Location is playing an increasing role in Wi-Fi networks. Most recently, Trapeze Networks announced a location-based radio frequency firewall application for its location appliance that works like a fence to prevent Wi-Fi devices in unauthorized areas – such as parking lots – from penetrating the WLAN.

The capability sounds reminiscent of Meru Networks' RF Barrier, which is not location-based, however, and is more about preventing signals from leaking out of a building rather than keeping out uninvited users.

However, Meru does make something it alternately calls an "RF Firewall," an "Air Firewall" and a "Layer 1 Firewall." According to Meru, the interface between receiver and processor modules in a Meru access point is synchronous, allowing a Meru AP to determine, as soon as the frame header is received and concurrent with the actual payload transmission, whether this frame is authorized or unauthorized. On that basis, it lets that frame through or blocks it over the air by causing a collision.

By contrast, real-time location systems (RTLS) locate, to within a few feet, a piece of equipment, object, person or animal that contains a mounted or embedded Wi-Fi tag or connection. Location methods include triangulation, trilateration, RF fingerprinting and time difference of arrival (TDOA). From there, policies can be enforced based on where the device is.

In addition to asset tracking and the security application from Trapeze, for example, mobile convergence company Agito Networks uses location to determine when a mobile session should be handed off from a Wi-Fi network to a cellular network and vice versa. Usually, this happens when a user passes through an outside doorway, away from the internal Wi-Fi network and into cellular coverage.

I'm also told use of location appliances for E911 applications – for quickly finding users of Wi-Fi phones in emergencies – are finally en route.

Many of the larger enterprise-class Wi-Fi suppliers offer location appliances:

* Aruba Airwave Wireless Management Suite with RTLS

* Cisco 3300 Series Mobility Services Engine (MSE) with Context-Aware Mobility Service

* Meru E(z) Location Manager

* Trapeze LA-200 Location Appliance (which has also been supersized to support 200 APs and 4,000 clients)

A separate appliance is usually required because of the extra processing needed for calculating and correlating location information.

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.