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LINK-11 NET OPERATING MODES
BUILDING A LINK-11 MESSAGE

Fire Controlman Volume 06-Digital Communications
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When Net Test is completed, all picket stations report their status to NCS. Then NCS directs all PUS to  switch  to  the  Roll  Call  mode  and  initiate  link operations. Net  Synchronization  and  Net  Test  are used in the initialization of the net. The normal mode of operation is Roll Call. The  above  scenario  has  introduced  you  to  several new   terms   and   modes   of   operation. These  are explained in detail in the following paragraphs. The  following  are  the  six  modes  of  Link-11 operation: Net Synchronization Net Test Roll  Call Broadcast Short  Broadcast Radio Silence Net Synchronization The   Net   Sync   mode   of   operation   is  used  to establish a uniform time base from which all net data communications   normally   initiate.   The   Net   Sync mode  is  usually  initiated  when  establishing  a  link  net after   all   operator   entries   have   been   properly completed.  The  Net  Sync  transmission  is  manually started  by  the  operator  on  the  NCS  platform  and continuously  transmits  the  Link-11  preamble  until stopped by the operator. The  preamble  consist  of  two  tones–the  605-Hz tone and the 2,915-Hz tone. During the transmission of Net Sync, the 2,915-Hz tone is periodically phased shifted 180 degrees. The time between these shifts is determined by the selected data rate and is called a frame. Each PU is equipped with a very accurate time base in the form of a frequency standard (internal or external).  When  the  NCS  transmits  Net  Sync,  each unit   receiving   the   transmission   synchronizes   its individual time base with the Net Sync signal. If the picket station is operating in the corrected sync mode, as is normally the case, the picket will check to see that it can recognize the Net Sync signal as a means of verifying  that  a  good  radio  link  has  been  established. If a picket is going to operate in the stored sync mode, it will align its stored frame timing to the timing of the NCS, using the received Net Sync signal. Since stored sync timing locks the picket to the time base of the   NCS,   data   from   other   pickets   may   be   lost. Therefore,  this  mode  should  only  be  used  during times of poor radio propagation or signal jamming. After the completion of Net Sync, the next operation performed in establishing a link is usually Net Test. Net Test Net  Test  provides  an  overall  evaluation  of  the  net and  equipment  performance.  When  you  are  operating in this mode, NCS will broadcast canned test data to all  pickets  within  the  net.  The  data  terminal  set contains a code generator that generates twenty-one 30-bit  data  words.  Once  all  the  words  in  the  word table have been generated, the process automatically starts  over  and  keeps  running  until  stopped  by  the operator. Net  Test  will  test  the  connectivity  between  all units and the operation of the DTS. Since it is a local test, Net Test does not check the interface between the CDS computer and the DTS. Net Testis also helpful to the technician for setting the audio input and output levels of the DTS or radio set. Roll Call Roll Call is the normal mode of operation. In this mode,   the   operator   on   the   NCS   platform   enters ownship’s   address   and   an   assigned   address   (PU number) for each PU in the proper switch position. When the link is initiated, each PU is polled for data. Polling consists of sending a call-up message. If the PU fails to respond, the call-up is repeated. If the PU still does not respond, it is skipped and the next PU is polled. When a PU recognizes its own address, the PU will transmit its data to all the participants in the link.  When  the  NCS  recognizes  the  end  of  the  PU 2-6






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