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24 E L EC TR I C AL CONNEC T I ON
W I N T E R 20 1 6
COPPER UP TO SPEED
T
wisted-pair copper telephone
cabling has been around for well
over 100 years. It forms part of the
infrastructure of all population centres
and extends to many rural and regional
areas as well.
Until about 1970, it was used almost
exclusively for the public switched
telephone network (PSTN) for
voice communications.
With the advent of computer
technology, its role has fundamentally
changed. Today, it is used for voice and
data communications, with data usage
increasing rapidly and telephony services
slowly declining.
In the early days of personal
computers, the PSTN carried the data
communications services, with access via
dial-up modem.
However, the rapid evolution of
computer technology called for faster and
faster communications. Soon, even the
fastest dial-up services, with a nominal
speed of 56kbps, proved to be too slow.
Today, data communications over
twisted pair is by means of xDSL services.
The plain old telephone service (POTS)
and the xDSL service share the same
telephone pair back to the exchange, but
otherwise run independent of each other.
The voice and data signals are
separated by splitters at the exchange
and at the customer’s premises.
SOME BASICS
A telephone service requires just one
pair of wires, on which voice signals
travel simultaneously in both directions.
The telephone instrument electronics
separate the signals, to make sure that
what you say is not fed back loudly to
your ear.
The transmission characteristics of
twisted-pair cable are such that signal
attenuation (loss) and crosstalk increase
with frequency.
The POTS service requires a bandwidth
of just 300Hz to 3.4KHz, and the circuit
from your telephone to the PSTN
is purposely limited to this range –
definitely not hi-fi, but quite satisfactory
for intelligible speech.
At this bandwidth, it is possible to
communicate up to about 5km using
a standard telephone operating on
standard 26AWG (0.4mm diameter) cable
pairs, or even further if a special long-line
telephone is used.
The limiting factor is the loop
resistance of the cable pair rather than
the signal loss. The telephone has to be
able to loop the line when the receiver is
lifted before it can dial out.
>
With data communications, the
required bandwidth is determined by
the speed of the data signal (in bits per
second) and also the signal encoding
scheme employed. As a general rule,
the higher the link speed, the more the
required bandwidth.
TECHNOLOGICAL DEVELOPMENT
ALLOWS VERY FAST DATA
OVER CONVENTIONAL
TELEPHONE CABLE.
GEORGE
GEORGEVITS
EXPLAINS.
COVER STORY: G.FAST