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a ship in a simplex mode terminated full duplex with a shore station. The ship may send or receive but not
do both at the same time.
FULL DUPLEX.—The full-duplex (fdx) mode is a method of operation in which
telecommunications between stations takes place simultaneously in both directions using two separate
frequencies. In other words, a ship may send and receive different messages at the same time. The term
"full duplex" is synonymous with "duplex."
BROADCAST.—Broadcast (bc) is the type of operation in which one station transmits information
on one or more channels directed to more than one station and/or unit. The broadcast system has no
provision for receipt or reply; however, special arrangements may require the receiving station to reply or
receipt for the message at a later time by other means. Broadcasts are the primary means of delivering
messages to the fleet. Since Navy units copying broadcasts are not required to receipt for messages
received, they can maintain radio silence while still receiving essential messages.
Message traffic is normally sent to the fleet by three methods: broadcast, intercept, and receipt. The
first two are "do not answer" methods; the third, as its name implies, requires a receipt from the addressee
(addee) for each message. Broadcast and intercept methods allow the fleet to preserve radio silence,
which is a great advantage from the standpoint of security. By the intercept method, a shore radio station
transmits messages to another shore station that repeats them back. Ships intercept and copy all of this
message traffic.
Broadcast is preferable to intercept chiefly because it is faster. It is the method by which nearly all
fleet traffic is handled. It uses radiotelegraph, radiotelephone, radio teletypewriter, and facsimile.
There is some similarity between civilian and naval broadcasts. Just as commercial stations in the
broadcast band transmit programs to radio receivers in the homes in their communities, Navy
communications stations broadcast messages to fleet units in their particular geographic areas. The
resemblance between Navy and commercial stations ceases there. Information broadcast by naval
communications stations is contained in chronologically numbered messages addressed to the ships. The
messages are copied by the fleet units, which check the serial numbers to ensure they have a complete
file. This checks and balances system ensures the ship has not missed any of the broadcast message
traffic.
Fleet broadcasts follow regular schedules. Messages are placed on the schedules in order of
precedence. If a message of higher precedence is given to a transmitter station while a lower precedence
message is being transmitted, the latter message may be interrupted to transmit the message of higher
precedence. All ships copy all messages appearing on the broadcast schedule they are guarding.
Messages are normally transmitted on several frequencies to make sure they are received. This gives
the receiving station the choice of frequency selection when considering time of day and atmospheric
conditions for best reception.
Q16. The naval communications system is made up of what two groups of communications?
Q17. What are the five most prominent communications modes of operation?
SWITCHED NETWORKS
The defense communications system (DCS) is composed of all worldwide, long-haul, government-
owned and leased point-to-point circuits, trunks, terminals, switching centers, control facilities, and
tributaries of military departments and other defense activities. In essence the DCS combines into a single
