70 Part I • Information Technology
The plug was pulled on a third proposed LEO satellite
system, named Teledesic, in October 2002. The original
plan for Teledesic, which was sponsored by Craig McCaw
(who built McCaw Cellular before selling it to AT&T), Bill
Gates (Microsoft), and Boeing, was to create a 288-satellite
network to provide low-cost, high-speed Internet access,
corporate networking, and desktop videoconferencing. The
number of satellites was later reduced to 30, each with a
larger “footprint” on the earth, but even that plan was
cancelled in 2002 before any Teledesic satellites were
launched. All these LEO satellite systems seemed like good
ideas at the time they were planned, but the expenses
involved were massive. Furthermore, the LEO systems took
so long from concept to deployment that competing, less
expensive technologies—such as cell phones, DSL, and
cable—had made massive inroads into the potential market
before the satellites were launched.
FIBER OPTICS The last and newest transmission medium—
fiber-opticcabling—is a true medium, not broadcast tech-
nology. Advances in optical technology have made it possi-
ble to transmit data by pulses of light through a thin fiber of
glass or fused silica. A light pulse can signal a 1 bit, while
the absence of a pulse signals a 0 bit. An optical transmis-
sion system requires three components: the light source,
either a light-emitting diode (LED) or a laser diode; the
fiber-optic cable itself; and a detector (a photodiode). The
light source emits light pulses when an electrical current is
applied, and the detector generates an electrical current
when it is hit by light.
Fiber optics are much faster than other media and
require much less space because the fiber-optic cable is
very small in diameter. Fiber-optic cables are more secure
because the cables do not emit radiation and, thus, are very
difficult to tap. They are also highly reliable because they
are not affected by power-line surges, electromagnetic
interference, or corrosive chemicals in the air. These
benefits are leading telephone companies to use fiber
optics in all their new long-distance telephone lines, lines
connecting central office sites, and most of their new local
lines from central office sites to terminuses located in
subdivisions. (The advantages of speed and security are
obvious; the size is important because many of the cable
ducts already installed lack room for other media but can
hold the thinner fiber-optic cabling.) The high cost of the
required equipment and the difficulty of dealing with the
tiny fibers make this an unattractive medium for most
LANs, except when it is used as a backbone to connect
multiple LANs and where very high speeds or high-security
needs exist.
Transmission speeds for fiber range up to 1 billion
bits per second (1 giga bps or 1 gbps) for large-diameter
fiber (50 to 100 micron^2 core, which does not include
any protective covering) to as high as 3,200 gbps for
small-diameter fiber (10 microns or less). The fact that
the smaller-diameter fiber has much larger capacity
might be surprising, but light reflections are greatly
reduced with a smaller fiber—the light ray bounces
around less—permitting higher transmission speeds. The
large-diameter fiber is multimode, meaning that several
light rays are traversing the fiber simultaneously, bounc-
ing off the fiber walls, while the small-diameter fiber is
single mode, with a single light ray at a time propagated
essentially in a straight line without bouncing. Single-
mode fiber, unfortunately, requires higher-cost laser
light sources and detectors than multimode fiber. In a
recent development, the light ray sent through a single-
mode fiber can be split into 80 or more different colors,
each carrying its own stream of data. In this process,
called dense wave division multiplexing, prisms are used
to send these multiple colors down a single fiber. For
example, some of the fiber currently being installed by
telephone companies is 8-micron single-mode fiber with
a transmission speed, using wave division multiplexing,
of 800 gbps. The outside diameter (including protective
covering) of this single-mode fiber is only 125 microns,
which is about one-fiftieth the outside diameter of a typ-
ical coaxial cable. Thus, both the speed and size advan-
tages of fiber optics are significant.Topology of Networks
The starting point for understanding networks is to
recognize that all telecommunications networks employ
one or more of the transmission media discussed previ-
ously. But what do the networks look like in terms of
their configuration or arrangement of devices and
media? The technical term for this configuration is the
topology of the network. There are five basic network
topologies—bus, ring, star, hierarchical or tree, and
mesh (see Figure 3.4)—plus an unlimited number of
variations and combinations of these five basic forms.BUS The simplest topology is the linear or bus topology.
With the bus, a single length of cable (coax, fiber, or
twisted pair) is shared by all network devices. One of the
network devices is usually a file server with a large data
storage capacity. An obvious advantage of the bus is the
wiring simplicity. A disadvantage is its single-point
failure characteristic. If the bus fails, nodes on either side
of the failure point cannot communicate with one another.(^2) A micron is one-millionth of a meter or one-thousandth of a millimeter.