CORDLESS TELECOMMUNIICATIIONS

 
CORDLESS TELECOMMUNIICATIIONS
The familiar cordless telephone, introduced in the early
1980s, has become a key factor in reshaping voice communications.
Since people cannot be tied to their desks, as much
as 70 percent of business calls do not reach the right person
on the first attempt. This situation has seen dramatic
improvement with cordless technology, which makes phones
as mobile as their users. Now almost 30 percent of business
calls reach the right person on the first attempt.
Cordless versus Cellular
Although cellular phones and cordless phones are both wireless,
they have come to assume quite distinct and separate
applications based on their areas of use and the differing
technologies developed to meet user requirements. Cellular
and cordless are implemented with their own standardsbased
technologies.
Briefly, cellular telephones are intended for off-site use.
The systems are designed for a relatively low density of
users. In this environment, macrocellular technology provides
wide area coverage and the ability to make calls while
traveling at high speeds. Cordless telephones, on the other
hand, are designed for users whose movements are within a
well-defined area, such as an office building. The cordless
user makes calls from a portable handset linked by radio signals
to a fixed base station (Figure C-7). The base station is
connected either directly or indirectly to the public network.
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Cordless Standards
The cordless system standards are referred to as CT0, CT1,
CT2, CT3, and DECT, with “CT” standing for Cordless
Telecommunications. CT0 and CT1 were the technologies for
first-generation analog cordless telephones. Comprising
base station, charger, and handset and intended primarily
for residential use, they had a range of 100 to 200 meters.
They used analog radio transmission on two separate channels,
one to transmit and one to receive. The potential disadvantage
of CT0 and CT1 systems is that the limited
number of frequencies can result in interference between
handsets, even with the relatively low density of residential
subscribers.
Also targeted at the residential user, CT2 represented an
improved version of CT0 and CT1. Using Frequency Division
Multiple Access (FDMA), the CT2 system splits the available
bandwidth into radio channels in the assigned frequency
domain. In the initial call setup, the handset scans the available
channels and locks onto an unoccupied channel for the duration of the call. Using Time Division Duplexing (TDD),
the call is split into time blocks that alternate between
transmitting and receiving.
The Digital Enhanced Cordless Telecommunications
(DECT) standard started as a European standard for cordless
communications, with applications that included residential
telephones and wireless Private Branch Exchange
(PBX) and wireless local loop (WLL) access to the public network.
Primarily, DECT was designed to solve the problem of
providing cordless telephones in high-density, high-traffic
office and other business environments.
CT3, on the other hand, is a technology developed by
Ericsson in advance of the final agreement on the DECT
standard and is designed specifically for the wireless PBX
application. Since DECT is essentially based on CT3 technology,
the two standards are very similar. Both enable the
user to make and receive calls when within the range of a
base station. Depending on the specific operating conditions,
this amounts to a distance of between 164 feet (50 meters)
and 820 feet (250 meters) from the base station. To provide
service throughout the site, multiple base stations are set up
to create a picocellular network. Signal handoff between the
cells is supported by one or more radio exchange units,
which are ultimately connected to the host PBX.
Both DECT and CT3 have been designed to cope with the
highest-density telephone environments, such as city office
districts, where user densities can reach 50,000 per block. A
feature called Continuous Dynamic Channel Selection (CDCS)
ensures seamless handoff between cells, which is particularly
important in a picocellular environment where several handoffs
may be necessary, even during a short call. The digital
radio links are encrypted to provide absolute call privacy.
The two standards, DECT and CT3, are based on multicarrier
Time Division Multiple Access/Time Division Duplexing
(TMDA/TDD). They do not use the same operating frequencies,
though, and consequently have different overall bit rates
and call-carrying capacity.
CORDLESS TELECOMMUNICATIONS 73
It is the difference in frequencies that governs the commercial
availability of DECT and CT3 around the world.
Europe is committed to implementing the DECT standard
within the frequency range of 1.8 to 1.9 GHz. Other countries,
however, have made frequencies in the 800- to 1000-
MHz band available for wireless PBXs, thereby paving the
way for the introduction of CT3.
Summary
Many of the problems arising from the nonavailability of
staff to a wired PBX can be avoided with cordless telephones.
They are ideal for people who by the very nature of their
work can be difficult to locate (e.g., maintenance engineers,
warehouse staff, messengers, etc.) and for places on a company’s
premises that cannot be effectively covered by a wired
PBX (e.g., warehouses, factories, refineries, exhibition halls,
dispatch points, etc.).
Akey advantage of cordless telecommunications is that it
can simply be integrated into the corporate telecom system
with add-on products and without the need to replace existing
equipment. Another advantage of cordless telecommunications
is that the amount of telephone wiring is dramatically
reduced. Since companies typically spend between 10 and 20
percent of the original cost of their PBX on wiring the system,
the use of cordless technology can have a significant impact on
costs. There is also considerable benefit in terms of administration.
For example, when moving offices, employees need
not change extension numbers, and the PBX does not have to
be reprogrammed to reflect the change.