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Q12. What are the four general types of communications services in the hf band?
Q13. A message transmitted on several frequencies at the same time is an example of what type of
transmission?
Q14. Physically separating receive antennas is an example of what technique?
Q15. When using frequencies above 30 megahertz, you are normally limited to using what range?
COMMUNICATIONS FUNDAMENTALS
Now that we have learned the Navy's fundamental use of the various frequency bands, we should
look at the types of communications links and their modes of operation. The Navy uses many modes of
operation; the type used is based upon the function of the circuit or network. These modes (or functions)
are combined to form a communications link. We will also discuss some of the actual networks the Navy
uses on a daily basis.
COMMUNICATIONS LINKS
A complex of links forms a major communications system. The naval communications system is
broken down into strategic and tactical groups.
Strategic
Strategic communications are generally world-wide in nature. They are operated on a common-user
(Navy, Army, Department of Defense, and so on) or special-purpose basis. A strategic system may be
confined within a specified area or limited to a specific type of traffic, but the configuration is designed so
that combined operations with other strategic systems are possible. As an example, we will look at the
automatic voice network, automatic digital network, and the defense special security communications
system later in this chapter.
Tactical
Tactical communications are usually limited to a specific area of operations and are used to direct or
report the movement of specific forces. Some tactical networks are used only for operational traffic;
others may be used for operational and administrative traffic. For instance, the task force, task-group, and
air-control networks are ordinarily used for operational traffic. Ship-to-shore networks and broadcast
networks serve both types of traffic.
Modes of Operation
Communications links have numerous modes of operation. In our discussion, a mode of operation is
identified as a link or path between two or more points that is capable of providing one or more channels
for the transmission of intelligence. Let's take a look at the five most common modes of operation.
SIMPLEX.—The simplex (splx) mode uses a single channel or frequency to exchange information
between two or more terminals. Communications is in one direction only.
HALF DUPLEX.—The half-duplex (hdx) mode has one-way flow of information between
terminals. Technical arrangements often permit transmission in either direction, but not simultaneously.
This term must be qualified to show s/o (send only), r/o (receive only), or s/r (send or receive).
SEMIDUPLEX.—The semiduplex (sdx) uses an arrangement of equipment where one terminal is
simplex configured and the other uses two channels or frequencies in full duplex. A clarifying example is
