Cordless telephone

History[edit]

Radio telephony (telephony without wires) 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 (in alphabetic order): 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 (although bulky) 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 Mass. 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 (1967).^[2]


In 1966, George Sweigert submitted a patent application for a "full duplex wireless communications apparatus". He was awarded U.S. patent 3,449,750 in June 1969.^[3] 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 (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 telephone network.^[4]


In 1977, Douglas G. Talley and L Duane Gregory were granted U.S. patent 4,039,760 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.^[5]


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.^[6] In 1995 digital spread spectrum (DSS) 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.^[7]

Many cordless phones in the 21st century are digital. Digital technology has helped provide clear sound and limit casual eavesdropping. Many cordless phones have one main base station and can add up to three or four additional bases. This allows for multiple voice channels 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 separate conversations with outside parties.


Manufacturers usually advertise that their higher frequency systems improve audio quality and range. In the ideal case, higher frequencies actually have worse signal propagation as shown by the basic Friis transmission equation, and path loss tends to increase at higher frequencies as well. Practical 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 adequate 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 handset 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, 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 limitation is caused by a number of issues, including the following:


Most manufacturers claim a range of about 30 metres (98 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, and wireless LAN; thus, it is likely that a cordless phone will suffer interference from signals broadcast by those devices, and also may itself generate interference. 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 reconfiguring the wireless LAN device to work in the 5.180 GHz to 5.320 GHz band.


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