link frequency. A picket unit (PU) address is obtained
from the net controller and inserted into the data
terminal set. An appropriate data mode is then selected
on the communication selector panel and the cipher data
selected amber for encrypted operation. Once these
steps are completed, the hardware is setup and ready for
the software initialization.
The data link system formats tactical information
into blocks of fixed length, called frames. Each frame
contains a 4-bit header block which serves as a control
code to tell the modem what type of data follows, and
26 bits of serial information. The 26 bits of data include
an address that identifies the originating PU or the
DNCU, type of information, the tactical data, and a
check sum (parity check).
Data Link Terms
There are four commonly used data link terms that
you must be familiar with to work with data link
systems. These four terms are discussed in the following
Participating unit is any ship, aircraft, or shore
station that is active in the data-link net.
Data Net Control Unit (DNCU) is the participating
unit that has overall control over the link net. Only one
DNCU maybe active in the net.
Picket Unit (PU) is any station in the net that is not
the DNCU. As many as 20 units are allowed as PUs.
Data Link Reference Position (DLRP) is the latitude
and longitude position that is used as a reference for all
data transmitted on the net. When a participating unit is
operating below 60 degrees north or south latitude, this
position must be within 1024 minutes of latitude of the
unit. Above 60 degrees north or south latitude, 2048
minutes of latitude is allowed.
Modes of Operation
Roll call is the normal system operation mode. The
DNCU sends out a message requesting each PU, in a
certain order, to respond with tactical data. The
transmitted data is available for use by all units in the
net, but is displayed only if the participating unit is
entered into the individual system. In the P-3C,
participating units must be entered in the data link
control tableau to have their data accepted for display
by the computer. All received data is sent to the central
computer, which then looks at the label portion of the
data message to determine if the information should be
ignored or displayed.
When acting as a DNCU, the participating unit
selects CONTROL and ROLL CALL on the DTS. The
hardware/software system automatically transmits the
request-for-transmit message to each unit entered in the
data link control tableau.
When a surface ship or an antisubmarine warfare
operating center (ASWOC) is DNCU, PICKET is
selected on the DTS. The hardware/software transmits
only when a request for data is received from the DNCU.
If no request is received, the modem does not transmit.
Two modes of operation used for data link net
transmission are LONG BROADCAST and SHORT
BROADCAST. LONG BROADCAST, when selected,
transmits continuously as long as the function remains
active. SHORT BROADCAST transmits a single set of
data when the switchlight is pressed. A broadcast net is
normally used to preserve emission control (EMCON)
conditions (roll call not being conducted by the DNCU)
until one of the PUs obtains significant information for
transmission to the net. The picket unit or DNCU then
transmits the data to the net continuously until
deactivated in the long broadcast mode or to the net
one-time in the short broadcast mode. The DNCU may
elect to use the broadcast functions while conducting
roll call to increase the probability of data reception by
pickets. Pickets do not use the broadcast functions
during roll call by the DNCU since the transmissions
will not be synchronized with the roll call and may
interfere with transmission from the DNCU.
When radio silence is selected, the DTS will receive
data only. Even if a request for data is received the
system cannot transmit. All information received is sent
to the central computer.
Learning Objective: Recognize components
and operating principles of a teletype system.
The teletypewriter system is an integrated
communications system that provides the ability to
transmit and receive encrypted information with
compatible ships, aircraft, and ground stations. The
keyboard portion of the teletype inputs data into a
There are basically two modes of teletype
operations: on-line and off-line. In the on-line mode,
data is sent from the central computer through the
teletype system. Received data is routed to the computer.
In the off-line mode, the characters that are typed into