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 paragraphs. 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 PU’s 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. TELETYPE SYSTEM 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 central computer. 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 1-22