertation, we propose a new multi-channel MAC protocol for MHs that are equipped with single transceiver or multi-transceivers in a multi-hop wireless network, which is named Self Adjustable Multi-channel MAC protocol (SAMMAC). The proposed scheme modified the RTS/CTS frames for multi-channel usage, and divides the transmission frame into channel negotiation and data transmission period. SAMMAC can contain different type of MHs (including multi-channel and single channel nodes) and achieve high throughput.
Channel assignment is an important issue in the multi-channel wireless networks to achieve high throughput. Previous works on multi-channel assignment can be classified into two categories: the algorithms-based and the negotiation-based schemes. The algorithm-based schemes treat channel assignment as the coloring problem and require complex computations. The negotiation-based schemes usually need a common control channel to negotiate data channels but fewer computations. However, they usually require time synchronization, which is hard to achieve in wireless networks. Moreover, the negotiation-based schemes have larger end-to-end delay in multi-hop communications due to message exchanges. This dissertation proposed an asynchronous scheme, named Hierarchical Clustering Channel Assignment (HCCA), for channel assignment based on clustering nodes into a hierarchy. Channels are allocated from the highest cluster to lower clusters. The HCCA is efficient to mitigate the interference of adjacent MHs.
We also design an algorithm to adjust the contention window according to the traffic condition, named Traffic Aware Multi-channel Medium Access Control (TAMMAC), which can exploit multiple channels with smart window increase and decrease rules to adjust the contention window size dynamically and properly according to network traffic condition to maximize the channel utilization. In addition, we design a hop-by-hop congestion control scheme for MANET with multi-channel usage, which is named Efficient Flow Control with Multi-channel (EFCM), to improve the whole network throughput. In this scheme, the intermediate nodes have higher priority than the source (or leaf) nodes to contend for the right of transmission to solve the intraflow contention. In order to solve the interflow contention, congestion control is taken in every node to construct a flow table, which restricts the number of packets of every flow passing by congested nodes.
The simulation results show that our schemes can effectively exploit multiple channels to achieve higher throughput than IEEE 802.11 and many existing multi-channel MAC protocols.

並列關鍵字

WLAN ； multi-channel ； self adjustable ； channel assignment ； congestion control ； QoS

參考文獻

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