Wireless Home Networking, Part V - Interference and Range Extension

By Joseph Moran

November 08, 2002

Computers always need more power, printers need more ink, and Wi-Fi networks, even at home, always need better range. Here's how to avoid some typical coverage problems and get farther from home.

You'd be hard pressed to name an electronic device that isn't susceptible to interference in one form or another. Interference, as you'd expect, a huge problem for wireless LAN devices as it can be for just about anything that transmits and receives radio signals.

In most cases, interference isn't a major problem when setting up a home WLAN, but it's almost always present to some extent, and the effects are usually mild, like reduced range. If your WLAN's range is considerably less than expected, some type of interference may be the cause.

A major cause of interference with any radio signals are the materials in your surroundings -- especially metallic substances, which have a tendency to reflect radio signals. Needless to say, the potential sources of metal around a home are numerous--things like metal studs, nails, building insulation with a foil backing and even lead paint can all possibly reduce the quality of the WLAN's radio signal.

Materials with a high density, like concrete, tend to be harder for radio signals to penetrate, absorbing more of the energy than more porous materials like wood or drywall. Other devices utilizing the same frequency can also result in interference with your WLAN. For example, the 2.4GHz carrier frequency used by 802.11b-based wireless products to communicate with each other is known as an ISM (Industrial, Scientific, Medical) band. This frequency band is set aside for the use of various types of products in these categories -- WLAN devices don't have this frequency all to themselves. In a home environment, other devices that use the 2.4GHz band include microwave ovens and certain cordless phones (the ones that are labeled 2.4GHz, not surprisingly). Does this mean that you can't have an 802.11b wireless network if you have a microwave oven or a 2.4GHz cordless phone? No. Like most consumer electronics devices, WLAN devices are designed to continue operating when interference is present; they simply degrade performance when conditions are not optimal. Interference from other devices being the prime example.

In a normal household scenario, an 802.11b wireless LAN may encounter interference from time to time. Although performance may suffer when the microwave oven or cordless phone is in use, in most cases you probably won't even notice, unless your microwave and phone are in use all the time.

One benefit of the 5GHz 802.11a WLAN devices is that they are not vulnerable to interference from other devices, because the 5GHz frequency range they utilize is not used by any other devices (at least for now).

On the down side, because of the type of modulation 5GHz wireless devices employ, their signals are more prone to be reflected and/or absorbed by materials in the home. This can have the same practical effect as active interference; that is, to weaken the signal, and reduce range and speed.

Extending Range

The antennas that come with most WLAN routers and access points are omni-directional, which means they radiate signals in every direction. This means that unless you put your access point in the center of a room at the center of your house, chances are that the signal may not reach every place you want it to. In most homes though, this isn't practical. Even if it were, it might still not be enough if you have a large house or if you want coverage in outside areas in addition to your interior. In these cases, you still have several options to extend the range of your wireless network.

If you're home is already wired for Ethernet, the easiest route would be to simply add one or more additional access points as close as possible to the dead spots. Of course, the vast majority of homes are not wired, so this course of action may not be possible, and it's useless to simply plug an access point into AC power without a tethering it to a wired LAN.

Another possibility is to replace the antenna or antennas on your wireless router or access point. When choosing a device with an eye toward upgrading the antenna later on, it is critically important to look at the number and type of antennas provided -- look for removable antennae, because an antenna that can't be removed can't be replaced.

Many 802.11b devices now feature a removable antenna, but not all, and most older models don't. At the moment you're not likely to find removable antennas on 802.11a devices. After you check the antenna type, check to see whether there are one or two. The latter is preferable, since it affords the possibility of replacing at least one of the two.

If you need better coverage in a particular area, you could replace an omni-directional antenna with a directional one, which may provide a more optimal coverage pattern. For example, if your access point is located on an exterior wall of your home, using a directional antenna pointed toward your home's interior would direct the signal where it is needed.

Another option is to replace the stock antenna with a larger and more powerful one. Doing so can not only help the WLAN signal reach far-flung locations, it will improve the strength and quality of the signal throughout your home.

Different types of replacement antennas are starting to become available from various WLAN vendors in both omni-directional and directional flavors, and at different signal strengths. In some cases, they may also include an amplifier to help strengthen the signal. If the maker of your router or access point doesn't offer an upgrade antenna, they can sometimes be purchased from third parties. It's also possible to build "homebrew" antenna using various common electronic components. However, the efficacy and legality of such antennas is far from certain.

Some WLAN vendors are now also beginning to ship access points that can double as repeaters -- well, one is for home networks, which is D-Link DWL-900AP+ AirPlus Enhanced 2.4GHz Wireless Access Point. By switching it into repeater mode, the unit can amplify and propagate the signal from another access point, thereby increasing range by 50%.

Finally, a new option for extending range that has recently become available is a WLAN access points that also connect to your home's AC wiring via the HomePlug standard. HomePlug lets you run an entire network off the powerline's in your home -- using a HomePlug access point would let wireless computers talk to the PCs on the powerline network.

These devices have a wireless antenna, but also use your home's AC wiring as a conduit to move data. If you had a room in your home that your WLAN access point could not reach, you could still use a wireless device there if you set up one of these powerline adapters. In order to use one, you need either a router with HomePlug powerline capabilities or a powerline-to-Ethernet bridge plugged into your router. With it, your wireless computer would communicate with the wireless antenna on the powerline adapter, which would then communicate via your AC wiring with a powerline device connected to your router. Siemen's Efficient Networks is one of the first companies with a Powerline Wireless Access Point.

802.11 Planet Conference Need more range extension info? Join us at the 802.11 Planet Conference & Expo, Dec. 3-5 in Santa Clara, CA. One of our sessions will cover Optimizing Coverage with High Performance Antennas.

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