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Finding people, places and things in wireless networks

Mar 20, 20062 mins
Cellular NetworksNetwork Security

* Array of measurements locates wireless clients

There are many emerging applications that involve tracking the whereabouts of a person or object in a wireless network. Among them are security, emergency response, asset management, and real-time contact applications.

There are also several technology approaches to location tracking. They take various measurements within the wireless environment to calculate where a client device is. Let’s take a brief look at a few of them, starting with the most basic.

* Nearest sensor. This is the simplest method, though by itself, it is the least precise. This capability, supported by most wireless network vendors in their management systems, determines the 802.11 access point (AP) or cellular base station to which a client device is associated. It assumes that this sensor is the closest sensor to the device. It then computes how far the signal radiates.

The diameter of the 360-degree radiation “cell” surrounding the sensor (in three dimensions, mind you) is as precise as this method alone gets, even presuming that the client does indeed associate with the nearest sensor. If an 802.11b/g AP has approximately a 100-by-100-foot coverage area, for example, the nearest-sensor method tracks the client to within a 10,000-square-foot area. Note, though, that a client might associate with a sensor a bit farther away if the nearest one is overloaded or its signal strength is otherwise not as strong.

* Triangulation/trilateration. The nearest-sensor measurement can be combined with others to pinpoint location more precisely. “Triangulation” measures the angles between three or more nearby sensors (or other reference points). Where they intersect is calculated as the client location. Precision within 50 meters is generally accepted for triangulation, according to Diana Kelley, senior analyst at Burton Group, based in Midvale, Utah. Trilateration measures the distance between sensors or other reference points, rather than the angles between them.

Next time: Getting more sophisticated with measurements.