Since they involve little cabling,
wireless systems will not be considered here
9.3 - Copper or glass fibre?
Copper cabling exists in two major types,
the unshielded twisted pair (UTP) and the shielded twisted
pair (STP). Coaxial cable is used in special applications
where radio and television signals are carried. UTP cables
usually consist of four twisted pairs of wires enclosed
in an overall sheathing. To reduce interference, each pair
has a different number of twists per unit length. These
cables can be used for alarm systems and voice-only telephones
(category 1) at the lower bandwidths, up to data applications
up to 100 MHz (category 5). Higher frequency cables have
more twists in a given length.
Unshielded twisted pair cables may be susceptible
to radio and electrical interference, this disadvantage
being reduced in the shielded twisted pair (STP) cable.
The shielding (of braiding or of foil) increases the size
and the cost of the cable, and also requires effective earthing
(in the USA they call it grounding). STP cables may also
have greater loss at higher frequencies, and are more expensive,
both to purchase and to install, not only because they are
larger, but because shielded connectors must be used. In
simple terms UTP cables are adequate for situations where
there is unlikely to be interference from fluorescent lighting
and machinery such as motors, welders, and so on.
Fibre optic cables (in the USA they spell
it fiber) have a very thin core of glass (usually ultra-pure
fused silica) with a protective sheath. Light is injected
into the cable by a laser or light-emitting diode, and changed
back to an electrical signal at the receiving end by a photocell.
The fibre-optic cable is able to carry signals over greater
distances than are copper cables, and at greater speed;
it can also be used over a greater bandwidth. Since it is
not an electrical system, it is immune from electromagnetic
and lightning interference. Other advantages over copper
cabling are vastly increased signal capacity (due to the
very high frequency of light), lower transmission losses,
lower cost of basic materials and much smaller cable size.
The reduced transmission loss can often allow simpler networks,
by having only one central point of administration with
all data cabling fed back to it. At the time of writing,
fibre networks are still more expensive than those with
copper cables, but the gap is narrowing. For very high bandwidth
applications fibre networks are the only solution. Category
5 systems can operate with copper cables, but some think
that they are better served by fibre optic systems. It seems
likely that when categories 6 and 7 arrive they will need
fibre to support them.