Cordless telephone

Cordless telephone

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A cordless telephone or portable telephone is a telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 with a wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...

 handset that communicates via radio waves
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...

 with a base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...

 connected to a fixed telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

, 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
Personal Handy-phone System
The Personal Handy-phone System , also marketed as the Personal Access System and commercially branded as Xiaolingtong in China, is a mobile network system operating in the 1880–1930 MHz frequency band, used mainly in Japan, China, Taiwan, and some other Asian countries and...

 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
Roaming
In wireless telecommunications, roaming is a general term referring to the extension of connectivity service in a location that is different from the home location where the service was registered. Roaming ensures that the wireless device is kept connected to the network, without losing the...

. In these models, base stations are maintained by a commercial mobile network operator
Mobile network operator
A mobile network operator , also known as mobile phone operator , carrier service provider , wireless service provider, wireless carrier, or cellular company, or mobile network carrier is a telephone company that provides services for mobile phone subscribers.One essential...

 and users subscribe to the service.

Unlike a corded telephone, a cordless
Cordless
The term cordless literally means without a cord and is generally used to refer to electrical or electronic devices that are powered by a battery or battery pack and can operate without a power cord or cable attached to a fixed electricity supply such as an outlet, generator, or other centralized...

 telephone needs mains electricity
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

 to power the base station. The cordless handset
Handset
On a telephone, the handset is a device the user holds to the ear to hear the audio sound. Modern-day handsets usually contain the phone's microphone as well, but in early telephones the microphone was mounted directly on the telephone itself, which often was attached to a wall at a convenient...

 is powered by a rechargeable battery
Rechargeable battery
A rechargeable battery or storage battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging anything from a button cell to...

, which is charged when the handset sits in its cradle.

History


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
George Sweigert
George H. Sweigert is widely credited as the first inventor to hold a patent for the invention of the cordless telephone. Google Patents link...

, an amateur radio operator and inventor from Cleveland, Ohio
Cleveland, Ohio
Cleveland is a city in the U.S. state of Ohio and is the county seat of Cuyahoga County, the most populous county in the state. The city is located in northeastern Ohio on the southern shore of Lake Erie, approximately west of the Pennsylvania border...

, is largely recognized as the father of the cordless phone. Google Patents link. http://www.google.com/patents?id=sidyAAAAEBAJ&dq=george+sweigert He submitted a patent application in 1966 for a "full duplex
Duplex (telecommunications)
A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....

 wireless communications appartus"[sic
Sic
Sic—generally inside square brackets, [sic], and occasionally parentheses, —when added just after a quote or reprinted text, indicates the passage appears exactly as in the original source...

]. The U.S. Patent and Trademark Office awarded him a patent in June 1969 (see below: Patents). Sweigert, a radio operator in World War II stationed at the South Pacific Islands of Guadalcanal
Guadalcanal
Guadalcanal is a tropical island in the South-Western Pacific. The largest island in the Solomons, it was discovered by the Spanish expedition of Alvaro de Mendaña in 1568...

 and Bougainville
Bougainville Island
Bougainville Island is the main island of the Autonomous Region of Bougainville of Papua New Guinea. This region is also known as Bougainville Province or the North Solomons. The population of the province is 175,160 , which includes the adjacent island of Buka and assorted outlying islands...

, developed the full duplex-concept for untrained personnel, to improve battlefield communications for senior commanders. He was also licensed as W8ZIS and N9LC in the amateur radio service. He also held a First Class Radiotelephone Operator's Permit issued by the Federal Communications Commission.

Sweigert was an active proponent for directly coupling consumer electronics to the AT&T
American Telephone & Telegraph
AT&T Corp., originally American Telephone and Telegraph Company, is an American telecommunications company that provides voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agencies. AT&T is the oldest telecommunications company...

-owned telephone lines in the late 1960s. (This was banned at the time; most telephones were made by Western Electric
Western Electric
Western Electric Company was an American electrical engineering company, the manufacturing arm of AT&T from 1881 to 1995. It was the scene of a number of technological innovations and also some seminal developments in industrial management...

 and rented to the customer by AT&T.) 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).

