Minimize WLAN Interference

Radio frequency (RF) interference can lead to disastrous problems on wireless LAN deployments. Many companies have gotten by without any troubles, but some have installations that don't operate nearly as well as planned. The perils of interfering signals from external RF sources are often the culprit. As a result, it's important that you're fully aware of RF interference impacts and avoidance techniques.

Impacts of RF interference

As a basis for understanding the problems associated with RF interference in wireless LANs, let's quickly review how 802.11 stations (client radios and access points) access the wireless (air) medium. Each 802.11 station only transmits packets when there is no other station transmitting. If another station happens to be sending a packet, the other stations will wait until the medium is free. The actual 802.11 medium access protocol is somewhat more complex, but this gives you enough of a starting basis.

RF interference involves the presence of unwanted, interfering RF signals that disrupt normal wireless operations. Because of the 802.11 medium access protocol, an interfering RF signal of sufficient amplitude and frequency can appear as a bogus 802.11 station transmitting a packet. This causes legitimate 802.11 stations to wait for indefinite periods of time before attempting to access the medium until the interfering signal goes away.

To make matters worse, RF interference doesn't abide by the 802.11 protocols, so the interfering signal may start abruptly while a legitimate 802.11 station is in the process of transmitting a packet. If this occurs, the destination station will receive the packet with errors and not reply to the source station with an acknowledgement. In return, the source station will attempt retransmitting the packet, adding overhead on the network.

Of course this all leads to network latency and unhappy users. In some causes, 802.11 protocols will attempt to continue operation in the presence of RF interference by automatically switching to a lower data rate, which also slows the use of wireless applications. The worst case, which is fairly uncommon, is that the 802.11 stations will hold off until the interfering signal goes completely away, which could be minutes, hours, or days.

Sources of RF interference

With 2.4 GHz wireless LANs, there are several sources of interfering signals, including microwave ovens, cordless phones, Bluetooth-enabled devices, FHSS wireless LANs, and neighboring wireless LANs. The most damaging of these are 2.4 GHz cordless phones that people use extensively in homes and businesses. If one of these phones is in use within the same room as a 2.4GHz (802.11b or 802.11g) wireless LAN, then expect poor wireless LAN performance when the phones are in operation. (Refer to a previous tutorial for results of testing interference from a cordless phone.)

A microwave operating within ten feet or so of an access point may also cause 802.11b/g performance to drop. Of course the oven must be operating for the interference to occur, which may not happen very often depending on the usage of the oven. Bluetooth-enabled devices, such as laptops and PDAs, will cause performance degradations if operating in close proximately to 802.11 stations, especially if the 802.11 station is relatively far (i.e., low signal levels) from the station that it's communicating with. The presence of FHSS wireless LANs is rare, but when they're present, expect serious interference to occur. Other wireless LANs, such as one that your neighbor may be operating, can cause interference unless you coordinate the selection of 802.11b/g channels.

Use tools to "see" RF interference

Unless you're Superman, you can't directly see RF interference with only your eyes. Sure, you might notice problems in using the network that coincide with use of a device that may be causing the interference, such as turning on a microwave oven and noticing browsing the Internet slow dramatically, but having tools to confirm the source of the RF interference and possibly investigate potential sources of RF interference is crucial. For example, MetaGeek's Wi-Spy is a relatively inexpensive USB-based Wi-Fi spectrum analyzer that indicates the amplitude of signals across the 2.4GHz frequency band. Figure 1 is a screenshot of the Wi-Spy display with a microwave oven operating ten feet away.

This clearly shows relatively high-level signals emanating from the microwave oven in the upper portion of the 2.4GHz frequency band, which indicates that you should tune any access points near this microwave oven to lower channels. To simplify matters, MetaGeek has an interference identification guide that you can use with Wi-Spy to help pinpoint interfering sources. The benefit of using a spectrum analyzer in this manner is that you can identify the interference faster and avoid guessing if a particular device is (or may) cause interference.

Take action to avoid RF interference

The following are tips you should consider for reducing RF interference issues:

1. Analyze the potential for RF interference. Do this before installing the wireless LAN by performing an RF site survey. Also, talk to people within the facility and learn about other RF devices that might be in use. This arms you with information that will help when deciding what course of action to take in order to reduce the interference.

2. Prevent the interfering sources from operating. Once you know the potential sources of RF interference, you may be able to eliminate them by simply turning them off. This is the best way to counter RF interference; however, it's not always practical. For example, you can't usually tell the company in the office space next to you to stop using their cordless phones; however, you might be able to disallow the use of Bluetooth-enabled devices or microwave ovens where your 802.11 users reside.

3. Provide adequate wireless LAN coverage. A good practice for reducing impacts of RF interference is to ensure the wireless LAN has strong signals throughout the areas where users will reside. If signals get to weak, then interfering signals will be more troublesome, similar to when you're talking to someone and a loud plane flies over your heads. Of course this means doing a thorough RF site survey to determine the most effective number and placement of access point.

4. Set configuration parameters properly. If you're deploying 802.11g networks, tune access points to channels that avoid the frequencies of potential interfering signals. This might not always work, but it's worth a try. For example, as pointed out earlier in this tutorial, microwave ovens generally offer interference in the upper portion of the 2.4GHz band. As a result, you might be able to avoid microwave oven interference by tuning the access points near the microwave oven to channel 1 or 6 instead of 11.

5. Deploy 5GHz wireless LANs. Most potential for RF interference today is in the 2.4 GHz band (i.e., 802.11b/g). If you find that other interference avoidance techniques don't work well enough, then consider deploying 802.11a or 802.11n networks. In addition to avoiding RF interference, you'll also receive much higher throughput.

The problem with RF interference is that it will likely change over time. For example, a neighbor may purchase a cordless phone and start using it frequently, or the use of wireless LANs in your area may increase. This means that the resulting impacts of RF interference may grow over time, or they may come and go. As a result, in addition to suspecting RF interference as the underlying problem for poor performance, investigate the potential for RF interference in a proactive manner.

Don't let RF interference ruin your day. Keep a continual close watch on the use of wireless devices that might cause a hit on the performance of your wireless LAN.

Courtesy of Wi-Fi Planet