Get The Interference Out Of Your WLAN

Wi-Fi networks, Bluetooth devices and cordless phones use radio waves that fall within a non-licensed portion of the frequency spectrum. This means that the Federal Communications Commission (FCC) in the U.S. and regulatory bodies in other countries allow usage in these public bands without coordination among users.

This situation occasionally results in radio wave signals from other sources hurting the performance of Wi-Fi networks, a problem generically known as RF interference. That's why it's important to consider all potential sources of RF interference when deploying a wireless LAN. If you don't, you are likely to see a precipitous drop-off in performance that, in some cases, you may not be able to fix.

Where Interference Comes From

Most RF interference impacts wireless LANs that operate in the 2.4GHz band, which 802.11b and 802.11g networks utilize. For example, a company may install an 802.11b/g wireless LAN and set all of the access points to the same channel.

In this case, the access points and the users associated with them politely take turns using the same part of the frequency spectrum. With only a few users active on the network, the impact on performance is barely noticeable. In fact, tests I've conducted indicate only 10 percent lower throughput when three or fewer users are actively sending packets. Higher utilization decreases throughput substantially more, however, due to greater latency in accessing the medium.A similar interference scenario is when neighboring companies and homes set their 802.11b/g access points to the same channels. In fact, a scan of the frequency spectrum in neighborhoods and office parks generally shows numerous access points set to channel 6, the usual vendor default channel. The impact of this form of interference is also slight, assuming there are few users active on the same channel, which is generally the case.

A more damaging form of RF interference comes from neighboring wireless LANs that use frequency hopping spread spectrum (FHSS), which are older networks often found in hospitals and warehouses. The problem is that FHSS transmits radio signals over the entire 2.4GHz band, which significantly impacts the performance of 802.11b/g systems. In fact, my testing in real-world situations shows that neighboring FHSS wireless LANs cause the performance of 802.11b/g systems to drop by more than 50 percent. The hitch with FHSS interference is that it doesn't go away unless you shut off the FHSS network. There's no room to tune an 802.11b/g access point to a channel that avoids the FHSS interference.

Bluetooth devices also use FHSS, but they transmit at much lower power than wireless LANs, and the interfering signals are only present when the Bluetooth device is in use. Generally, Bluetooth only causes significant interference to 802.11b/g users who are near the edge of the coverage of a particular access point and within a few feet from an operating Bluetooth device.

At my desk, for example, I use Bluetooth to synchronize a PDA to a desktop PC. I don't notice any drops in performance when simultaneously browsing the Web from my wireless laptop, but the access point is only fifteen feet away. If there are lots of Bluetooth devices in use, then 802.11b/g users operating in areas where signal strength is low will likely experience considerably lower throughput.

2.4GHz cordless phones, especially those using FHSS, also offer a potential for interference with 802.11b/g networks. As with FHSS wireless LANs, cordless phones transmit RF over the entire 2.4GHz band, and you can't tune an access point to a channel that avoids them. I've found, for instance, that when using my FHSS cordless phone, synchronizing e-mail and browsing the web from my laptop via 802.11b/g gets a little slower. Tests indicate a 25% drop in measured throughput while the phone is in use.Cordless phones that utilize direct sequence spread spectrum (DSSS) are easier on 802.11b/g wireless LANs. The direct sequence models only transmit signals over one third of the 2.4GHz band, similar to 802.11b/g wireless LANs. This makes it possible to tune 802.11b/g access points away from the interference with these models.

Interference from microwave ovens is notorious for lowering the performance of 802.11b/g wireless LANs. But, this only happens while the oven is operating, which may not be very often. The severity of the impact on throughput also depends on the emission frequency of the oven and the access point channel setting. Wireless-Nets' testing, for example, shows that an oven centered near channel 9 of the 802.11b/g spectral space causes throughput of 802.11b/g users to drop by 70 percent within 20 feet of the oven.

For instance, I've sat in our break room using a wireless laptop and find that browsing the Web gets very slow and sometimes stops when someone is zapping a frozen dinner. If the access point is set to channel 1, then the hit on performance is not noticeable because the microwave oven emissions only cover roughly one third of the 2.4GHz band, and setting the access point to channel 1 keeps the signals far enough apart to avoid interference.

Getting The Interference Out

Fortunately, in most cases it's possible to appreciably limit, if not eliminate, RF interference.Start by thoroughly assessing the potential for RF interference before installing a wireless LAN. RF analysis tools, such as those from Airmagnet and Berkeley Varitronics Systems, aid in pinpointing potential interference sources.

If interfering signals are found, then try configuring the access points to lessen impacts of the interference. For example, set access points operating near microwave ovens to channels that are four or five channels from the center frequency of a microwave oven. If it's not possible to reduce RF interference to a livable level, then consider using 802.11a which operates in the 5GHz band, which is virtually free from sources of interference.

Jim Geier is the principal of Wireless-Nets, Ltd., a consulting firm focusing on the implementation of wireless mobile solutions and training.