Fast and Efficient Scanning Mechanisms in Wireless Local Area Networks

I completed my honours in 2006 in fast scanning mechanisms with 802.11 networks. This study investigated why 802.11 wireless devices took so long to scan or find other AP when roaming. It was found that the primary cause of large scanning delays was due to channel overlap in the 2.4 GHz band. Wireless clients scann all channels, including the overlapping channels in the 2.4 GHz band which are only separated by 5 Mhz. We discovered that often respond to transmissions on these nearby overlapping channels. Due to the channel probing mechanism standardized in 802.11, this actually prolongs scanning. This was an interesting finding as we are unaware of any prior work suggesting that two devices can communicate given a channel separation of 5 MHz. We also proposed and analysed some mechanisms to support fast scanning for low power devices. The thesis and papers can be found in the publications section.

Abstract

The proliferation of Wireless LANs and the increasing integration of voice into data networks have created the potential for VoWLANs (Voice over Wireless Local Area Networks). This technology has immense cost saving potential and the ability to provide better service and functionality. However, before the integration of VoWLAN is possible, handoff delays must be reduced. Currently, the connectivity transition that occurs when wireless stations move between APs (Access Points) is too long, causing poor voice quality and call dropouts. This research examines a layer two handoff delay known as the scanning delay. Experiments are performed to measure and capture the scanning process. Through the analysis of packet captures, a previously unacknowledged scanning delay is discovered. It is found that channel overlap is the predominant cause of scanning delays in the 2.4 GHz band. Furthermore, in anticipation of the need for 5 GHz equipment in municipal wireless LANs, passive scanning mechanisms are also explored. Fast passive scanning mechanisms are proposed and prototyped.