digital data pulses into a form acceptable to the
various types of communications channels.
equipment most often transmits digital data over a
distance by varying a continuous analog signal in
amplitude, frequency, or phase.
Communications channels that can pass data in
two directions (transmit and receive) are known as
Single-direction channels are
simplex channels. Duplex channels may operate in
one of the following two modes: half-duplex or
fill-duplex. Half-duplex channels transmit data in
one direction, pause, and then receive data coming
from the opposite direction. Full-duplex channels, on
the other hand, can transmit and receive data
TYPES OF COMMUNICATIONS CHANNELS
In the fleet and at shore activities, you will
encounter several forms of communications channels.
The most common channels are landlines and radio
Landlines are physical lines or cables that connect
the digital equipment. Originally, landlines referred
to telephone lines and were limited to carrying analog
audio frequencies (voice frequencies). For digital
information to be carried over these lines, the
The signal can be varied in amplitude
(amplitude modulation), frequency (frequency
modulation), or phase (phase modulation) to convey
the data. We will discuss each type of modulation
later in this chapter.
A digital signal has a limited set of values (1 or 0,
true or false, etc.).
A limited number of discrete
pulses can be transmitted in a fixed period. The
unique sequence of the bits represents the data.
Digital equipments (computers and peripherals)
within a system normally communicate with each
other in pure digital pulses (serial and parallel).
Transmitting digital information over a distance
requires the use of special equipment to convert
characteristics of one or more tones or carriers in the
audio-frequency range had to be modified in
amplitude, frequency, or phase.
Today, telephone lines are commonly used in
many network applications. Bulletin boards, such as
BUPERS ACCESS, use existing telephone lines; but
many landline-based systems use dedicated lines.
Dedicated lines are common in local area networks
(LANs). In a LAN system, several computers are
joined together to share information with all the users
on the system. System connections are made using
coaxial, dual-coaxial, fiber-optic, or twisted-pair
cable. The type of cable depends on several factors,
such as the number of users on the LAN and the
maximum distance between workstations.
The device used to convert the digital data into a
form usable by the communications channel and back
to digital data is known as a modem.
Modem is an acronym for MOdulator
Demodulator. The modulator function converts the
data of the transmitting system into discrete
modifications of the tone or carrier signals. The de-
modulator converts the data-carrying tone or carrier
signal into digital data for the receiving system.
Radio waves have been used for teletype and
voice communications for many decades.
advantages of radio-based systems are that they are
more mobile and can communicate over barriers such
as large bodies of water. Tactical information links,
like those we will cover in chapter 2, are almost
Radio communications are based on frequency
ranges or radio-frequency bands. The frequency
range of the carrier frequency determines the
operational characteristics of the system. Table 1-1
illustrates the international frequency bands and their
uses. The tactical digital information systems used by
the Navy generally use portions of the hf and uhf