Cordless Telephone - Performance

Performance

Manufacturers usually advertise that higher frequency systems improve audio quality and range. Higher frequencies actually have worse propagation in the ideal case, as shown by the basic Friis transmission equation, and path loss tends to increase at higher frequencies as well. More important influences on quality and range are signal strength, antenna quality, the method of modulation used, and interference, which varies locally.

"Plain old telephone service" (POTS) landlines are designed to transfer audio with a quality that is just enough for the parties to understand each other. Typical bandwidth is 3.6 kHz; only a fraction of the frequencies that humans can hear, but enough to make the voice intelligible. No phone can improve on this quality, as it is a limitation of the phone system itself. Higher-quality phones can transfer this signal to the handset with less interference over a greater range, however. Most cordless telephones, though, no matter what frequency band or transmission method is used, will hardly ever exactly match the sound quality of a high-quality wired telephone attached to a good telephone line. This constraint is caused by a number of issues, including the following:

1. Sidetone: hearing one's own voice echoed in the receiver speaker
2. A noticeable amount of constant background noise (This is not interference from outside sources, but noise within the cordless telephone system.)
3. Frequency response not being the full frequency response available in a wired landline telephone

Most manufacturers claim a range of about 30 m (100 ft) for their 2.4 GHz and 5.8 GHz systems, but inexpensive models often fall short of this claim.

However, the higher frequency often brings advantages. The 900 MHz and 2.4 GHz band are increasingly being used for a host of other devices, including baby monitor, microwave oven, Bluetooth, wireless LAN; thus, it is likely that a cordless phone will suffer interference from signals broadcast by those devices. It is also possible for a cordless phone to interfere with the 802.11a wireless standard, as the 802.11a standard can be configured to operate in the 5.8 GHz range. However, this can easily be fixed by configuring the device to work in the 5.180 GHz to 5.320 GHz band.

The recently allocated 1.9 GHz band is reserved for use by phones that use the DECT standard, which should avoid interference issues that are increasingly being seen in the unlicensed 900 MHz, 2.4 GHz, and 5.8 GHz bands.

Many cordless phones in the early 21st century are digital. Digital technology has helped provide clear sound and limit eavesdropping. Many cordless phones have one main base station and can add up to 3 or 4 additional bases. This allows for multiple voice paths that allow 3-way conference calls between the bases. This technology also allows multiple handsets to be used at the same time and up to 2 handsets can have an outside conversation.