Filed 03/11/1974, Douglas G. Talley and L Duane Gregory applied for and were granted ( 08/02/1977) US Patent 4039760 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.

In the 1980s, a number of manufacturers, including Sony
Sony
, commonly referred to as Sony, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan and the world's fifth largest media conglomerate measured by revenues....

, 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
Nickel-cadmium battery
The nickel–cadmium battery ' is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes....

; the base unit was powered by household current, typically via an AC adaptor. 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
AA battery
An AA battery is a standard size of battery. Batteries of this size are the most commonly used type of in portable electronic devices. An AA battery is composed of a single electrochemical cell...

 or AAA cells in place of the more expensive traditional proprietary telephone batteries. Since the 1980s, several companies have entered the cordless-phone market: VTech
VTech
VTech is the common name of Video Technology Ltd. , a Hong Kong-based manufacturer of consumer electronics...

, Uniden
Uniden
is a Japanese company in the wireless communication industry.-History:Uniden was established on February 7, 1966 by its founder Hideo Fujimoto...

, Philips
Philips
Koninklijke Philips Electronics N.V. , more commonly known as Philips, is a multinational Dutch electronics company....

, Gigaset and Panasonic
Panasonic
Panasonic is an international brand name for Japanese electric products manufacturer Panasonic Corporation, which was formerly known as Matsushita Electric Industrial Co., Ltd...

. They advertise many new features, a few provided by the phone and most provided by the network.

Frequencies


In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, seven frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

 bands have been allocated by the Federal Communications Commission
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...

 for uses that include cordless phones. These are:
  • 1.7 MHz (1.64 MHz to 1.78 MHz & up to 5 Channels, AM System)http://electronics.howstuffworks.com/question320.htm
  • 43–50 MHz (Base: 43.72-46.97 MHz, Handset: 48.76-49.99 MHz, allocated in 1986 for 10 channels, and later 25 Channels, FM System)
  • 900 MHz (902–928 MHz) (allocated in 1994)
  • 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. Channel selection had to be done manually by the user, and transmitted just above the AM broadcast band. These models are no longer in production, and are considered obsolete due to being very susceptible to eavesdropping and interference, especially from fluorescent lighting and automobile ignition systems.

43-50 MHz cordless phones had a large installed base by the early 1990s, and featured shorter bendable antennas plus auto 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
Voice inversion
Voice Inversion scrambling is an analog method of obscuring the content of a transmission. It sometimes used in public service radio, automobile racing, cordless telephones and the Family Radio Service. Without a descrambler, the transmission makes the speaker sound like "Donald Duck"...

 as a basic form of scrambling to help limit unauthorized eavesdropping
Eavesdropping
Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...

.

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, 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
Direct-sequence spread spectrum
In telecommunications, direct-sequence spread spectrum is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact that the carrier signals...

 and FHSS
Frequency-hopping spread spectrum
Frequency-hopping spread spectrum is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver...

 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.

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
Friis transmission equation
The Friis transmission equation is used in telecommunications engineering, and gives the power received by one antenna under idealized conditions given another antenna some distance away transmitting a known amount of power. The formula was derived in 1945 by Danish-American radio engineer Harald T...

, and path loss
Path loss
Path loss is the reduction in power density of an electromagnetic wave as it propagates through space. Path loss is a major component in the analysis and design of the link budget of a telecommunication system....

 tends to increase at higher frequencies as well. More important influences on quality and range are signal strength
Signal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...

, antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

 quality, the method of modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...

 used, and interference, which varies locally.

