Cordless telephone
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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 metres 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. There are also some advanced cordless phone systems that can operate without power. In the event of a power outage, the phone will convert itself into a standard telephone that gets its power from the landline. However, in this mode, only a handset attached directly to the base could work. All wireless handsets would not function.
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.
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Frequencies
In the United States, there are five frequency bands which have been allocated by the Federal Communications Commission for use by cordless telephones. These are:
- 1.7 MHz [1]
- 27 MHz (allocated in 1980)
- 43–50 MHz (allocated in 1986)
Modern telephones are all manufactured to use the following bands:
- 900 MHz (902–928 MHz) (allocated in 1990)
- 2.4 GHz (allocated in 1998)
- 5.8 GHz (allocated in 2003)
Performance
Communication companies usually advertise that higher frequency systems improve audio quality and range, though this is an oversimplification. Though higher frequencies can pass through some materials more easily, and the bands are less crowded, the primary factors that determine quality and range are signal strength 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.
The system's operating range (regardless of frequency) depends on the strength of the signal, not the frequency. (For instance, FM radio stations are typically in the range of 70–120 MHz, yet they are able to provide city-wide coverage due to their high-power broadcasting antennas.) Most manufacturers claim a range of about 30 m (100 ft) for their 2.4 GHz and 5.8 GHz systems but, not surprisingly, most inexpensive models fall short of this claim.
However, there are some clear 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's likely that the signal from the base unit will interfere with 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 communication so it is more immune to interference.
Also, higher frequencies are, in theory, less likely to be blocked by objects such as walls and other household objects.
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, or 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.
See also
- Digital Enhanced Cordless Telecommunications (DECT) in Europe
- DCTS in North America
- Personal Handy-phone System (PHS) in Japan and China
- Mobile phone
- Radio frequency
External links
- Review of Frequency Allocations for Cordless Telephones
- How Cordless Telephones Work
- Information about Digital Spread-Spectrum cordless phones
- Cordless Phone History