Information Technology Reference
In-Depth Information
wires can be affected by nearby electric fields, such as might be gen
erated by electric motors, fluorescent lights, other electrical appli
ances, or even weather patterns such as lightning. One way to keep
these effects to a minimum is to twist the two wires of a circuit to
gether. This twisting makes each wire in a pair subject to roughly
the same amount of interference, and the effects in the two wires
tend to cancel each other out. With no twisting, outside interference
affects one wire in a pair more than the other wire, and transmis
sions can experience much extra electrical noise or interference. For
this reason, wires often are twisted into pairs within cables, and a
cable with eight wires is organized into four pairs of wires (with
each pair twisted together). Standards prescribe how much twisting
of wire is needed to be effective, and a pair of wires with adequate
twisting is called, not surprisingly, a
twisted pair
.
The fourth type of wire, called
coaxial cable
, is shown at the
right side of Figure 8.1. In this form, one wire forms a central core,
and another wire is braided around the outside, with insulation be
tween the inner core and outside braid. In this configuration, the
outer wire shields the inner core from electrical interference; electri
cally, interference cannot penetrate the outer braid to affect voltages
and currents at the core. Although coaxial cable used to be used ex
tensively in computer applications, it now is used primarily for
video connections.
Now that you know something of the basic types of cable, we
consider how to use these wires to connect computer equipment. In
principle, you could connect your computer to another device using
any wires and any plugs at their ends. However, there is great practi
cal advantage in utilizing a standard plug, sometimes called a
connec-
tor
. A standard plug, for example, could be used for many different
computers, telephones, and other devices, simplifying the need for ex
tensive inventories of different plugs. Also, when a standard plug is
used widely, manufacturers can take advantage of economies of mass
production to produce the plugs reasonably cheaply. Cheap plugs
that work well for a variety of our needs make us, the consumers,
happy . . . and also safe; if a connector must meet specific standards
before it's sold, then we can feel reasonably confident that the wiring
is done correctly and that the insulation will work properly.
With the need for standardization, the Federal Communications
Commission of the United States maintains a registry of standard
jacks and how they are to be wired. Connectors and wires specified
in this registry are called
registered jacks
. If you use headphones for
your radio or CD player, those headphones likely come with a jack
