How to Block WLAN Interference

To provide additional unlicensed spectrum for high-speed data services, the FCC recently allocated 300 MHz of spectrum under the U-NII (Unlicensed National Information Infrastructure) label, which traverses frequencies from 5.15 to 5.35 GHz and 5.725 to 5.825 GHz.

Devices that use unlicensed spectrum range from garage door openers and cordless phones to WLAN equipment. For businesses, WLAN devices let enterprises provide mobility for their workers and increase productivity. But there's a price to pay with this wireless freedom: Because these devices use the same spectrum, they can interfere with one another.

Most radio devices, such as cordless phones and other analog devices, communicate over narrowband transmission, using little bandwidth and a high-power concentration. But narrowband transmission is susceptible to jamming and interference because the frequency it uses is static. Just as a jet airplane taking off can drown out a conversation, a higher-power device, such as an MRI machine, an RF (radio frequency) lighting system or even a garage door opener, can drown out a cordless phone if both are operating on the same frequency.

That's why WLANs instead use spread-spectrum technology--it maximizes bandwidth usage and minimizes interference with other devices. In contrast to narrowband, spread spectrum uses a wider frequency range than is needed to send data and uses less power. It's more difficult for other devices to intercept the signal because spread spectrum is spread out and looks like meaningless noise to narrowband devices. Wi-Fi, digital cordless phones and Bluetooth devices all use spread spectrum.

But even with spread spectrum, Bluetooth can still interfere with WLANs, because both use the same RF area. Bluetooth, the popular standard for wireless personal-area networks, uses a form of spread spectrum called FHSS (frequency-hopping spread spectrum), which eliminates the concept of channels and uses the entire 2.4-GHz spectrum (see "Bluetooth Spreads Out" chart). Instead of transmitting inside a specific frequency range, it "hops" around to different frequencies approximately 1,600 times a second. FHSS use eliminates interference because even if some of the spectrum is used by another device, the signal can be transmitted again on the next frequency selected with minimal performance reduction.To gauge the impact of Bluetooth transmissions on an 802.11b/g WLAN, I conducted a simple open-air test in Network Computing's Syracuse University Real-World Labs®, monitoring how a Bluetooth file transfer would affect WLAN throughput. I used two Hewlett-Packard 5150 Pocket PCs compatible with the Bluetooth 1.1 specification, an 802.11g Cisco 1200 Access Point and a Cisco CB21AG client card. The AP was positioned 10 feet from the client card, and the Bluetooth devices were in between. To measure throughput, I used Ixia's Chariot, a traffic-generation and -monitoring tool. Throughput degraded on average approximately 15 percent for each 802.11b/g channel I tested (1, 6 and 11). The performance hit was not noticeable with basic Web surfing, but would have been had I needed high wireless throughput for file sharing or network backups.

The Bluetooth Special Interest Group recently ratified the Bluetooth 1.2 specification, which will include a feature for preventing interference. The new AFH (Adaptive Frequency Hopping) specification will let Bluetooth limit the pseudo-random frequencies Bluetooth uses when interference is detected. That way, Bluetooth communications will "play nice" with other transmissions within the 2.4-GHz band.

Wi-Fi, Interrupted

There are other potential sources of interference to your WLAN. Picture this: You're surfing the Internet over your WLAN and the phone rings and knocks you offline. If this sounds familiar, chances are the culprit is a 2.4-GHz cordless phone, which uses narrowband transmission for mobility and can cut out your wireless connection.

I tested an analog 2.4-GHz cordless phone from VTech in our labs. After using a spectrum analyzer to study the phone's transmission, I found that the phone transmits on 24.12 GHz in the middle of 802.11b/g Channel 1. How would this affect a wireless network? After some simple testing, I found that the phone severely disrupts a wireless connection on Channel 1. The other nonoverlapping channels, 6 and 11, are not affected, because the interfering device uses a narrow bandwidth of the spectrum.

After performing my initial tests within close proximity of the interfering device, I moved the phone farther away and tested within an open-air environment on 802.11b/g Channel 1. With the phone at a distance of 50 feet, it caused a 99 percent throughput drop, proving that this device almost completely disrupts a Wi-Fi connection when it's close by (see the heat map graphic). At distances of 100 and 150 feet away, the throughput drop tapered off at 20 percent and 5 percent, respectively.

Lesson learned: Avoid interactions between 802.11b/g networks and analog 2.4-GHz cordless phones. Each vendor's implementation could use any portion of the 2.4-GHz spectrum, which makes conflicts with Wi-Fi networks unpredictable.

