Wi-Fi vs. cordless phones

The Wi-Fi was going great .. until we made a phone call with the cordless, and then the signal from the Netgear router would drop from the iMac until the call was done. I had heard that 2.4 GHz cordless phones had the potential to interfere with Wi-Fi, which also operates in the 2.4 GHz frequency range.

I went into testing mode. I took the cordless back to The Back Room, where This Old PC connects to the router about 9 feet away via an internal Wi-Fi card (Airlink Plus from Fry’s). No trouble there to stay connected to the router while making a phone call (cordless phone base station is in the house about 30 feet away).

Could it be the weaker wireless connection in the house just can’t take the telephone interference? I got a 900 MHz phone, hooked that up in the house. No interference. Would I have to get rid of my Panasonic 2.4 GHz just to achieve Wi-Fi nirvana. I noticed on the display for the 900 MHz handset that I was on Channel 01. You can change channels on a cordless telephone? My Panasonic 2.4 GHz doesn’t have a display at all … but it DOES have a channel-changing button.

One click on the channel button, and my Wi-Fi interference problem was gone.

Here’s where the problem begins:

  • 802.11b/g frequency range in the U.S.: 2.412 GHz to 2.462 GHz
  • Cordless phone frequency range: 2.4 GHz to 2.4835 GHz
  • So they’re pretty much right in the same place — and that could mean trouble. Since both the router and the telephone have a number of different channels available, it is a matter of changing the channel on one or the other to reduce or eliminate the interference. I believe you can change channels on the router by logging onto the admin page. On the phone, as I say above, it’s as easy as hitting the Channel button.

    One thing that tipped me off to the problem was the marketing for the new cordless phones, which operate in the 5.8 GHz range. The boxes say, “won’t interfere with Wi-Fi” … yeah … unless you plan to use the new 802.11a standard:

  • 802.11a frequencies in the U.S.: 5.170 GHz to 5.320 GHz
  • Cordless phones: 5.725 GHz to 5.850 GHz
  • Now, there’s no direct overlap there, so it might not be a problem, but some real-world testing is needed to make sure — and I suspect there will be 2.4 GHz phones on the market for awhile.

    Interestingly enough, in my research (yeah, it’s Web surfing, but I call it “research”), I found out that most 2.4 GHz cordless phones transmit one way at that frequency, the other way at 900 MHz, to avoid interference: You see, since a telephone call is what’s called “full duplex” — meaning you can talk and listen to the other caller at the same time (and hear your own voice simultaneously through the earpiece) without saying “over” like a radio operator — the phones need to operate on the different frequency bands to a) avoid interference and b) extend battery life of the telephone handset (which I imagine transmits with 900 MHz and receives at 2.4 GHz for that reason).

    An interesting quote from Hello Direct:

    To keep neighbors from constantly hearing each other’s conversations, the FCC initially limited the output wattage for cordless phones to just .001 watt. But when digital and spread spectrum technologies (SSTs) made eavesdropping a less
    valid concern by scrambling signals or dividing them across multiple bandwidths,
    the allowable wattage for cordless phones was increased to 1 watt. This action made for clearer calls and increased a cordless phone’s range three- to fourfold. Meanwhile, the increased wattage covered up the fact that higher-frequency signals require more power to transmit.

    When 5.8 GHz phones were introduced, the allowable wattage was not increased—and here is where the buyer must beware. Because transmitting signals at a higher frequency requires more power, some 5.8 GHz phones use the new frequency only for the
    base-to-handset transmission. Then, to make sure a handset’s battery has a reasonable life, handset-to-base transmissions are sent on the older 2.4 GHz frequencies.

    2 Although it will be some time until it is introduced, a protocol using 5.8 GHz technology is in development. This new protocol, 802.11a, will be able to send high-quality video. It will also interfere with the new phones.