provides the reference for the first frame of data.
Each succeeding frame becomes the phase reference
for the following frame.
Figure 2-6.—The phase reference frame added to the
preamble with normal data tone levels.
Information Segment
The information segment of the Link-11 message
is composed of control code frames and message data
frames. Control code frames consist of a start code,
a stop code, and an address code. Each control code
is two frames in length and performs a specific
function. Control codes are not sent to the CDS
computer.
START CODE.— The start code is a two-frame
code that follows the phase reference frame, as shown
Figure 2-7.—The start code added to the Link-11
transmission.
in figure 2-7. When sensed by the DTS, the start code
causes the DTS to send a prepare-to-receive data
interrupt to the CDS computer.
MESSAGE DATA FRAMES.— Message data
frames contain the tactical data being disseminated
and follow the start code, as shown in figure 2-8. The
number of message data frames depends on the
amount of tactical information the unit transmits. The
24 bits of data contained in each frame is sent to the
CDS computer.
Figure 2-8.—The message data frames added to the
Link-11 transmission.
STOP CODE.— The stop code is a two-frame
code that follows the data message in a Link-11
transmission and is shown in figure 2-9. There are
two types of stop codes: the control stop code and the
picket stop code. The control stop code is used in
messages originated by NCS (NCS report) and
indicates that a picket address code follows the stop
code. The picket stop code indicates to the NCS that
the picket unit
has completed its message
transmission. Both the control stop code and picket
stop code cause the receiving DTS to send the End-
of-Receive interrupt to the CDS computer.
LINK-11 MESSAGE FORMATS
The formats of the messages transmitted by the
Link-11 system vary with the mode of operation.
Roll Call Mode Messages
In the Roll Call mode, the unit designated as the
net control station sends out two types of messages.
These are the NCS call-up message (interrogation)
and the NCS report (message with interrogation). A
third message, the picket reply message, is sent by
picket unit in response to interrogation messages.
2-8