If 2.4-GHz cordless phones are a necessity, go with the more expensive but less interfering digital 2.4-GHz phones. These phones use DSS (digital spread spectrum) technology and provide greater range, security and resistance to interference. These phones are typically branded as digital, DSS or FHSS.I tested a Siemens digital 2.4-GHz cordless phone that uses FHSS. The phone randomly selects a small portion of the spectrum to use, transmits and then moves on to another area. When it's close to a Wi-Fi network, the device can cause as much as a 10 percent throughput drop on each of the nonoverlapping 802.11b/g channels (1, 6 and 11). But when you put as little as 50 feet between the device and the network, interference becomes negligible.

If maximum 802.11b/g performance is not critical for your organization, digital 2.4-GHz cordless phones make sense. Their potential for interference is small and generally predictable. I don't recommend them if you're using a Wi-Fi network, however. If you don't want any interference between 802.11b/g networks and cordless phones, consider using an old 900-MHz.

Or you can use a new 5.8-GHz digital cordless phone, like the Panasonic product I tested. Panasonic promises the phone won't interfere with 802.11b/g, but I wanted to determine if it interacts with a 802.11a WLAN, which uses the UNI 1-3 bands in the 5-GHz range. The only channels that should interfere with this phone would be in the UNI 3 band because the phone transmits using FHSS at 5.725 to 5.850 GHz in the highest ISM band.

I tested the phone's impact on throughput with Cisco's RM21 802.11a radio upgrade on a Cisco 1200 Series AP. As in the other tests, I kept the AP and client stationary, and moved the cordless phone and base to distances of five, 50, 100 and 150 feet. The most dramatic throughput drop occurred on channels 153, 157 and 161, which are contained in the area where the UNI-3 and ISM bands overlap. Although the average throughput drop was only 10 percent, it's best not to use this phone along with an 802.11a network on the UNI-3 channels.So, to manage interference from cordless phones, there are a few simple rules. If you're running an 802.11b/g wireless network, use 5.8-GHz cordless phones. If you're running an 802.11a wireless network, use 2.4-GHz cordless phones. And if you're deploying an 802.11a/b/g wireless network, use 5.8-GHz cordless phones and stay away from channels 153 through 161.

To control interference, you must control the devices causing the disruptions. So if you limit cordless phone usage to a spectrum you're not using for data, you'll eliminate interference and gain peace of mind.

Wi-Fi vs. Wi-Fi

Other WLANs operating on the same channel or even in the same spectrum also can interfere with your wireless network. This interference, called co-channel interference, is caused by frequency reuse and is most prevalent in 802.11b/g networks. Because the 802.11b/g standard allows for only three nonoverlapping channels, frequencies must be reused within the same building when more than three APs are required. Even if you can keep a safe distance among your APs, co-channel interference from neighboring businesses or homes, or even from rogue APs your employees use, can hurt your WLAN's performance.

To simulate co-channel interference, I set up two Cisco 1200 APs on the same 802.11b/g channel and performed throughput testing when the devices were five, 50, 100 and 150 feet away from each other. On average, the throughput drop was 33 percent, which dropped to 25 percent at 150 feet. Once I moved the interfering AP out of range, throughput on the original AP returned to its maximum levels.You can also get co-channel interference between overlapping channels in the 802.11b/g spectrum. Channel 1, for example, operates between 24.01 GHz and 24.23 GHz, and Channel 2 uses 24.06 GHz to 24.28 GHz. That means 17 MHz is used by both channels. Because only one device can transmit on any given frequency, they are effectively sharing 77 percent of their bandwidth, which causes a sharp drop in WLAN performance.

To demonstrate the effects of overlapping channel interference, I set up two APs in the lab and kept one on a constant channel (11) while varying the channel (1 through 11) of the interfering AP (see "Losing Ground," below). By measuring the throughput drop on the original AP, I saw what overlap can do: The most prominent throughput reduction was on channels 7 through 11, an average of 40 percent. On nonoverlapping channels 1 though 6, the degradation was an average of 20 percent.

Although channels 1 through 6 are marketed as nonoverlapping for Channel 11, an RF phenomenon called side lobes--basically, power leakage into unintended frequencies--causes them to give off minimal interference. But this is eliminated at greater distances.

Although many devices can interfere with WLANs, adjacent WLANs are the most overlooked culprit. The frequency in which WLAN devices operate is valuable and finite, so carefully plan and deploy your architecture and radio devices to avoid potential interference.

Jameson Blandford is a lab associate at the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected].0