"Plain old telephone service
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

" (POTS) landline
Landline
A landline was originally an overland telegraph wire, as opposed to an undersea cable. Currently, landline refers to a telephone line which travels through a solid medium, either metal wire or optical fibre, as distinguished from a mobile cellular line, where transmission is via radio waves...

s 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
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

. This constraint is caused by a number of issues, including the following:
  1. Sidetone
    Sidetone
    Sidetone is audible feedback to someone who is speaking. The term is most used in telecommunication contexts.-Telephony:In telephony, sidetone is the effect of sound that is picked up by the telephone's mouthpiece and in real-time introduced at a low level into the earpiece of the same handset,...

    : hearing one's own voice echoed in the receiver speaker
  2. A noticeable amount of constant background noise (This is not interference from outside sources, but noise within the cordless telephone system.)
  3. Frequency response
    Frequency response
    Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...

     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
Baby monitor
A baby monitor, also known as a baby alarm, is a radio system used to remotely listen to sounds made by an infant. The transmitter unit, equipped with a microphone, is placed near to the child. It transmits the sounds the baby makes by radio waves to a receiver unit with a speaker carried by, or...

, microwave oven
Microwave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...

, Bluetooth
Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...

, wireless LAN
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...

; 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
ISM band
The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....

 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 3 or 4 additional bases. This allows for multiple voice paths that allow 3-way conference call
Conference call
A conference call is a telephone call in which the calling party wishes to have more than one called party listen in to the audio portion of the call. The conference calls may be designed to allow the called party to participate during the call, or the call may be set up so that the called party...

s between the bases. This technology also allows multiple handsets to be used at the same time and up to 2 handsets can have an outside conversation.

Security


Many analog phone signals are easily picked up by radio scanner
Scanner (radio)
A scanner is a radio receiver that can automatically tune, or scan, two or more discrete frequencies, stopping when it finds a signal on one of them and then continuing to scan other frequencies when the initial transmission ceases....

s, 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
Eavesdropping
Eavesdropping is the act of secretly listening to the private conversation of others without their consent, as defined by Black's Law Dictionary...

. 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
Pseudorandomness
A pseudorandom process is a process that appears to be random but is not. Pseudorandom sequences typically exhibit statistical randomness while being generated by an entirely deterministic causal process...

 way. Spreading the signal out over a wider bandwidth is a form of redundancy
Redundancy (information theory)
Redundancy in information theory is the number of bits used to transmit a message minus the number of bits of actual information in the message. Informally, it is the amount of wasted "space" used to transmit certain data...

, and increases the signal-to-noise ratio
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

, 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
Pseudorandom number generator
A pseudorandom number generator , also known as a deterministic random bit generator , is an algorithm for generating a sequence of numbers that approximates the properties of random numbers...

 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
Wireless access point
In computer networking, a wireless access point is a device that allows wireless devices to connect to a wired network using Wi-Fi, Bluetooth or related standards...

 or base station
Base station
The term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...

 that supports the same technology. Also required is a call management function and a gateway to the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

 (PSTN). This may or may not be integrated in the base-station. A Voice over IP
Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...

 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
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

 or offer optional encryption.

Integration with mobile phones


Some cordless phones can use the Bluetooth
Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...

 standard to ring and answer calls on behalf of a mobile (cellular) phone near the base station.

Patents


See also

  • Carterphone
  • Digital Enhanced Cordless Telecommunications
    Digital Enhanced Cordless Telecommunications
    Digital Enhanced Cordless Telecommunications , usually known by the acronym DECT, is a digital communication standard, which is primarily used for creating cordless phone systems...

     (DECT) in Europe
    Europe
    Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

  • Fixed-Mobile Convergence Alliance
  • Mobile phone
    Mobile phone
    A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

  • Personal Handy-phone System
    Personal Handy-phone System
    The Personal Handy-phone System , also marketed as the Personal Access System and commercially branded as Xiaolingtong in China, is a mobile network system operating in the 1880–1930 MHz frequency band, used mainly in Japan, China, Taiwan, and some other Asian countries and...

     (PHS) in Japan
    Japan
    Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

     and China
    China
    Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

  • Radio frequency
    Radio frequency
    Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...

  • Spread spectrum
    Spread spectrum
    Spread-spectrum techniques are methods by which a signal generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth...


External links