Dueling with Cordless Phones

By Jim Geier

December 18, 2003

Your home's 2.4GHz cordless phones have been known to cause interference to wireless LANs. Here's some practical knowledge on the problem based on real world testing.

As I've been discussing lately, radio frequency (RF) interference causes lower throughput in wireless LANs (WLANs). It's important that you learn as much as possible about what causes interference and what to do to minimize its impacts in order to maximize performance. Here we'll focus on the interference generated by 2.4GHz cordless phones.

Today, you can purchase cordless phones that operate in a variety of frequency bands: 900MHz, 2.4GHz, and 5GHz. The phones causing the greatest potential for interfering with WLANs, especially 802.11b and 802.11g, are ones based on 2.4GHz -- the same frequency used by 802.11b/g. I recently purchased a 2.4GHz cordless phone for my office, which prompted me to do some interference testing.

The access point in use for the testing was a Cisco 350 set to maximum transmit power of 100mW. The access point is located about 30 feet from my office.

As a wireless client, I used a laptop with AirMagnet monitoring software. AirMagnet is able to continually transmit packets of a specified size for a period of time while measuring signal strength, noise levels, packet retransmission rates, and throughput.

The cordless phone included I bought is an AT&T model number 2462. The phone's documentation doesn't identify the type of modulation or transmit power, but I imagine that it's probably frequency hopping spread spectrum (FHSS) <DEFINE: FHSS> operating at 100mW.

As a baseline, I took some initial measurements without operating the cordless phone and with the access point set to channel 1. The access point signal level resulting from the beacons was -62dBm, sufficient for solid 11Mbps associations. Throughput tests indicated 607 packets per second (pps) while continuously sending 1,532 byte frames over a period of 15 seconds.

After turning the cordless phone on within one foot from the laptop, a typical scenario at my desk, I found that the throughput dropped to 416pps. This is a 31 percent drop in performance, which has a fairly significant impact on Web browsing. Picture-intensive sites paint in slower, but it wasn't too troublesome. Keep in mind that the laptop was the only wireless client active on the network, though. Multiple active users would result in even more sluggish performance.

I continued the same throughput tests with the access point set to channels 6 and 11. I didn't find any significant variation in results. The throughput dropped to around 400pps with the phone on regardless of the access point channel. This supports my assumption that the phone uses FHSS, which spreads the signal power over the entire 2.4GHz band. As a result, you can't adjust access point to a channel that avoids the interference.

Before wrapping up the testing, I ran throughput tests from the laptop while farther away from the cordless phone. While someone was using the phone about 20 feet away with several walls in between, I reran the throughput tests. At this distance, I found that the throughput would decline only 7 percent, which didn't have any noticeable affect on Web browsing.

As a result, the impact from cordless phones seems to be an issue that's fairly local to the cordless phone handset. Just keep in mind that the interference is just as great when operating the wireless device near the base station, even if the phone is far away.

A surprising problem was that voice conversations over the cordless phone would crackle substantially while throughput tests were being run within close proximity to the phone or the base station. In fact, the quality of the audio was so bad that it would lead to discontinuing the call. This was much worse than the delays that the phone was inflicting on Web browsing.

Although the tests didn't result in complete failure of the WLAN, I wouldn't recommend deploying both 2.4GHz WLANs and phones in the same facility. Instead, lean toward using 5GHz phones.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers training focusing on WLANs.



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