| Summary: | 碩士 === 國立中興大學 === 電機工程學系所 === 99 === With the development of IEEE 802.11 wireless network in recent years, mobile devices equipped with wireless network capability have become very popular. The wide deployment of wireless network access points (AP) forms a wireless network with very large coverage. To enable a mobile device connecting to the wireless network uninterruptedly, we need fast and reliable handoff procedures between AP’s. The handoff delay can be divided into three parts: channel scan, authentication, and re-association. Among these, the channel scan time accounts for the largest proportion of handoff time.
Random scan is a channel scan method in IEEE 802.11 wireless networks design to reduce the channel scan time. It divides a beacon interval into three sections. In each beacon interval, one section in randomly selected and a randomly selected channel is scanned for available AP’s. Normal transmission can be conducted in the remaining sections. It takes three beacon intervals to scan all the sections. The sequences for channels and sections are randomly generated. Each station switches to the target channel according to the sequences.
To extend the studies of random scan, this thesis focuses on handoff simulation of random scan and passive scan and compares the differences between the two. To alleviate the problem that scanning time may not be enough in random scan when the moving speed is high, we add a threshold to activate channel scan before handoff is required. In simulations, we find that random scan can reliably find suitable AP to switch and significantly shorten the channel scan time at various speeds.
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