A cordless phone or portable phone is a phone where the handset is portable and communicates with the body of the phone over the radio, rather than attached by a cable. The base station is connected to the telephone network through a telephone line as a cable phone, and also serves as a charger to charge the phone battery. The range is limited, usually to the same building or short distance from the base station. They use spectrum without operation and operate in one of the following ranges:
1880_1900 MHz (DECT_ Europe) 1850_1910 MHz (PACS_USA)
The base station at the customer's premises is what distinguishes cordless phones from cell phones. Current wireless cordless standards, such as PHS and DECT, have blurred a clear line between cordless phones and phones by applying handoff handoffs; various advanced features, such as data transfer; and even, on a limited scale, international roaming. In special models, base stations are managed by commercial mobile network operators and users subscribe to those services.
In 1994, digital cordless phones in the 900 MHz frequency range were introduced. Digital signals allow the phone to be more secure and reduce intercepts - relatively easy to eavesdrop on analogue cordless phone calls. In 1995, digital spread spectrum (DSS) was introduced for cordless phones. This technology allows digital information to spread between several frequencies between the receiver and the base, making it almost impossible to eavesdrop on the cordless conversation.
Unlike a wired phone, a cordless phone requires primary power to power the base station. This cordless handset is powered by a rechargeable battery, which is charged when the handset is stored in its place.
Video Cordless telephone
Histori
A jazz musician named Teri Pall invented a cordless phone version in 1965 but could not market his invention, because a distance of 2 miles (3.2 km) caused a radio signal to interfere with plane communication. In 1968, he sold his rights to a cordless phone to a manufacturer who modified it for practical use.
Bell Labs Lineless Telephone 1963-1967
Beginning in 1963, a small team of Bell Laboratories engineers was given the task of developing a fully functional and fully functional Lineless duplex phone. The team includes (in alphabetical order): S.M. Baer, ​​â € <â € In 1964, the breadboard model worked in the laboratory. During 1964-1965 this was refined and packed for testing around the building and land of Bell Labs Holmdel N.J. The system is operated under an experimental license on a crystal-controlled channel in bands 35 and 43 MHz using FM, low power transmitter and sensitive superhet receiver. Full control of all phone functions, including hooks and on-off calls is provided through an unparalleled tone control system. The end user of the system is packaged to look like a normal (though large) phone handset. The other end - base station is a small box connected to a regular telephone network. (See additional photographs at Bell Laboratories Record the article below.Note mainly "dial".) Around 50 units were built at the Western Electric model store in Andover Mass for field lanes at two Bell System locations in the Boston and Phoenix region. The entire project is described in Bell Laboratories Record, Volume 45 (1967). In 1966, George Sweigert applied for a patent for "full duplex wireless communications appartus". He was awarded US 3449750 in June 1969 Dia (see below: Patents ). Sweigert, a World War II radio operator stationed in the South Pacific Islands of Guadalcanal and Bougainville, developed a complete duplex concept for untrained personnel, to improve battlefield communications for senior commanders. Sweigert is an active proponent to directly combine consumer electronics into the AT & amp; T-owned in the late 1960s. (This is prohibited at the time; most phones are made by Western Electric and leased to customers by AT & T.) The Carterfone coupler, a raw device for connecting two-way radios with a telephone, causes a reversal of the Federal Communications Commission banning direct clutch equipment consumers to the telephone line (known as Carterfone landmark decisions) on June 26, 1968. Original cordless phones, such as Carterfone, are acoustically connected to the Public Switched Telephone Network (PSTN). In 1977, Douglas G. Talley and L Duane Gregory provided US 4039760 Ã, for duplex voice communication links including controls therefore provided between stations bases connected directly to telephone exchange telephone lines and mobile units comprising a cordless small phone instrument containing transmitter, receiver and control circuits supported by rechargeable batteries. A single logic tone is transmitted and detected for all logical controls for ring signals, on-hook and off-hook signals and call pulses. Maps Cordless telephone
Frequency
In the United States, seven frequency bands have been allocated by the Federal Communications Commission for use that includes cordless phones. This is:
- 1.7Ã, MHz (1.64-1,78 MHz, up to 5 channels, AM). Cordless phones manufactured after October 1, 1984 are not permitted to use this band and are required to use a newer 43-50 MHz frequency (higher).
