Q: How accurately can the radio signal from an access point (which supports attenuation by varying the power output, such as some Buffalo access points) be attenuated, contained or controlled?
- Derry M.

The Wizards gaze into their crystal ball and respond:

Greg Murphy, Airwave Wireless
Controlling the radio signal from an access point is very important. You may want to increase the power of the signal to cover a wider area. Just as often, you may want to create a very small radio footprint to avoid broadcasting outside your facility or to reduce interference with surrounding access points.

The strength of the radio signal from an access point can typically be tightly controlled through the access point’s own configuration options and the use of an external antenna (including the cable and connectors) or attenuator.

Keep in mind that controlling the radio signal from the access point is only half the battle. Your client devices are always going to be the wild card in your wireless network planning, since most administrators have little control over client power output. If your goal is to prevent the radio signal from being transmitted outside your building, a high-powered client device next to a wall or window will still leak its signal outside the building. Similarly, if you are hoping to turn down the power on your access points to try to thwart eavesdroppers and “parking-lot intruders,” you need to remember that the attacker may thwart your best efforts by pointing a high-gain directional antenna through a window.

Michael Montemurro, Chantry Networks
Several factors determine how radio frequency attenuation is controlled. Depending on the hardware, there are usually registers that let you vary the transmit power of the access point. The Buffalo access point lets you adjust the output power in discrete steps (typically they support between four and eight levels). The degree and range of adjustment that you can make depends on a number of hardware characteristics. The output power and the receive sensitivity need to be closely matched so you can transmit packets to wireless clients that you can hear, and visa versa.

Another mechanism to control or contain the coverage area of an access point is to fix its transmit rate. A standard access point can extend its RF range by dropping its transmit rate as the client moves away. If you fix the access point rate to 11 Mbit/s for 802.11b, you effectively decrease the coverage range because the access point will not be able to extend its range by dropping the transmit rate to 5.5 Mbit/s, 2 Mbit/s, or 1 Mbit/s.

Finally, another mechanism to control or contain a signal is to change the antenna on the access point (if it has external antennae). By changing the antenna, you can effectively contain the coverage area by changing its shape.

Scott Haugdahl, WildPackets
The signal can be controlled fairly accurately, given the ability to control the physical location (with respect to interior or exterior walls, windows, elevators, and other obstructions), power output, antenna type and antenna direction (directional vs. omni-directional). If security or containing the signal is a concern, a larger problem lies with clients - none of the above need apply. Clients can propagate signals nearly anywhere.