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.
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.
"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:
There has been the rare exception, however, where a certain cordless telephone sounds indistinguishable from a quality wired landline telephone. Usually these are rare examples of a particular phone model, thereby being a 'fluke" rather than the norm.
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 operates 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 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 an outside conversation.
To an analog receiver like a scanner, a DSS signal sounds like bursts of noise. Only the base unit with the same pseudorandom number generator can receive the signal, and it chooses from one of thousands of such unique generators each time the handset is returned to the cradle.
Additionally, the digital nature of the signal increases its tolerance to noise, and some even encrypt the digital signal for even more security.
Sweigert was an active proponent for directly coupling consumer electronics to the AT&T-owned telephone lines in the late 1960s (which was banned at the time). The Carterphone, a crude device for interconnecting a two-way radio with the telephone, led to the reversal of the Federal Communications Commission ban on direct coupling of consumer equipment to phone lines (known as the 1968 landmark Carterphone decision) on June 26, 1968. The original cordless phones, like the Carterphone, were acoustically (not electrically) connected to the Public Switched Telephone Network (PSTN).
In the 1980s, a number of manufacturers, including Sony, introduced cordless phones for the consumer market. Typically, they used a base station that was connected to a telephone line and a handset with a microphone, speaker, keypad, and telescoping antenna. The handset contained a rechargeable battery, typically NiCd; the base unit was powered by household current, typically via a wall wart. The base included a charging cradle, which was generally a form of trickle charger, on which the handset rested when not in use.
Some cordless telephones now utilize two rechargeable AA or AAA batteries in place of the more expensive traditional proprietary telephone batteries.
Cordless phones became commercially feasible with the breakup of the Bell System's monopoly on land-line telephone service around 1984. Before the breakup, all telephones were made by Western Electric and rented to the customer.
Since the 1980s, several companies have entered the cordless-phone market: V-Tech, Uniden, Philips, and Panasonic. They advertise many new features: a few provided by the phone and most provided by the network.