A cordless telephone or portable telephone is a telephone with a wireless handset that communicates via radio waves with a base station connected to a fixed telephone line, usually within a limited range of its base station (which has the handset cradle). The base station is on the subscriber premises, and attaches to the telephone network the same way a corded telephone does.

The base station on subscriber premises is what differentiates a cordless telephone from a mobile telephone. Current cordless telephone standards, such as PHS and DECT, have blurred the once clear-cut line between cordless and mobile telephones by implementing cell handover, various advanced features, such as data-transfer and even, on a limited scale, international roaming. In these models, base stations are maintained by a commercial mobile network operator and users subscribe to the service.

Unlike a corded telephone, a cordless telephone needs mains electricity to power the base station. The cordless handset is powered by a rechargeable battery, which is charged when the handset sits in its cradle.

Frequencies

In the United States, seven frequency bands have been allocated by the Federal Communications Commission for uses that include cordless phones. These are:

• 1.7 MHz (Up to 6 Channels, AM System)1
• 27 MHz (allocated in 1980, Up to 10 Channels, FM System)
• 43–50 MHz (allocated in 1986, Up to 25 Channels, FM System, Base Transmitter: 43.72-46.97MHz, Handset Transmitter: 48.76-49.99MHz)
• 900 MHz (902–928 MHz) (allocated in 1990)
• 1.9 GHz (1920-1930 MHz) (developed in 1993 and allocated U.S. in October 2005)
• 2.4 GHz (allocated in 1998)
• 5.8 GHz (allocated in 2003 due to crowding on the 2.4 GHz band).

Virtually all telephones sold in the US use the 900 MHz, 1.9 GHz, 2.4-GHz, or 5.8 GHz bands, though some legacy phones remain in use on the 27- and 43-50-MHz bands. There is no specific requirement for any particular transmission mode on 900, 2.4, and 5.8, but in practice, virtually all newer 900 MHz phones are inexpensive, bare-bones analog models; digital features such as DSSS and FHSS are now generally only available on the higher frequencies.

Some cordless phones advertised as 5.8 GHz actually transmit from base to phone on 5.8 GHz and transmit from phone to base on 2.4 GHz or 900 MHz, to conserve battery life inside the phone.

The recently allocated 1.9 GHz band is used by the popular DECT phone standard and is considered more secure than the other shared frequencies.

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.