Interference from Cordless Phones

By Jim Geier

April 15, 2003

Everywhere you look people are using cordless phones to enable mobility as they talk to customers, vendors, and friends. Learn about the potential for cordless phones to interfere with WLANs and how you can make the two coexist.

More and more households and businesses today are using cordless phones, made by a variety of companies, including Panasonic, Siemens, and Sony. The cordless phones supposedly sound clearer and have much better range than wired phones. Of course they also provide mobility.

Cordless phones operate on 3 different frequencies, 900MHz, 2.4GHz, and 5GHz. Do those last two frequencies sound familiar? Yep, at least the 2.4GHz and 5GHz frequencies fall in the same frequency bands as 802.11b and 802.11a wireless LANs, respectively -- and the use of the phones can cause significant RF interference to your WLAN.

Potential 802.11b Interference

The majority of cordless phones in use today are 2.4GHz models. These phones, innocent as they may seem, reap devastating effects on 802.11b WLANs. Finding cases where 2.4GHz phones severely interfere with 802.11b WLANs is usually an easy task. Some 802.11b WLANs have even been totally shut down by somebody simply answering a 2.4GHz cordless phone. Why? The answer lies in the technology.

Most 2.4GHz phones use frequency hopping spread spectrum (FHSS) technology that operates within the 2.4GHz spectrum. FHSS hops from frequency-to-frequency across the entire 2.4GHz spectrum. 802.11b WLANs, on the other hand, use direct sequence spread spectrum (DSSS), which transmits within approximately one third of the 2.4 GHz spectrum. Because FHSS jumps across the entire spectrum while DSSS stays in only one portion, a FHSS 2.4GHz will clobber an 802.11b network, causing interference or even failure.

Potential 802.11a Interference

The jury is still out on whether 5GHz cordless phones have any detrimental effects on 802.11a WLANs. If interference issues do arise, however, they will most likely be easier to correct than with 802.11b. 802.11a uses more non-conflicting channels within the 5GHz spectrum as compared to 802.11b in the 2.4GHz spectrum. 802.11a enjoys the luxury of 12 independent, non-overlapping channels. 802.11b has 11 channels in the U.S., but only three don't overlap (channels 1, 6, and 11).

Resolutions to Consider

What can you do to eliminate the effects a cordless phone has on your WLAN? There are a number of solutions, some of which are guaranteed to work and others that may significantly lower the interference level. The fail-safe solutions all consist of replacing equipment.

If your interference is coming from a 2.4GHz phone, the sure fire solution is to change to a 900MHz or 5GHz phone or switch the WLAN to 802.11a. Both methods will eliminate the interference. These solutions are the most effective, but also the most expensive.

Before spending lots of money on new hardware, consider the following less expensive (but possibly less effective) approaches:

  • With 802.11b systems, try changing the channel on the access point (at least try 1, 6, and 11). Anecdotally, people have found that they have the least interference on channel 11.
  • With 802.11a systems, change to any one of the channels that are not in use by the cordless phone.
  • Change the location of the access point or the cordless phone base. Maximize the distance between the cordless phones and WLAN devices, especially WLAN users operating at the fringes of access point range.
  • If you have the option, use an external, remote antenna on your computer. This will let you place the antenna as far away as possible from a cordless phone.
  • Operate the cordless phone with the antenna in the lowered position if possible. This will dampen the RF signal coming from the cordless phone and reduce the amount of interference.

There are no guarantees that these actions will work, but they may cut the interference enough to allow acceptable performance. The most attractive aspect of these solutions, however, is the fact that they are free.

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 workshops on deploying WLANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.



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