A Dynamically Refocusable Sampling Infrastructure for 802.11 Networks


Udayan Deshpande. A Dynamically Refocusable Sampling Infrastructure for 802.11 Networks. PhD thesis, Dartmouth College Computer Science, Hanover, NH, May 2008. ©Copyright Udayan Deshpande. Available as Dartmouth Computer Science Technical Report TR2008-620.


The edge of the Internet is increasingly wireless. Enterprises large and small, homeowners, and even whole cities have deployed Wi-Fi networks for their users, and many users never need to--- or never bother to--- use the wired network. With the advent of high-throughput wireless networks (such as 802.11n) some new construction, even of large enterprise build- ings, may no longer be wired for Ethernet. To understand Internet traffic, then, we need to understand the wireless edge. Measuring Wi-Fi traffic, however, is challenging. It is insufficient to capture traffic in the access points, or upstream of the access points, because the activity of neighboring networks, ad hoc networks, and physical interference cannot be seen at that level. To truly understand the MAC-layer behavior, we need to capture frames from the air using Air Monitors (AMs) placed in the vicinity of the network. Such a capture is always a sample of the network activity, since it is physically impossible to capture a full trace: all frames from all channels at all times in all places. We have built a monitoring infrastructure that captures frames from the 802.11 network. This infrastructure includes several “channel sampling” strategies that will capture repre- sentative traffic from the network. Further, the monitoring infrastructure needs to modify its behavior according to feedback received from the downstream consumers of the captured traffic in case the analysis needs traffic of a certain type. We call this technique “refocusing”. The “coordinated sampling” technique improves the efficiency of the monitoring by utilizing the AMs intelligently. Finally, we deployed this measurement infrastructure within our Computer Science building to study the performance of the system with real network traffic.

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Keywords: [security] [wifi]

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