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What’s the best way to combat WLAN interference?

Mar 15, 20044 mins
Network SecurityWi-Fi

Q: We are receiving outside interference from multiple wireless LANs, and possibly other culprits. What is the best way for us to combat this interference? Are directional antennas helpful? – Donald, Alabama.

Bob Friday, Airespace:

The majority of non-proprietary wireless networking products available today follow 802.11 standards, which operate in the unlicensed 2.4 and 5.2 GHz bands. As a result, devices operating in these ranges are open to all different kinds of interference coming from a variety of sources such as other legitimate 802.11 networks to Bluetooth devices.

Older generations of WLANs are not effectively equipped to adapt to various forms of interference. These systems require an administrator to adjust channel and power on each access point manually; this can be like searching for your keys in the dark. In these situations, the easiest measure one can take to try to maintain the network is to change the channels of access points in troubled areas to frequencies bearing less interference. This may solve your immediate problem in that location, but may cause co-channel interference in other areas of your enterprise, which will force you to change channel on all of your other access points.

You can also try to manipulate your power levels or experiment with directional antennas. But these actions just change your coverage area, rather than establishing a clearer signal between an access point and the client device. Your best bet is to invest in more advanced infrastructure or management software that intelligently reacts to interference, and automatically change channel and power.

Newer generations of WLANs based on switch architectures are more intelligent at identifying and adapting to interference. These systems centralize management of access points, enabling network-wide control and visibility of configurations. At a minimum, these products should identify the presence of interference and suggest configuration modifications to improve network performance.

Remember that there are two different types of interference in the 802.11 world. The first type is co-channel interference. This may be coming from the harmless coffee shop network just next door that is operating on the same channel as one of your access points. The second type of interference is non-802.11 noise – this comes from Bluetooth or a cordless phone operating in the same band as your wireless network.

Regardless of the interference type – your wireless network must be able to react and adjust to it accordingly.

Dan Simone, Trapeze Networks:

Interference can be a significant issue on WLANs that run in the 2.4 GHz bands, which include the 802.11b and 802.11g radio specifications. The proliferation of 802.11b WLANs that only have three non-overlapping channels, along with interference from other devices in the 2.4 GHz spectrum, such as wireless phones and microwave ovens, make that part of the airwaves quite crowded.

The main reason that 2.4 GHz WLANs interfere with each other is that the spectrum provides for only three non-overlapping channels: 1, 6, and 11. Clients and access points use the same channel to communicate with each other, and planning a large network with many access points, or being in an area with many WLANs, rapidly creates interference as multiple access points on the same channel hear each other’s transmissions.

Try approaching your neighbors to see whether they can turn down their power and/or coordinate channel selection. Directional antennas will help client connectivity only if your own coverage is particularly weak in a specific area of your building. You’ll still hear their access points on your network, even if you implement directional antennas and direct the signal toward the inside of your building.

Outside of forming a neighborhood cooperative or putting up an RF shield around your building, your best option is probably to move your WLAN to 802.11a. This radio technology runs in the 5 GHz band, which is much less crowded in terms of other devices and provides more than 12 non-overlapping channels for WLAN service.

Israel Drori, Legra Systems:

Directional antennas won’t help because they don’t affect the source of your problem, which is the source of the interference. Your network needs mechanisms that automatically detect rogue access points, which are sources of interference, and automatically switch channels to avoid it. Many routers and radios on the market have them built in, but you have to make sure they’re turned on. Consider using 802.11a, which has a less crowded frequency band.