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