RF fingerprinting pinpoints location

Pinpointing wireless clients makes it easier to secure and manage wireless LANs. However, traditional technologies used for indoor 802.11 location tracking come up short on accuracy.

WLANs typically have used closest access point (closest AP) or triangulation technologies to track location. A third technology, radio frequency (RF) fingerprinting, uses intelligent algorithms to improve the accuracy of these previous technologies - locating 802.11 devices to within a few yards. Each of these technologies is typically implemented through a stand-alone WLAN management system that collects and processes real-time information gathered by WLAN infrastructure components, which include access points and centralized WLAN controllers.

With closest AP, an IT administrator submits a query to a WLAN management system to find a client based on its media access control address. The WLAN management system checks all access points to see where the device is associated. Because an 802.11b/g access point has roughly a 100-by-100-foot coverage area, locating the client by the closest AP method tracks it to within a 10,000-square-foot area, or the space of about 100 cubicles in a building.

With triangulation, IT administrators use a WLAN management system to query all access points on a wireless network to see which ones "hear" the target user's 802.11 signal. Access points that hear the desired device respond to the WLAN management system with their Received Signal Strength Indicator (RSSI) information. The WLAN management system then draws coverage circles around each of these access points, with each circle reflecting the border of the signal strength at which the access point received the signal from the user/device. The point where these circles converge is assumed to be the location of the desired device.

RF fingerprinting improves on the other location-tracking approaches by taking into account the effects that a building or people will have on an RF signal - characteristics such as reflection, attenuation and multi-path. This makes wireless device location tracking more detailed, precise and reliable.

Access points use RF fingerprinting technology to collect information pertaining to RF topology via a site survey, which creates a grid identifying how every part of a building floor plan looks to all access points. A grid point can be as small as a half-foot. To determine the radio frequency at each grid point, a WLAN management system must first predict how the RF will interact with the building.

The WLAN management system then creates an RF fingerprint with detailed topology information and RF data by tracing rays from every access point in the network and accounting for reflection and multi-path to any given destination. The RF prediction involved in RF fingerprinting takes into account the attenuation and reflection off walls and other objects into a building. The WLAN management system then creates a database of coordinates, recording how each access point views that area from a signal strength perspective - creating an RF fingerprint for each coordinate.