Convergence Channel: Deploying Wireless Networks
We’ve spoken before in this column about the technologies behind wireless transmission of data. This time around, I’d like to discuss some things to keep in mind when actually looking at implementing and deploying a wireless network.
With convergence continuing at its rapid pace, more users are trying to cram more systems onto the network. Often, a wired infrastructure just might not be available. It may already be at maximum capacity or may not even exist where you need to place a device. These conditions may make it cost prohibitive to install a new hard-wired network, so wireless then becomes a very attractive alternative.
Expect the Unexpected
There is only one thing you can guarantee when considering a wireless network … nothing is guaranteed.
The nature of a wireless system of any kind is unpredictable. There are many things that can interfere with a wireless link. You’ve seen examples of this at home. With some WiFi systems, devices such as microwaves can interfere with your signal. Remember that the unlicensed wireless products you buy for your commercial applications generally use the same frequencies as consumer-level equipment (more on frequencies later).
Interfering devices can come in many shapes and forms. The most prevalent today are other wireless networks in the immediate area. As I mentioned above, the frequencies allotted for wireless networking require no licenses to operate on, and are unregulated by the FCC. As such, “rogue” networks can show up anywhere, at any time.
These interfering devices aren’t limited to microwaves. Other ubiquitous products that can interfere with a wireless network are cordless phones and Bluetooth devices. I’ve personally experienced a drop in WiFi signal when using a Bluetooth headset next to the computer.
This means you can do all the site surveys you want and do everything possible to ensure that your client has a clear shot at a wireless connection, but in a matter of days, weeks, months, etc., that clear shot will be lost if a neighboring company places a new WiFi station in its warehouse.
Don’t get me wrong, wireless site surveys are a valuable tool. They can show you where not to put a radio, or what frequencies can’t be used at that point in time, but they should never be used to tell a customer they will have clear sailing from that point forward.
Entering the Fresnel Zone
When most people picture a wireless link between two devices in their mind, they see a straight line. In fact, we describe two radios able to see each other uninterrupted as a clear “line of sight.” However, in the real world of physics, the path that the radio waves take between the transmitter and receiver is much more than just a straight line.
Take a look at the diagram on this page. It is a visual depiction of the Fresnel zone. Basically, the radio waves radiate out in an elliptical pattern, widening out at the middle distance between the originating transmission and receiving sides. The Fresnel zone highlights the fact that an object need not completely block the line of sight between the transmitter and receiver to complicate matters. That tree doesn’t have to grow above the line of sight. It just has to block enough of the Fresnel zone to cause an issue.
Generally, it is recommended that an object block no more than 20 percent of the Fresnel zone area (‘b’ in the diagram), although more obstruction may be tolerated depending on factors such as the power of the radios, gain of the antennas and distance between the two.
This is one of those areas that can be intimidating for the average security installer as it takes some very advanced math to accurately determine the Fresnel zone of a wireless link. This is why it’s important to have a wireless expert do a site survey and anticipate as many obstructions as possible.
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