Link-16 Data Exchange Link-16 transmits data serially using 70-bit data words. During the transmit time slot, either three, six, or 12 data words can be transmitted. The number of words transmitted depends on whether the standard, packed-2,  or  packed-4  data  packing  structure  is  used. The  number  of  words  that  compose  a  Link-16 message is variable but is normally 1, 2, or 3 words. There are three types of messages: fixed format, free text, and variable format. The fixed format messages are called J-series messages and are used to exchange tactical information. Free text messages are used for voice  communications,  while  the  variable  format messages  are  user  defined  in  length  and  content. Variable format messages are not used by the Navy. JTIDS  Architecture There  are  several  features  of  the  JTIDS architecture  that  have  resulted  in  improved communications   of   the   Link-16   system. These features include the following: Nodelessness Security Network  participation  groups NODELESSNESS.— A node is a unit required to maintain communications of a data link. In Link-11, the NCS is a node. If the NCS goes down, the entire net is inoperative. Link-16 does not need a dedicated station.  When  the  Link-16  net  is  established,  a  single JU transmits a  Network  Time  Reference  (NTR). The  time  established  by  this  unit  is  the  network system time. All other units in the net use the NTR message to synchronize with the network. Once the NTR  and  the  network  have  been  established,  the network  can  continue  to  operate  regardless  of  the participation of any particular unit. SECURITY.—   The  security  of  the  Link-16 system is vastly improved over that of the Link-11 system. In   Link-16,   both   the   data   and   the transmissions are encrypted. Data is encrypted by a device   similar   to   Link-11,   using   a   specified cryptovariable for message security. The security of the  data  transmission  is  provided  by  the  use  of  a second  cryptovariable  that  controls  the  transmitted waveform.  Frequency  hopping  to  prevent  jamming  is one  of  the  features  of  the  security  system.  The transmission security also   provides for   the introduction of jitter and a pseudo-random noise to be added  to  the  waveform.  The  addition  of  jitter  and noise, along with the frequency hopping, makes the transmitted  signal  extremely  difficult  to  detect  and jam. N E T W O R K P A R T I C I P A T I O N GROUPS.— The time slots of a Link-16 network can be broken down into separate Network  Participation Groups(NPGs).  An NPG is defined by its function and  determines  the  types  of  messages  that  are transmitted  on  it.  Some  of  the  NPGs  used  by  the Navy are as follows: Surveillance Electronic Warfare Mission Management Weapons Coordination Air Control Fighter-to-Fighter Secure  Voice Precise  Participant  Location  and  Identification (PPLI) and Status By  dividing  the  net  into  NPGs,  each  JU  can participate on only the groups that support the mission of the unit. Most Navy Command and Control (C²) units,  both  ships  and  aircraft,  operate  on  all  the defined NPGs except the Fighter-to-Fighter NPG. Link-16 New Capabilities The  increased  size  of  the  Link-16  enables  the reporting  of  up  to  three  times  as  much  tactical information  as  was  available  under  the  Link-11 5-6