Cordless telephones
| Cordless telephones are one of those minor miracles of
modern life -- with a cordless phone, you can talk on the
phone while moving freely about your house or in your yard.
Cordless phones have many of the same features as standard
telephones. The main difference is that cordless phones
do not have a cord from the handset to the phone base unit.
In cordless telephone this wire is replace with a wireless
radio link. A cordless telephone is basically a combination
telephone and radio transmitter/receiver.A cordless phone
has two major parts: base and handset. The base is attached
to the phone jack through a standard phone wire connection,
and as far as the phone system is concerned it looks just
like a normal phone. The base receives the incoming call
(as an electrical signal) through the phone line, converts
it to an FM radio signal and then broadcasts that signal.
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The handset receives the radio signal from the base, converts
it to an electrical signal and sends that signal to the speaker,
where it is converted into the sound you hear. When you talk
to handset microphone, the handset transmits the audio in the
same way to base that then sends that audio to telephone line.
The base and handset operate on a frequency pair that allows
you to talk and listen at the same time, called duplex frequency.
There are many generations of cordless telephones in use. Cordless
phones first appeared around 1980 in USA. Those earliest cordless
phones operated at a frequency of 27 MHz. In 1986, the Federal
Communications Commission (FCC) granted the frequency range
of 47-49 MHz for cordless phones, which improved their interference
problem and reduced the power needed to run them. However, the
phones still had a limited range and poor sound quality. In
1994, digital cordless phones in the 900 MHz frequency range
were introduced. Digital signals allowed the phones to be more
secure and decreased eavesdropping (is was pretty easy to eavesdrop
on analog cordless phone conversations). In 1998, the FCC opened
up the 2.4 GHz range for cordless phone use.
Other countries have also cordless phone systems that operate
at different frequency ranges. For example in Europe there has
been systems like CT1, CT2 and DECT.CT1 is a simple analogue
system that operates at 47 MHz band (8 channels). Also versions
for 31 MHz and 900 MHz operation exist (more channels here).CT2
is a more modern system that uses digital radio communications
at 864.1-868.1 MHz frequency range. CT2, the second generation
of cordless phones, uses a digital speech path in any one of
the forty 100KHz wide RF channels in the frequency range 864-868MHz.
Instead full duplex operation is obtainedby the use of a digital
technique known as Time Division Duplex (TDD). With TDDthe two
halves of a telephone conversation are first converted into
digital formand then they are divided into a number of small
data packets. Each packet isthen compressed to one half of its
original size before the two sets of data are interleaved on
the same carrier frequency. The CT2 specification defines a
Common Air Interface (CAI), which means that all CT2 handsets
and base stations can communicate with each other, regardless
ofmanufacturer.
The modulation methid enployed is two-level FSK with frequencydeviations
of (a) 14.4 to 25.2 kHz above the carrier frequency representingbinary
1 (b) 14.4 to 25.2 kHz below the carrier frequency representing
binary 0. A single RF channel is used for both directions of
transmission using the 'ping-pong' version of TDD. Speech signals
in either direction of transmissionare sampled and coded into
digital form at 32kbits/s. The 2ms duration samplesare transmitted
at 72kbits/s in 1ms bursts to allow the bits to be compressedinto
packets of data of 1ms duration. Forty RF channels are available
do that CT2 is a combined FDMA/TDD system. DECT is a digital
telecommunication system standardized by ETSI. It operates at
1880-1900 MHz frequency (uses TDMA modulation). DECT supports
20-500 meters range with both voice and data communications
(nowadays the main use is voice).
The Digital European Cordless Telephone system uses a cellular
radio-like technologyThe DECT system uses a three dimension
cellular layout in which there may be cellsabove and below one
another as well as side by side layout and is designed for high
density use. The DECT system uses the frequency band 1.88-19GHz
and this band is divided up into ten separate carrier frequencies.
In turn, each carrier frequency is divided into 23 time slots,
any two of which are used for a conversation The system provides
32kbit/s voice channels using TDD. DECT uses FDMA/TDMA/TDD techniques
to provide 120 duplex channels using 10 separate carrier frequncies
and multiplexing 12 send channels and 12 receive channels onto
each carrier. The bit rate per channel is 1152kbits/s and the
modulation is GMSK with a frequency deviation of +/- 228kHz
and a carrier spacing of 1728kHz. PHS is a personal communications
system, which supports bothprivate use (i.e. for use as cordless
telephone or wireless PABX extension) andpublic use (i.e. for
use in the public PHS service). It is widely used in Japan.PHS
generally operates at 1895-1906.1 MHz which isdesignated for
PHS private use in Japan. PHS equipment that is designed towork
in the private and public PHS band operate at 1895-1918.1MHz.
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