Signal quality degrades for several reasons,
including attenuation, crosstalk, and impedance.
Attenuation
Attenuation is the decrease in signal strength,
measured in decibels (dB) per 100 feet. Such loss
happens as the signal travels over the wire. Attenuation
occurs more quickly at higher frequencies and when the
cable’s resistance is higher.
In networking environments, repeaters are
responsible for cleaning and boosting a signal before
passing it on. Many devices are repeaters without
explicitly saying so. For example, each node in a token-
ring network acts as a repeater. Since attenuation is
sensitive to frequency, some situations require the use
of equalizers to boost different-frequency signals the
appropriate amount.
Crosstalk
Crosstalk is interference in the form of a signal from
a neighboring cable or circuit; for example, signals on
different pairs of twisted wires in a twisted pair cable
may interfere with each other. A commonly used
measure of this interference in twisted-pair cable is
near-end crosstalk (NEXT), which is represented in dB.
The higher the dB value, the less crosstalk and the better
is the cable.
Additional shielding between the carrier wire and
the outside world is the most common way to decrease
the effects of crosstalk.
Impedance
Impedance, which is a measure of electrical
resistance, is not directly a factor in a cable’s
performance. However, impedance can become a
factor if it has different levels at different locations in a
network. In order to minimize the disruptive effects of
different impedances in a network, special devices,
called baluns, are used to equalize impedance at the
connection.
Impedance does reflect performance indirectly.
The higher the impedance, the higher is the resistance;
the higher the resistance, the greater is the attenuation at
higher frequencies.
Line Conditioning
Line conditioning tries to eliminate the effects of
certain types of distortions on the signal. It becomes
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more necessary as transmission speeds increase. Two
types of line conditioning are available:
C conditioning tries to minimize the effects of
distortion related to signal amplitude and
distortion due to envelope delay.
D conditioning tries to minimize the effects of
harmonic distortion in addition to the amplitude
and envelope delay distortions handled by type C
conditioning.
A line driver is a component that includes a
transmitter and a receiver; it is used to extend the
transmission range between devices that are connected
directly to each other. In some cases a line driver can be
used in place of a modem, for short distances of 10 miles
or less.
To test a particular section of cable, you can use a
line-testing tool. A line monitor is a low-end line-
testing tool that tells you if the line is intact. A high-end
line-testing tool can do very precise measurements
using time domain reflectometry (TDR). A TDR is a
device used to test the integrity of a section of cable
before the cable is even unwound. This diagnostic
method uses a signal of a known amplitude and
duration, which is sent along a stretch of cable.
Depending on the amount of time the signal takes to
return and on the cable’s nominal velocity of
propagation, the TDR can determine the distance the
signal traveled and whether there are any shorts or
opens in the cable.
BACKBONE CONNECTIONS
In addition to the inherent problems of the cabling,
backbone connections add the problems that come with
the use of connectors. They have only a limited number
of times that they can be connected before they have to
be replaced. These connectors are used in several
places along the backbone, each presenting one more
place for trouble to start. Some of the places that
connectors are used are:
At the server
At the repeater, concentrator, and the gateway
Along the backbone for each drop or tap
At the splice and coupler (used with fiberoptic)
Each of the connections uses a different type of
connector, each with its own limitations. For example:
