Chapter 3 • Telecommunications and Networking 65The notion of bandwidth, or capacity, is important
for telecommunications. For example, approximately
50,000 bits (0s and 1s) are required to represent one page
of data. To transmit 10 pages using a 56,000 bps (56 kbps)
modem over an analog telephone line would take about
nine seconds. If one were transmitting a large data file
(e.g., customer accounts), that bandwidth or capacity
would be unacceptably slow. On the other hand, to trans-
mit these same 10 pages over a 1 million bps (1 mbps)
DSL line would take only half of a second. Graphics
require approximately 1 million bits for one page. To
transmit 10 pages of graphics at 56 kbps over an analog
telephone line would take a little under 3 minutes, while
transmitting these same 10 pages of graphics over a
1 mbps DSL line would take only 10 seconds. One hour of
high-definition video requires approximately 3 billion
bytes (3 gigabytes), which would take nearly five days to
transmit over a 56 kbps analog telephone line—obviously
no one would ever do this! This high-definition video
would still take 6^2 / 3 hours over a 1 mbps DSL line, but
the time drops to a more reasonable 13^1 / 3 minutes over
a 30-mbps fiber-to-the-premises line. The bandwidth
determines what types of communication—voice,
data, graphics, stop-frame video, full-motion video, high-
definition video—can reasonably be transmitted over a
particular medium.
Types of Transmission Lines
Another basic distinction is between private (or dedicated)
communication lines and switched lines. The public
telephone network, for example, is a switched-line system.
When a communication of some sort (voice or data) is sent
over the telephone network, the sender has no idea
what route the communication will take. The telephone
company’s (or companies’) computers make connections
between switching centers to send the communication over
the lines they deem appropriate, based on such factors as
the length of the path, the amount of traffic on the various
routes, and the capacity of the various routes. This
switched-line system usually works fine for voice commu-
nications. Data communications, however, are more sensi-
tive to the differences in line quality over different routes
and to other local phenomena, such as electrical storms.
Thus, a data communication sent from Minneapolis to
Atlanta over the telephone network might be transmitted
perfectly at 11 A.M., but another communication sent from
Minneapolis to Atlanta 15 minutes later (a different
connection) might be badly garbled because the communi-
cations were sent via different routes.
One way to reduce the error rate is through private
lines. Most private lines are dedicated physical lines leased
from a common carrier company such as Verizon, Sprint
Nextel, or AT&T. A company might choose to lease a line
between Minneapolis and Atlanta to ensure the quality of
its data transmissions. Private lines also exist within a
building or a campus. These are lines owned by the organ-
ization for the purpose of transmitting its own voice and
data communications. Within-building or within-campus
lines for computer telecommunications, for example, are
usually private lines.
The last basic idea we wish to introduce is the
difference among simplex, half-duplex, and full-duplex
transmission. With simplex transmission, data can travel
only in one direction. This one-way communication is
rarely useful, but it might be employed from a monitoring
device at a remote site (e.g., monitoring power consump-
tion) back to a computer. With half-duplex transmission,
data can travel in both directions but not simultaneously.
Full-duplex transmission permits data to travel in
both directions at once, and, therefore, provides greater
capacity, but it costs more than half-duplex lines. Ordinary
telephone service is full-duplex transmission, allowing
both parties to talk at once, while a Citizen’s Band (CB)
radio provides half-duplex transmission, allowing only one
party to transmit at a time.Transmission Media
A telecommunications network is made up of some physi-
cal medium (or media) over which communications are
sent. Five primary media are in use today: twisted pair of
wires, coaxial cable, wireless, satellite (which is a special
form of wireless), and fiber-optic cable.TWISTED PAIR When all uses are considered, the most
common transmission medium is a twisted pairof wires.
A twisted pair consists of two insulated copper wires,
typically about 1 millimeter thick, twisted together in a
long helix. The purpose for the twisting is to reduce
electrical interference from similar twisted pairs nearby.
Most telephones are connected to the local telephone
company office or the local private branch exchange
(PBX) via a twisted pair. Similarly, many LANs have been
implemented by using twisted pair wiring to connect the
various microcomputers and related devices. For example,
Category 5e cabling—which consists of four twisted pairs
in a single cable jacket—is currently used for many new
high-speed LANs.
The transmission speeds attainable with twisted
pairs vary considerably, depending upon such factors as
the thickness of the wire, the distance traveled, and the
number of twisted pairs in the cable. On the analog voice
telephone network, speeds from 14,400 to 56,000 bps are