A cordless telephone or portable telephone replaces the handset cord with a radio link. The handset communicates with a base station connected to a fixed telephone line. The range is limited usually to the same building or some short distance from the base station. The base station 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.
In 1994, digital cordless phones in the 900 MHz frequency range were introduced. Digital signals allowed the phones to be more secure and decreased eavesdropping—it was relatively easy to eavesdrop on analog cordless phone conversations. In 1995, digital spread spectrum (DSS) was introduced for cordless phones. This technology enabled the digital information to spread in pieces over several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on the cordless conversations.
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.
A jazz musician named Teri Pall invented a version of the cordless phone in 1965 but could not market her invention as its two-mile range caused radio signals to interfere with aircraft. She sold her rights to the cordless phone in 1968 to a manufacturer who modified it for practical use.
George Sweigert submitted a patent application in 1966 for a "full duplex wireless communications appartus". He was awarded US 3449750 in June 1969 He (see below: Patents). Sweigert, a radio operator in World War II stationed at the South Pacific Islands of Guadalcanal and Bougainville, developed the full duplex concept for untrained personnel, to improve battlefield communications for senior commanders.
Sweigert was an active proponent for directly coupling consumer electronics to the AT&T-owned telephone lines in the late 1960s. (This was banned at the time; most telephones were made by Western Electric and leased to the customer by AT&T.) The Carterfone coupler, 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 landmark Carterfone decision) on June 26, 1968. The original cordless phones, like the Carterfone, were acoustically (not electrically) connected to the Public Switched Telephone Network (PSTN).
Douglas G. Talley and L Duane Gregory were granted US 4039760 in 1977 for a duplex voice communication link including controls therefore as provided between a Base Station connected directly to a telephone line of a telephone exchange and a Mobile Unit consisting of a small, compact cordless telephone instrument containing transmitter, receiver and control circuits powered by a rechargeable battery pack. A single logic tone is transmitted and detected for all logical control for ring signals, on-hook and off-hook signals and dial pulses.
- 1.7 MHz (1.64 MHz to 1.78 MHz & up to 5 Channels, AM System)
- 43–50 MHz (Base: 43.72-46.97 MHz, Handset: 48.76-49.99 MHz, allocated in November 1984 for 10 channels, and later 25 Channels, FM System)
- 900 MHz (902–928 MHz) (allocated in 1993)
- 1.9 GHz (1880–1900 MHz) (used for DECT communications outside the U.S.)
- 1.9 GHz (1920-1930 MHz) (developed in 1993 and allocated U.S. in October 2005, esp. re DECT 6.0)
- 2.4 GHz (allocated in 1998)
- 5.8 GHz (allocated in 2003 due to crowding on the 2.4 GHz band).
1.7 MHz cordless phones were the earliest models available at retailers, generally identifiable by their large metal telescoping antennas. Channels just above the AM broadcast band were selected manually by the user. Some of the frequencies used are now part of the expanded AM radio band can be clearly heard by anyone with an AM radio. There are reports of people still using these phones and even using them as make-shift AM radio stations that can be heard for a couple of city blocks. These models are no longer in production. They are considered obsolete because they are susceptible to eavesdropping and interference, especially from fluorescent lighting and automobile ignition systems. However, before the popularity of cellular phones, a person under the right conditions could get 1/2 mile or more range out of these AM systems. These phones are sharing the 49.8 Mhz band (49.830 - 49.890) and are directly shared with baby monitors.
43–50 MHz cordless phones had a large installed base by the early 1990s, and featured shorter flexible antennas and automatic channel selection. Due to their popularity, an over crowding of the band led to an allocation of additional frequencies, thus manufacturers were able to sell models with 25 available channels instead of just 10 channels. Despite being less susceptible to interference, these models are no longer in production and are considered obsolete because these frequencies are easily heard on practically any radio scanner. Advanced models began to use voice inversion as a basic form of scrambling to help limit unauthorized eavesdropping.
