Managing Information Technology

64 Part I • Information Technology

variations in human speech characteristics) and was insen-
sitive to minor degradations in the signal quality. On the
other hand, computer data consist of a string of binary
digits, or bits—a string of zeros and ones—to represent the
desired characters. The form of computer data does not
mesh well with analog transmission. First, only two
distinct signals—representing zeros and one—need to be
sent, and second, the data are extremely sensitive to degra-
dations in signal quality. Noise in a telephone line could
easily cause a zero to be interpreted as a one or vice versa,
and the entire message might become garbled. Because of
this problem with noise, data cannot be sent directly over
the analog telephone network.
Two solutions are possible to the problem of trans-
mitting computer data. The original solution, and one that
is still used, is to convert the data from digital form to
analog form before sending it over the analog telephone
network. This conversion is accomplished by a device
called a modem, an abbreviation for a modulator/
demodulator (see Figure 3.1). Of course, the data must be
reconverted from analog form back to digital form at the
other end of the transmission line, which requires a second
modem. The use of modems and the analog telephone
network is an acceptable way to transmit data for many
applications, but it is severely limited in terms of transmis-
sion speeds and error rates.
The second and longer-term solution to the problem
of transmitting computer data is to develop digital
networksspecifically designed to directly transmit a digi-
tal signal consisting of zeros and ones. Digital networks
have the advantages of potentially lower error rates and
higher transmission speeds, and modems are no longer
necessary. Because of these advantages, the networks that
have been specifically created for the purpose of linking
computers and computer-related devices are digital.
Furthermore, the telephone network is gradually being
shifted from an analog to a digital network. Digital services
such as ISDN and DSL (to be explored later in this
chapter) are now available in many parts of the United
States for users seeking higher-speed access to the Internet
over the public telephone network.
This shift of the telephone network from analog to
digital is due in part to the increasing volume of data being
transmitted over the network, but there is also a significant

advantage to transmitting voice signals over a digital

network. Digital voice transmission can provide higher-

quality transmission—less noise on the line—just as digi-

tal recording provides higher-fidelity music. Most of our

telephone instruments are still analog devices, so the signal

sent from the instrument to the nearest switching center

(which may be operated either by the telephone company

or by your own organization) is still an analog signal. These

telephone switches, however, are rapidly being converted

from analog to digital switches. When the analog voice

signal arrives at a digital switch, it is converted to a digital

voice signal for transmission to a digital switch somewhere

else, which may be across town or across the country.

Thus, an increasing proportion of the voice transmission

between switching centers is digitized. In the future, our

telephone instruments will also be digital devices, so the

entire telephone network will eventually become digital.

Speed of Transmission

Whether the signal is digital or analog, another basic

question is the speed of transmission. Please note that by

speed we do notmean how fast the signal travels in terms like

miles per hour, but rather the volume of data that can be

transmitted per unit of time. Terms such as bandwidth,baud,

andHertz(Hz) are used to describe transmission speeds,

whereas a measure such as bits transmitted per second (bits

per second, or bps) would be more understandable. Happily,

the three terms mentioned previously are essentially the same

as bits per second in many circumstances. In common

usage,bandwidthis just the circuit capacity. Hertzis

cycles per second, and baudis the number of signals sent per

second. If each cycle sends one signal that transmits exactly

one bit of data, which is often the case, then all these terms

are identical. To minimize any possible confusion, we will

talk about bits per second (bps) in this chapter. In information

technology publications, baud was formerly used for

relatively slow speeds such as 2,400 baud (2,400 bps) or

14,400 baud (14,400 bps), while Hertz(with an appropriate

prefix) was used for higher speeds such as 500 megaHertz

(500 million bps) or 2 gigaHertz (2 billion bps). More

recently, the term baudhas fallen into disfavor, but Hertzis

still widely used in PC advertisements. For clarity, we will

stick with bpsin this chapter.

MICROCOMPUTER MODEM MODEM SERVER

Transmission

Line

Analog

Signal

Digital

Signal

Digital

Signal

FIGURE 3.1 The Use of Modems in an Analog Network