Cordless telephone
A cordless telephone or portable telephone is a portable telephone handset that connects by radio to a base station connected to the public telephone network. The operational range is limited, usually to the same building or within some short distance from the base station.
A cordless telephone differs functionally from a mobile phone in its limited range and by depending on the base station on the subscriber premises. Current cordless telephone standards, such as PHS and DECT, have blurred the once clear-cut line between cordless and mobile telephones by implementing cell handoff ; various advanced features, such as data-transfer; and even, on a limited scale, international roaming. In specialized models, a commercial mobile network operator may maintain base stations and users subscribe to the service.
Unlike a corded telephone, a cordless telephone needs mains electricity. The cordless handset contains a rechargeable battery, which the base station re-charges when the handset rests in its cradle.
History
Radio telephony predated cordless phones by at least two decades. The first, MTS, or Mobile Telephone Service went into service in 1946. Because the range was intended to cover the widest possible service area, capacity was extremely low, and the early tube technology made equipment rather large and heavy. The second generation radio telephone, or IMTS, or Improved Mobile Telephone Service became active in 1964.Beginning in 1963, a small team of Bell Laboratories engineers were tasked with developing a practical and fully functional duplex wireless telephone. The team included : S.M. Baer, G.C. Balzer, J.M. Brown, W.F. Clemency, M. Rosenthal, and W. Zinsmeister, under the direction of W.D. Goodale, Jr.
By 1964, breadboard models were working in the lab. During 1964-65 these were refined and packaged to test around the Bell Labs Holmdel N.J. facilities. The system operated under an experimental license on crystal controlled channels in the 35 and 43 MHz bands using FM, a low power transmitter and a sensitive superhet receiver. Full supervision of all telephone functions, including on-off hook and dialing was provided via an out of band tone supervision system. The model developed for home use was designed to look like a standard telephone handset. The base station was a small box connected to a standard telephone network. About 50 units were built in a Western Electric model shop in Andover for field trials in two Bell System locations in the Boston and Phoenix area. The overall project was described in the Bell Laboratories Record, Volume 45.
In 1966, George Sweigert submitted a patent application for a "full duplex wireless communications apparatus". He was awarded in June 1969. 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. The telephone companies at the time did not permit third-party equipment to be connected to their lines; 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 on June 26, 1968. The original cordless phones, like the Carterfone, were acoustically connected to the public telephone network.
In 1977, Douglas G. Talley and L. Duane Gregory were granted for a duplex voice communication link including controls 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.
Cordless phones became widely used in home and workplaces during the early 1980s. According to The New York Times, the number of cordless phones sold in the United States grew from 50,000 in 1980 to 1 million in 1982. They quickly became popular because of their convenience and portability, despite fears that their reliance on radio signals would make them vulnerable to eavesdropping or other malfeasance.
In 1994, digital cordless phones in the 900 MHz frequency range became available to consumers. These new types of phones provided better audio quality because it could filter out interference and its signals could penetrate walls more easily. 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 was introduced for cordless phones. This technology enabled the spreading of the digital voice transmission over multiple frequencies, improving privacy and reducing interference between different subscribers.
Frequencies
United States
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 Cordless phones manufactured after October 1, 1984, are not allowed to use this band and were required to use the newer 43-50 MHz frequencies, although older telephones, on the older frequency pairs, could still be used.
- 27 MHz, near the Citizens Band Radio service with some frequencies being 26.010, 26.050, 26.380, 26.419 and 27.095 MHz. These were initially paired with the 1.7 MHz frequencies, then, later, with the 49 MHz frequencies. Signals were FM - frequency modulation.
- 43–50 MHz Allocated in December 1983, and approved for use in mid-1984 for 10 channels. 15 additional channels allocated April 5, 1995.
- 900 MHz
- 1.9 GHz
- 2.4 GHz
- 5.8 GHz
1.7 MHz cordless phones were the earliest models available at retailers, and are 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, and can be heard by anyone with an AM radio. There are reports of people still using these phones, and using them as makeshift AM radio stations that can be heard for a couple of city blocks. These models became obsolete because they are susceptible to eavesdropping, and interference from fluorescent lighting and automobile ignition systems. However, under ideal conditions they could have or more range.
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 overcrowding of the band led to an allocation of additional frequencies; thus manufacturers were able to sell models with 25 channels instead of just 10 channels. Although less susceptible to interference than previous AM units, these models are no longer in production and are considered obsolete because their 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. These phones share the 49.8 MHz band with some wireless baby monitors.
900 MHz cordless phones are rarely sold but have a huge installed base. Features include even shorter antennas, up to 30 auto selecting channels, and higher resistance to interference. Available in several varieties; analog, analog spread spectrum, digital, and digital spread spectrum, most being sold today are low-cost analog models, which are still susceptible to eavesdropping. 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 as the phone goes out of range of the base. Newer Digital Spread Spectrum 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 among 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 older analog and digital models.
Virtually all new cordless phones sold in the US use DECT 6.0 on the 1.9 GHz band, though legacy phones can remain in use on the older 900 MHz, 2.4 GHz and 5.8 GHz bands. There is no specific requirement for any particular transmission mode on the older bands, but in practice many legacy phones also have digital features such as DSSS and FHSS.
Some cordless phones formerly 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.
The 1.9 GHz band is used by the DECT 6.0 phone standard and is considered more secure than the other shared frequencies. The vast majority of new cordless phone devices sold in North America, whether connected by landline or to mobile phones, now use DECT 6.0. However, DECT 6.0's late start compared to DECT elsewhere has led to a large installed base of legacy cordless phones using other frequencies, many of which remain in use today despite increasingly common interference with the ever growing use of Wi-Fi, Bluetooth and other unlicensed digital radio standards, especially at 2.4 GHz and 5.8 GHz.