競爭式無線網路之多頻道排程演算法

Abstract

IEEE 802.11 protocol DCF 採用CSMA/CA機制使得各個節點可以透過競爭的方式來取得媒介的使用權，當競爭者越來越多，DCF的效能會因為碰撞的機會變高而逐漸的下降；DCF主要用於單一頻道上媒介存取的控制，多通道的環境可以將兩個發送源分散在不同的實體頻道上而且不會干擾到彼此，在現有的技術下，頻道的切換會帶來短暫的空隙，在這段時間內收發器沒有辦法做接收或是傳送的功能，過度的切換頻道反而會使得整體的效能降低；MCS (Multi-channel Scheduling)是一個建立在多頻道環境下的鏈結層管理方式，透過收發器和佇列的管理，目的在於建立一個無斷裂的連結網路以及根據網路流量的分佈來挑選一個適合傳輸的頻道來減少節點間的干擾以及減少頻道切換的次數，MCS能夠有效的利用多頻道的特性來增加網路整體的吞吐量，並且在延伸到更多數的收發器時，我們加入了收發器排程的關連性來加深網路的連結性，以及加速各個偵測頻道的速度來讓頻道狀態保持在可使用的狀態。

IEEE 802.11 performs CSMA/CA mechanism based on the DCF protocol for medium access control. In a single-channel environment, as the network becomes crowded, system throughput suffers due to increased random collisions. By introducing multiple non-overlapping (non-interfering) radio channels in the system, we should be able to redistribute traffic loads over those channels such that contention level in a single channel can be effectively reduced. However, utilizing multiple channels with a single radio transceiver requires channel-switching strategy, at the cost of switching latency. In addition, network may be partitioned due to inappropriate channel switching policy. In this thesis, we propose a link-layer Multi-channel Scheduling (MCS) scheme to manage the channel switching activity. MCS improves system throughput by decreasing the frequency of channel switching and distributing traffic to less contended channel, while keeping the network connected (partition-proof). We also extend the MCS strategy to utilize multiple transceivers. In this case, wireless stations can get in touch with each other faster, and network connectivity can be further enhanced.