an insulating layer, a shielding layer, and a weather
proof outer jacket, as shown in figure 1-10. It is almost
as easy to install as twisted-pair, and is the preferred
medium for many of the major local-area networks.
Coaxial cable is used extensively in local-area networks
whenever the distance involved is relatively short,
generally less than 2 miles for baseband LANs and 10
miles for broadband LANs. It is used in both baseband
and broadband networks. Wait a minute! You say you
want to know what the terms baseband and broadband
mean and how they relate to networks? Not to worry;
we explain them to you a little later in the text, but for
now, all you need to know is that they both deal with the
way data is transmitted (in the form of electrical signals)
through some type of medium.
Fiber Optic Cable
Fiber optic cable is to coaxial cable is to twisted-
pair as the F-18 Hornet is to the Corvette is to the model
T. It is the newest of the communication mediums, one
that was spurred by the development of laser
technology. Fiber optic cable (shown in fig. 1-11)
consists of thousands of clear glass fiber strands, each
approximately the thickness of a human hair.
Transmission is made possible by the transformation of
digital data into modulated light beams, which are sent
through the cable by a laser light-emitting diode (LED)
type device at incredibly fast speeds. Transmission
rates available (as of 1990) range up to approximately 1
billion (or giga) bits per second (Gbps), with speeds
over 2 Gbps possible. When thinking in terms of
frequencies, light frequencies are extremely high. They
are approximately 600,000 times that of the highest
television channel. In terms of data communications,
the higher the frequency of the signal, the more
information it can carry. Put simply, every hairlike fiber
within a fiberoptic cable has the capacity to carry many
hundreds of local-area network channels
simultaneously. When dealing with fiber optic cable,
you will hear such terms as:
Figure 1-10.—Coaxial cable,
l
l
*
Figure 1-11.—Fiber optic cable.
Monomode— Single fiber cable
Multimode— Several fibers within a cable
Graded index— A variation of multimode
Some of the major advantages of fiber optics over
wire media include speed, size, weight, longevity, and
resistance to tapping without being noticed. Since it
carries no electrical current, it is immune to electrical
interference of any kind, and there is no worry of it
being a shock hazard.
One big disadvantage of fiber optic is the tighter
restrictions on how much the cable can be bent. Other
disadvantages include higher cost, and the inability to
add on new workstations while other stations are active.
Although it is relatively easy to splice the fiber optic
cable and add new stations, the network or a portion of
the network must be down while preparing the splice.
On the other hand, if your activity has serious
interference problems, or has a need for absolute
network security, or the need to send signals several
miles, then fiber optics might be the only solution.
Cable Selection
About now, you may be asking yourself, why all the
fuss over transmission speeds? Why not just simply
choose the cheapest transmission medium available and
use it? It may not be the ideal situation, but it would get
the job done, right? This is true; and with that in mind,
we ask you this question. Would you put regular
unleaded gasoline in your brand new car that happens to
have a high-performance engine? The engine may not
run as well as you would like, but it would get the job
done, right? The same is true of transmission speeds and
the different levels of speed within a computer system.
To put it another way, the speed of transmission is very
much related to the type of transmission medium used
between stations in a network.
1-17
