Cordless telephone

A cordless telephone or portable telephone is a telephone with a wireless handset which communicates with a base station connected to a fixed telephone landline (POTS) via radio waves and can only be operated close to (typically less than 100 meters of) 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 is connected to the base station when not in use.

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.

Frequencies

In the United States, there are seven frequency bands that have been allocated by the Federal Communications Commission for uses that include cordless phones. These are:

1.7 MHz (Upto 6 Channels, AM System)^*
27 MHz (allocated in 1980, Upto 10 Channels, FM System)
43–50 MHz (allocated in 1986, Upto 25 Channels, FM System)
900 MHz (902–928 MHz) (allocated in 1990)
1.9 GHz (1920-1930 MHz) (allocated in October 2005)
2.4 GHz (allocated in 1998)
5.8 GHz (allocated in 2003)

Virtually all telephones in the US use the 900 MHz, 2.4 GHz, or 5.8 GHz bands.

The recently allocated 1.9 GHz band is used by the popular DECT phone standard from Europe.

Performance

Communication companies usually advertise that higher frequency systems improve audio quality and range. However, in reality 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. Other factors that determine quality and range are signal strength, antenna quality, and the method of modulation used.

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 kHz; only a fraction of the frequencies that humans can hear, but it is 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 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, there are some advantages of moving into higher frequency. 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 suffere interference from signals broadcast by those devices. The 5.8 GHz band is less crowded, currently being used only for the less popular 802.11a wireless standard and military communications so it is less prone to interference.

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.

Security

Many analog phones 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 greatly 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, which means better 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.

Wireless phone handsets

Wireless phone handsets exist, designed for connection with a local wired service, not using tranditional mobile phone networks, most commonly using digital technologies being DECT, 2.4 GHz unlicensed spectrum or 802.11a/b/g standards-based wireless LAN technology. The wireless phone handset does per definition need to 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 better reach, but with potential suffering confidentiality and voice quality. Most digital systems have inherent encryption or offer optional encryption.

Patents

— Telegraph — A. G. Bell

— Duplex Radio Communication and Signalling Appartus — G. H. Sweigert

External links

Consumer electronics | Telephony

Schnurlostelefon | Téléphone sans fil | コードレス電話 | 无绳电话

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