900 MHz cordless phones are still sold today and have a huge installed base. Features include even shorter antennas, up to 30 auto selecting channels, and higher resistance to interference. Available in three varieties; analog, analog spread spectrum (100Khz bandwidth), digital, and digital spread spectrum, with most being sold today as budget analog models. Analog models are still susceptible to eavesdropping, However, older used models can still be found that are fully capable of receiving this spectrum. Digital variants can still be scanned, but are received as a digital hiss and therefore are difficult to eavesdrop upon. Digital transmission is immune to static interference but can experience signal fade (brief silence) as the phone goes out of range of the base. Digital Spread Spectrum (DSS) variants spread their signal over a range of frequencies providing more resistance to signal fade. This technology enabled the digital information to spread in pieces over several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on the cordless conversation. The FCC only allows DSS model phones to transmit at the full power of 1 watt, which allows increased range over analog and digital models.
Virtually all telephones sold in the US use the 900 MHz, 1.9 GHz, 2.4-GHz, or 5.8 GHz bands, though legacy phones can remain in use on the older bands. There is no specific requirement for any particular transmission mode on 900, 1.9, 2.4, and 5.8, but in practice, virtually all newer 900 MHz phones are inexpensive analog models with digital features such as DSSS and FHSS generally available only 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.
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:
- Sidetone: hearing one's own voice echoed in the receiver speaker
- A noticeable amount of constant background noise (This is not interference from outside sources, but noise within the cordless telephone system)
- 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 three or four additional bases. This allows for multiple voice paths that allow three-way conference calls between the bases. This technology also allows multiple handsets to be used at the same time and up to two handsets can have an outside conversation.
Many analog phone signals are easily picked up by radio scanners, allowing anyone within range to listen in on conversations (though this is illegal in many countries). Though many such analog models are still produced, modern digital technology is available to reduce the risk of eavesdropping. Digital Spread Spectrum (DSS) typically uses frequency hopping to spread the audio signal (with a 3 kHz bandwidth) over a much wider range of frequencies in a pseudorandom way. Spreading the signal out over a wider bandwidth is a form of redundancy, and increases the signal-to-noise ratio, yielding longer range and less susceptibility to interference. Higher frequency bands provide more room for these wide-bandwidth signals.
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.
Wireless phone handsets
Roaming wireless phone handsets exist which are not tethered to any particular base station, but which also do not use traditional mobile (cellular) phone networks. These most commonly use digital technologies like DECT, 2.4 GHz unlicensed spectrum, or 802.11a/b/g standards-based wireless LAN technology. The wireless phone handset must connect to a wireless access point or base station that supports the same technology. Also required is a call management function and a gateway to the public switched telephone network (PSTN). This may or may not be integrated in the base-station. A Voice over IP service can be used by phones that use wireless data access points, thus using a broadband Internet connection to defer the connection to the PSTN to a remote gateway operated by the service provider, close to the call's destination. Analog equivalents do exist and can provide longer reach, but with potential loss of confidentiality and voice quality. Most digital systems have inherent encryption or offer optional encryption.
Integration with mobile phones
Some cordless phones can use the Bluetooth standard to ring and answer calls on behalf of a mobile (cellular) phone near the base station.
- Digital Enhanced Cordless Telecommunications (DECT) in Europe
- Fixed-Mobile Convergence Alliance
- Mobile phone
- Personal Handy-phone System (PHS) in Japan and China
- Radio frequency
- Spread spectrum
- Patently female: from AZT to TV dinners : stories of women inventors and their breakthrough ideas, Ethlie Ann Vare, Greg Ptacek
- US 3449750 DUPLEX RADIO COMMUNICATION AND SIGNALING APPARATUS FOR PORTABLE TELEPHONE ... G. H. SWEIGERT
- Google Patents link
- US patent 174465, A. G. Bell, "Telegraph", issued 1876-03-07
- US patent 775337, ROBERTO LANDELL DE MOTYRA, "WIRELESS TELEPHONE", issued 1904-11-22
- US patent 3449750, G. H. Sweigert, "Duplex Radio Communication and Signalling Appartus", issued 1969-06-10
- Patents Link
|Wikimedia Commons has media related to Cordless telephones.|
- Review of Frequency Allocations for Cordless Telephones
- Carterphone Decision
- How Cordless Telephones Work
- Information about Digital Spread-Spectrum cordless phones