Cordless telephone

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A cordless telephone with base.

A cordless telephone or portable telephone is a telephone with a wireless handset which communicates via radio waves with a base station connected to a fixed telephone line (POTS) and can only be operated near (typically less than 100 meters) its base station, such as in and around the house. Unlike a standard telephone, a cordless telephone needs household mains electricity to power the base station. The cordless handset is powered by a battery which is recharged by the base station when the handset sits in its cradle.

Modern cordless telephone standards, like PHS and DECT, have blended the once clear-cut line between cordless and mobile telephones by supporting cell handover, various advanced features like data transfer and even, on a limited scale, international roaming. In these deployment models, base stations are maintained by a commercial mobile network operator and users subscribe to the service.

1 Frequencies
2 Performance
3 Security
4 Wireless phone handsets
5 History
6 Patents
7 See also
8 External links

[edit] Frequencies

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 (Up to 6 Channels, AM System)[1]
27 MHz (allocated in 1980, Up to 10 Channels, FM System)
43–50 MHz (allocated in 1986, Up to 25 Channels, FM System, Base Transmitter: 43.72-46.97MHz, Handset Transmitter: 48.76-49.99MHz)
900 MHz (902–928 MHz) (allocated in 1990)
1.9 GHz (1920-1930 MHz) (developed in 1993 and allocated U.S. in October 2005)
2.4 GHz (allocated in 1998)
5.8 GHz (allocated in 2003 due to crowding on the 2.4GHz band)

Virtually all telephones now sold in the US use the 900 MHz, 2.4 GHz, or 5.8 GHz bands, though some legacy phones remain in use on the 27 and 43-50 MHz bands. There is no specific requirement for any particular transmission mode on 900, 2.4, and 5.8, but in practice virtually all 900 MHz phones are inexpensive, bare-bones analog models; digital features such as DSSS and FHSS are generally only available on the higher frequencies.

One must be careful when looking to purchase 5.8 GHz phones. In actuality, only high-end 5.8 GHz hardware transmits both ways on that frequency. Most so-called 5.8 GHz cordless phones 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 from Europe.

[edit] 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, 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 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 part 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 the frequency band used, or the transmission method, will hardly ever exactly match the sound quality of a high quality wired telephone attached to a good phone line. This can be caused by a number of issues including:

Sidetone, hearing one's own voice echoed in the receiver speaker.
A noticeable amount of constant background noise. (This is not interference from outside souces, but noise within the cordless telephone system.)
Frequency response not being the full frequency response available in a wired landline telephone. There has been the rare exception however where a certain cordless telephone sounds indistinguishable from a quality wired landline telephone, but this is very rare. Usually these are rare examples of a particular phone model, thereby being a 'fluke" rather than the norm.

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 the cordless phone will suffer interference from signals broadcast by those devices. It is a common misconception that it interferes with the 802.11a wireless standard, as the 802.11a standard operates most commonly in the 5.180 GHz to 5.320 GHz band whereas this is 5.8 GHz. However, 802.11a devices can operate in the 5.8GHz range if configured to do so.

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.

[edit] Security

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.

[edit] Wireless phone handsets

Wireless phone handsets exist, designed for connection with a local wired service, not using traditional mobile phone networks, most commonly using digital technologies: namely, DECT, 2.4 GHz unlicensed spectrum or 802.11a/b/g standards-based wireless LAN technology. The wireless phone handset (per definition) 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 telephony network (PSTN), this may or may not be integrated in the base-station. 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.

[edit] History

George Sweigert, an amateur radio operator and inventor from Cleveland, Ohio, is largely recognized as the father of the cordless phone. He submitted a patent application in 1966 for a "full duplex wireless communications appartus". The US Patent and Trade Office awarded him a patent in June of 1969 (see Patents). Sweigert, a radio operator in World War II stationed at the South Pacific Islands of Guadalcanal and Bouganville, developed the full duplex concept for non-trained 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 A.T.& T. owned telephone lines in the late 1960s (which was banned at the time). 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 phone, like the Caterphone, was acoustically (not electrically) connected to the Public Switched Telephone Network (PSTN).