By Jim Geier
January 11, 2002
RF interference will continue to plague current and future 802.11b implementations unless an adequate site survey is conducted – preferably prior to actual network installation. Let’s take a look at the primary culprits and available corrections.
RF interference is still plaguing wireless LAN deployments. Many companies have gotten by without any problems, but some have installations that don’t operate 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 impact and avoidance techniques.
What is the impact of RF interference?
As a basis for understanding the impact of RF interference in wireless LANs, let’s quickly review how 802.11 stations (radio cards and access points) access the 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 protocol is somewhat more complex, but this gives you enough of the basic concepts.
RF interference involves the presence of unwanted, interfering RF signals that disrupt normal system 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 until the interfering signal goes away.
To make matters worse, an interfering signal generally 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 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 delays and unhappy users. In some causes, 802.11 will attempt to continue operation in the presence of RF interference by automatically switching to a lower data rate, which 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 that may cause problems
For 2.4 GHz wireless LANs, there are several sources of interfering signals, including microwave ovens, wireless phones, Bluetooth enabled devices, and other wireless LANs. The most damaging of these are 2.4 GHz wireless phones that people are starting to use in homes and some companies. If one of these phones is in use within the same room as an 802.11b wireless LAN, then expect poor wireless LAN performance.
Microwave ovens operating within 10 feet or so of an access point or radio-equipped user will generally just cause 802.11b performance to drop. Bluetooth enabled devices, such as laptops and PDAs, will also 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 802.11 and 802.15 standards groups, however, are working on a standard that will enable the coexistence of Bluetooth and 802.11 devices. Other wireless LANs, such as one that your neighbor may be operating, can cause interference unless you coordinate the selection of 802.11b channels.Take action to avoid RF interference
What can be done about RF interference? Here are tips you should consider:
- Analyze the potential for RF interference. Do this before installing the wireless LAN by performing an RF site survey using tools we’ve discussed in a previous article. Also, talk to people within the facility and learn about other RF devices that might be in use.
- Prevent the interfering sources from operating. Once you know the potential sources of RF interference, you could 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 tell the company in the office space next to you to shut off their wireless LAN; however, you might be able to disallow the use of Bluetooth-enabled devices or microwave ovens where your 802.11 users reside.
- Provide adequate wireless LAN coverage. One of the best remedies for 802.11b RF interference is to ensure the wireless LAN has strong signals throughout the areas where users will reside. If wireless LAN signals get too weak, then interfering signals will be more troublesome. Of course this means doing a thorough RF site survey to determine the most effective number and placement of access point.
- Set configuration parameters properly. If you’re deploying 802.11b networks, then tune access points to channels that avoid the frequencies of interfering signals. This might not always work, but it’s worth a try. For 802.11 frequency hopping systems, try different hopping patterns. By the way, the newer 802.11e MAC layer, slated for availability sometime in 2002, offers some built-in RF interference avoidance algorithms.
- Deploy the newer 802.11a wireless LANs. Most potential for RF interference today is in the 2.4 GHz band (i.e., 802.11b). If you find that other interference avoidance techniques don’t work well enough, then consider deploying 802.11a networks. At least for the foreseeable future, you can avoid significant RF interference in 802.11a’s 5 GHz band. You’ll also receive much higher throughput; however, the limited range requires additional access points and higher costs.
Stay tuned; next time we’ll investigate potential returns on investment when deploying wireless LANs in enterprise environments.
Author Biography: Geier provides independent consulting services to companies developing and implementing wireless networks. He is the author of the book, Wireless LANs (2nd Edition), and his Online Guide to wireless networking is located at www.wireless-nets.com/guide.htm.