I had an opportunity to do some Wi-Fi interference testing with several Plantronics 2.4GHz cordless phones. I found that the cordless phones, when ever switched on (with or without a dial tone), would always transmit a 10MHz wide signal in the 2.4GHz ISM band, which is roughly 90MHz wide. I could clearly see from a spectrum analyzer that turning on a second phone would produce another 10MHz wide signal in an unoccupied part of the band. Apparently, the phones would search for the least congested area and tune its transmitter to that frequency. With a fairly clean spectrum, with no Wi-Fi or Bluetooth devices operating, I could turn on six of the phones, which filled approximately two thirds of the spectrum.

Based on the way that the phones were operating, the use of cordless phones (at least the Plantronics model I tested) will reduce the capacity for supporting Wi-Fi signals. Because a Wi-Fi access point uses one third of the spectrum, the operation of six phones would leave enough room for a single access point. If there is a need for operating nine phones, however, in the same general area as an access point, then interference between the access point and the phones (the three using a frequency that overlaps with the access point) will occur. The result is distorted voice heard over the phone when talking to someone and probably a much higher frame retransmission rate at the access point.

Thus, it’s a good idea to investigate whether there are any cordless phones in operation where you’re planning to deploy a Wi-Fi network. Keep in mind, however, that the various cordless phone models operate differently. I’ve found through other testing that some even use frequency hopping spread spectrum, which distributes RF signals over the entire 2.4GHz band. This leaves no room for Wi-Fi access points.