- 27 MHz, near the Radio Citizens Band (CB) service with some frequencies are 26,010, 26,050, 26,380 and 26,419 MHz. It was originally paired with a frequency of 1.7 MHz, then, then, with a frequency of 49 MHz. The signal is FM frequency modulation.
- 43-50 MHz (Base: 43.72-46.97 MHz, Handset: 48.76-49.99 MHz, FM) Allocated in December 1983, and approved for use in mid-1984 for 10 channels. 15 additional channels allocated 5 April 1995.
- 900Ã, MHz (902-928Ã, MHz, allocated in 1993)
- 1.9Ã, GHz (1880-1900 MHz, used for DECT communications outside the US)
- 1.9Ã, GHz (1920-1930 MHz, developed in 1993 and allocated in October 2005, especially with DECT 6.0)
- 2.4 GHz (2400-2500 MHz, allocated in 1998)
- 5.8Ã, GHz (5725-5875 MHz, allocated in 2003 due to the crowd on the 2.4Ã ©, GHz band)
Due to the beginning of the previous frequency allocation crowd that caused the user to stop using telephone equipment operating at that frequency, the tape is now relatively clear. Currently, radio monitoring fans note the use of older equipment with phone activity in US AM broadcast bands, some 27 MHz frequencies and most of the older 43-50 MHz frequencies.
The 1.7 MHz cordless phone is the earliest model available at retailers, and is generally identifiable by their large metal telescoping antenna. The channel just above the AM broadcast band is manually selected by the user. Some of the frequencies used now become part of the expanded AM radio waves, and can be heard clearly by anyone with an AM radio. There are reports of people still using this phone, and even using them as an emergency AM radio station that can be heard for several city blocks. These models are no longer in production, and are considered obsolete because they are susceptible to intercepts and interference, especially from fluorescent lamps and car ignition systems. However, a person under the right conditions can get 0.5 miles (0.80 km) or more range from this AM system.
The 43-50 MHz cordless phone had a large installed base in the early 1990s, and featured a shorter flexible antenna and automatic channel selection. Due to their popularity, the density of the band caused an additional frequency allocation; so manufacturers are able to sell models with 25 channels instead of just 10 channels. Although less susceptible to interference from previous AM units, these models are no longer produced and are considered obsolete because their frequencies are easily heard in almost any radio scanner. Advanced models began to use voice inversion as a basic form of randomization to help limit unauthorized tapping. This phone shares the 49.8 MHz band (49,830 - 49,890) with multiple wireless baby monitors.
900Ã, MHz Cordless phones are still on sale today and have a large installed base. Features include shorter antennas, up to 30 automatic selection channels, and higher noise resistance. Available in several varieties; analog, analog spread spectrum (100-kHz bandwidth), digital, and digital spread spectrum, the most widely sold today are cheap analog models, which are still vulnerable to eavesdropping. Digital variants can still be scanned, but accepted as digital hiss and therefore difficult to tap. Digital transmission is immune to static interference but can experience a fading signal (brief silence) when the phone is out of range of the base. Newer Spread Spectrum (DSS) variants propagate their signals over a wide range of frequencies, providing more resistance to fading signals. This technology allows digital information to spread between several frequencies between the receiver and the base, making it almost impossible to eavesdrop on the cordless conversation. The FCC only allows DSS model phones to transmit a full power of 1 watt, enabling increased reach through older analog and digital models.
Nearly all new phones sold in the US use bands 900Ã, MHz, 1.9Ã, GHz, 2.4-GHz, or 5.8Ã, GHz, although legacy phones can still be used on older bands. There are no special requirements for certain transmission modes on 900, 1.9, 2.4, and 5.8, but in practice, almost all newer 900 MHz phones are cheap analog models; some digital features such as DSSS and FHSS are generally only available at higher frequencies.
Some cordless phones advertised as 5.8 GHz actually transmit from base to phone at 5.8 GHz and send from phone to base at 2.4 GHz or 900 MHz, to save phone battery.
The 1.9Ã9 band, the recently allocated GHz is used by popular DECT phone standards and is considered more secure than other shared frequencies.
Performance
Many cordless phones in the early 21st century are digital. Digital technology has helped provide clear voice and limit eavesdropping. Many wireless phones have one main base station and can add up to three or four additional bases. This allows multiple voice channels that allow conference calling three directions between bases. This technology also allows multiple handsets to be used at the same time, and up to two handsets can have separate conversations with outsiders.
