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Together with the TAM the cordless telephone is the most widely
used of the new type of simple telephone.
The cordless telephone has a base unit connected to the line
and communicates with its handset via radio transmissions.
Thus the user can move around freely when handling calls,
provided they do not go beyond the transmission range of the
base unit. The handset uses rechargeable batteries which require
periodic recharging , unusually on the base unit. These batteries
will require replacement at yearly intervals to maintain the
best performance from the handset.
As developments are made the handsets have become smaller
and speech quality improved, together with a move beginning
from the original analogue system to digital methods.
Both types rely upon radio waves being able to carry information
between the base and handset. Therefore the correct positioning
of the base itself will have an effect upon the range or coverage
that is available for the handset. The best locations are
those central to the area in which the handset is needed to
operate, away from obstacles , the elements and sources of
interference.
The analogue cordless telephone has changed little in its
basic operation since they were first introduced. The electronics
have become more efficient allowing longer times away from
the base station for charging, improved speech quality using
signal processing and are more highly featured.
The transmission characteristics are regulated and define
the operating range between the handset and base. Transmission
from base to handset is a 1.7Mhz signal and from handset to
base is 47Mhz at a power level of 6mV.
This signalling works best when used in direct line of sight
with the base station as it is easily blocked by intervening
obstacles and affected by the construction of these obstacles.
It can also be affected by proximity to other electrical equipment
such as televisions, computers, lighting and fridges which
can reduce the range and quality of the speech across the
radio link.
In ideal conditions the maximum range between base and handset
will not usually exceed 100m and may be reduced considerably
by the presence of the factors outlined above.
Digital cordless telephones provide the features of analogue
cordless telephones but with the improved performance of digital
technology. These are reduced interference and improved speech
quality.
These are achieved by digitising the speech, representing
it as a sequence of 0’s and 1’s and transmitting
these between base and handset. The digital data is used to
reconstruct the speech at the receiving end. This overcomes
all but the most severe interference from the transmission
because the coding methods used allow the signal to be reconstructed
to a true copy of the source by correcting the received signal
before rebuilding the speech. Severe interference will block
the signals in the same way as analogue cordless.
The overall range is the same as an analogue cordless, however
the performance is improved.
CT2 (Cordless Telephone 2), DECT (Digital Enhanced Cordless
Telephone) and the CAI (Common Air Interface) are all operation
technologies and specifications applied to the operation of
digital cordless telephones over the years.
CT2 was the second-generation cordless telephone technology.
It provides interference free cordless operation, even inside
buildings making it useful when connected to PBXs. Many CT1,
first generation (analogue) cordless telephones, could not
function well inside large buildings and were more adversely
affected by the interference from the building structure.
An extension to CT2 was the CAI or Common Air Interface. This
was a standardised system which allows multiple handsets from
different manufacturers to operate via the same base station.
This was envisaged to be the path to cheap, mobile communications
at home, the office and out and about with users having a
single handset which would be able to use base stations in
its immediate vicinity if the appropriate access rights were
available. This means that at home the owners base is used
and calls are charged against their account in the usual way.
At the office the same handset automatically becomes an extension
of the employers PBX with all the relevant functionality and
calls being charged to the employers account. When out of
the office it would log onto public base stations to allow
calls to be made and billed to a customer account in a similar
way to cellular telephones. This mobile network was trialed
in the UK but failed due to lack of subscribers because user
could only make and not receive calls. Subsequent cellular
network developments brought the more functional ETACS (Extended
Total Access Communication System) and now the GSM cellular
systems within the reach of non-business users.
DECT is a Digital Enhanced Cordless Telephone standard which
is intended to provide mobile access to cellular services
throughout the world if it is adopted by the appropriate authorities.
It is superseding the older systems in use today and has similarities
to GSM in its method of working. The handsets will share many
features of CT2 CAI telephones, being usable at home and work,
though early plans to allow roaming to mobile networks have
not yet materialised. Though a full realisation is some years
away at this time DECT equipment is currently sold for PBX
systems and domestic use by several manufacturers.
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