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REMOTE  CONTROL  UNIT
JTIDS  Architecture

Fire Controlman Volume 06-Digital Communications
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Transmission Protocols Since Link-16 exchanges much of the same data that  is  used  in  both  Link-11  and  Link-4A,  a  brief comparison of the architectures, the capacities, and the data rates of the three systems is useful. During normal operation, Link-11 operates using the  protocols  of  the  Roll  Call  mode.  In  this  mode, each  participating  unit  is  polled  by  the  NCS  to transmit  data.  On  completion  of  data  transmission, the unit returns to the receive mode and the next unit is polled until all units have been polled. This cycle is  continuously  repeated.  Link-11  messages  are  called M series messages. Link-4A   uses   the   time-division   multiplexing principle with a command-and-response protocol to enable  the  operator  to  control  multiple  aircraft independently  on  the  same  frequency. Link-4A messages sent to the controlled aircraft are referred to as V series messages  and  messages  received  from  the controlled aircraft are called R series messages. Link-16 uses the Time-Division  Multiple  Access (TDMA)  principle  of  data  communications.  Using this  architecture  with  time  interlacing  provides  the system  with  multiple  and  apparently  simultaneous communications nets. Instead of assigning each unit a  PU  number,  Link-16  assigns  each  unit  a  JTIDS Unit number, or JU. The JU identifies the units and determines  a  preassigned  set  of  time  slots  that designate when the unit transmits and receives data. Each  time  slot  is  1/128  of  a  second,  or  7.8125 milliseconds, in duration. When a JU transmits data, the frequency that the data   is   transmitted   on   is   changed   every   13 microseconds  (µsec),  according  to  a  predetermined pseudo-random  pattern. Link-16  uses  51  different frequencies   for   data   exchange.   This   frequency hopping   adds  to  the  security  and  integrity  of  the system by making it nearly impossible to jam. Link-16 Nets Link-16 has the capability to handle multiple nets. A  Link-16  net  is  a  group  of  participants  sharing 5-5 mutually   beneficial   tactical   information. For example, using the Link-11 system, a net is formed by a  group  of  participants.  These  participants  operate  on the same frequency. A separate net is formed when another group of participants operates on a different frequency. The  second  net  would  be  used  by participants  involved  in  a  fleet  exercise  that  wouldn’t want  the  exercise  data  to  interfere  with  the  normal tactical  net.  The  controlling  station  and  aircraft  using Link-4A is also a net. Link-16 has the ability to form multiple nets. The Link-16 system has 128 numbers used to designate particular nets (00-127). Net number 127 is reserved to indicate a stacked net. A stacked net is formed by setting up the time slots so that they have the same set, initial slot number, and recurrence rate. When the system is initialized, the use of net number 127 indicates a stacked net is to be used and the operator can   then   specify   locally   which   net   to   use   for operations.    Figure  5-5  illustrates  the  concept  of  a stacked net used for air control. Net 1 is a group of aircraft controlled by the ship, while Net 3 is a group of aircraft controlled by an E-2. If the E-2 requires additional  aircraft,  the  ship  can  direct  the  aircraft under  its  control  to  the  E-2. As  the  aircraft approaches the E-2, the pilot can switch to Net 3 and immediately become an active participant in the new net.  Even  though  the  operator  has  several  nets available  to  monitor  or  use,  a  single  terminal  can transmit or receive on only one of them for each time slot.  Stacked  nets  are  possible  because  the  frequency- hopping  pattern  is  different  for  each  net.  Examples  of stacked nets are voice nets and control nets. Figure  5-5.—Stacked  nets  using  Link-16.






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