Manufacturers usually advertise that their higher frequency system improves audio quality and range. In an 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. The practical effects on quality and range are signal strength, antenna quality, modulation methods used, and interference, which vary locally.
"Plain old telephone service" (POTS) landlines are designed to transfer audio of sufficient quality for the parties to understand each other. Typical bandwidth is 3.6 kHz; only a fraction of the frequency that humans can hear, but enough to make sound understandable. No phone handset can improve this quality, as this is a limitation of the phone system itself. Higher quality phones can transfer this signal to the handset with less interference in the larger range. Most cordless phones, no matter what frequency band or transmission method is used, will hardly ever be exactly the same as the quality of a high quality cable phone that connects to a good phone line. This limitation is caused by a number of issues, including the following:
- Sidetone: hear your own voice echoing in receiver speakers
- The amount of background noise is constant (this is not a nuisance from an outside source, but noise in a cordless phone system)
- The frequency response is not the full frequency response available on the cable phone line
Most manufacturers claim a range of about 30 meters (98Ã, ft) for systems of 2.4 GHz and 5.8 GHz, but inexpensive models often fail to meet this claim.
However, higher frequencies often bring benefits. The 900 MHz and 2.4 GHz bands are increasingly used for a number of other devices, including baby monitors, microwave ovens, Bluetooth, and wireless LANs; thus, it is possible that the cordless phone will be subject to interference from signals broadcast by the device, and may also cause interference. It also allows for cordless phones to interfere with 802.11a wireless standards, as 802.11a standards can be configured to operate within the 5.8 GHz range. However, this can be easily fixed by reconfiguring the wireless LAN device to work in 5,180 GHz to 5.320 GHz ribbons.
The more recent 1.9Ã ©, GHz bands are reserved for use by mobile phones that use the DECT standard, which should avoid the increasingly visible interference problems in the unlicensed 900 MHz, 2.4 GHz and 5.8 GHz band frequencies.
Security
Many analog phone signals are easily picked up by radio scanners, allowing anyone within reach to hear the conversation (though this is illegal in many countries). Although many such analog models are still in production, modern digital technology is available to reduce the risk of tapping. Digital Spread Spectrum (DSS) typically uses frequency hopping to spread audio signals (with 3kHz bandwidth) over a much wider frequency range by pseudorandom means. Spreading out signals over a wider bandwidth is a form of redundancy, and improves signal-to-noise ratio, resulting in longer range and less susceptibility to interference. The higher frequency band gives more space for this wide bandwidth signal.
For analog receivers such as scanners, the DSS signal sounds like a blast of sound. Only basic units that use suitable pseudorandom numbers can decode the signal, and select from one of thousands of unique codes each time the handset is returned to the cradle. In addition, the digital nature of the signal increases its tolerance to noise, and some systems even encrypt digital signals for added security.
wireless phone handset
There are roaming wireless phone handsets that are not moored to a particular base station, but also do not use traditional cellular (cellular) networks. It most often uses digital technologies such as DECT, 2.4 GHz without spectrum, or 802.11a/b/g standard wireless LAN technology. The wireless phone handset must be connected to a wireless access point or base station that supports the same technology. Also required is the call management function and gateway to the public switched telephone network (PSTN). It may or may not be integrated in the base station. The Voice over IP service can be used by telephones using wireless data access points, thus using a broadband Internet connection to delay connections to the PSTN to a remote gateway operated by the service provider, close to the destination of the call. Equivalent analogue exists and can provide longer range, but with the potential for loss of confidentiality and sound quality. Most digital systems have inherent encryption or offer optional encryption.
Health and safety
See also
- Carterphone
- Fixed Mobile Convergence Alliance
- Mobile
- Personal Handy-phone System (PHS) in Japan and China
- Spread spectrum
References
Patent
US Patent 174465, AG Bell, "Telegraph", published 1876-03-07 US Pat. 775337, ROBERTO LANDELL DE MOTYRA, "WIRELESS TELEPHONE", published 1904-11-22External links
- Review Frequency Allocation for Non-Wired Phone
- Carterphone Decision
- How The Cordless Phone Works
- Information about the Spread-Spectrum Digital cordless phone
Source of the article : Wikipedia
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