A voice switch self-test and fault location read- out (built-in test). An audio indication to the user when the voice switch  built-in-test  (BIT)  detects  a  trunk  line short. A   technical   control   monitor   phone,   incorpo- rated into the voice switch, which can access all voice  channels. SYSTEMS  EQUIPMENT  AND  LOCATION The   following   paragraphs   provide   a   brief description   of   the   major   equipment   included   in   a typical   SAS   installation.   Figure   2-13   illustrates   the audio path of the system and the order in which we will discuss the equipment. User Station Equipment User  station  equipment  is  located  in  operations centers  throughout  the  ship,  like  the  bridge,  combat information  center  (CIC),  flag  plot,  secondary  corm, and   other   stations   where   exterior   voice   com- munications  are  required  by  the  ship’s  mission.  This equipment consists of telephone sets, audio amplifiers, loudspeakers,   headsets,   recorders,   audio   jackboxes, Naval   Tactical   Data   System   (NTDS)   consoles   and intercom units, and local switching devices for added system  flexibility. Voice Switching Equipment The   voice   switching   equipment   is   a   major component of the SAS. It is the interface and primary switch  between  the  user’s  equipment  and  all  crypto  and plain   subsystems.   It   is   designed   for   very   high interchannel   isolation,   which   is   a   TEMPEST requirement for all equipment that handle both secure and  nonsecure  signals  at  the  same  time.  (The  ASAS and MSAS use different switches for this purpose.) Crypto and Plain Subsystems The  various  crypto  and  plain  subsystems  are located   in   the   main   communications   spaces. Cryptographic devices and other “red” equipment are located in a secure area within these spaces. There are five crypto and plain subsystems used within the SAS: NESTOR,   VINSON,   PARKHILL,   FLTSATCOM secure voice, and PLAIN ONLY. Additional classified information  on  these  subsystems  is  available  on  a need-to-know  basis. Transmitter and Receiver Transfer Switchboards These  equipment  are  part  of  the  overall  exterior communications  switching  system  and  are  located  in the   main   communications   spaces,   generally   in   the vicinity  of  the  technical  control  working  area.  The switchboard   equipment   group   interconnects   crypto and  plain  subsystem  equipment  with  the  appropriate radio equipment. You should recall from chapter 1 that these switchboards are also the interconnecting points for   other   subsystems   within   the   overall   exterior communications   system.   Therefore,   they   are   not unique to the SAS. Transmit and Receive Radio Equipment These equipment may be located in both the main communications spaces and in separate rooms located in  various  parts  of  the  ship.  This  equipment  group consists  of  the  various  transceivers,  transmitters,  and receivers   used   for   voice   nets.   The   more   common transceivers  you  will  encounter  are  the  AN/SRC-20 series,   AN/VRC-46,   AN/WSC-3(V)3   and   (V)7, AN/URC-93,  and  AN/WSC-6.  Common  transmitters include  the  AN/URT-23,  AN/URT-24,  T-1322/SRC, and  AN/GRT-21.  Common  receivers  are  the  R-1051 series,   R-1903,   and   AN/GRR-23.   For   additional information   on   individual   equipment,   refer   to   that equipment’s technical manual. TELETYPEWRITER/FACSIMILE   EQUIPMENT In  chapter  1,  we  discussed  different  methods  of voice communications. Sometimes, the message is too long for practical transmission by voice, so to get your message or idea across, you may need to use a chart, map,  or  photograph.  Teletype  (tty)  and  facsimile (FAX) equipment allow us to do this. In the following paragraphs, we will discuss some common   terms   associated   with   tty   and   FAX equipment. We will then look at some basic systems. MODES OF OPERATION There   are   two   basic   modes   of   teletypewriter operation: asynchronous (start-stop) and synchronous. The  asynchronous   mode  is  the  most  common.  The synchronous  mode  is  used  primarily  in  high-speed  data systems. Asynchronous  Mode In this mode, the receiving device is only allowed to run for one character and is then stopped to await the start signal for the next character. Any differences in speed between the transmitting and receiving devices can  only  accumulate  during  the  time  assigned  to  one 2-15