AUTOMATIC DIGITAL NETWORK The Automatic Digital Network (AUTODIN) is a worldwide computerized communications system. AUTODIN provides for the transmission of narrative and data pattern traffic on a store-and-forward basis. AUTODIN provides reliable, secure, and efficient communications.  AUTODIN  also  incorporates  error detection and contains the highest speed transmission equipment  currently  available.  AUTODIN  is  part  of  the Defense  Communications  System  (DCS)  and  is managed  by  the  Defense  Communications  Agency (DCA). Interface  equipments  translate  all  AUTODIN inputs  into  common  machine  language,  making AUTODIN  compatible  with  many  computer  codes, speeds, and media, such as cards and tapes. Because of this,  communications  equipment  within  the  NTS  can  be integrated into the AUTODIN system. AUTODIN Switching Centers The backbone of the AUTODIN system is the Automatic Switching Center (ASC). There are eight ASCs in the continental United States and five ASCs overseas (Europe and the Pacific). The  ASCs  are  interconnected  into  a  digital  network by trunk lines. Each center has local lines that link it to each  subscriber  (communications  center)  terminal. Messages entering the AUTODIN system at any of the subscriber  terminals  are  forwarded  through  their respective   switching   centers.   The   ASCs   accept messages   from   subscribers,   determine   the classifications  and  precedence  of  the  messages,  and relay the messages to the addressed subscribers. AUTODIN Operational Modes There  are  five  AUTODIN  system  operational modes. These modes provide variation of speed and operation   capabilities   based   on   the   equipment configurations of the message center subscribers. The following paragraphs describe each mode: Mode I —A  duplex  operation  with  automatic error  and  channel  controls.  Mode  I  operation allows  independent  and  simultaneous  two-way operation  between  two  stations.  The  channel control characters acknowledge receipt of valid line blocks and messages or allow return of error information to the subscriber. The terminal (switching  center)  responds  automatically  to these  characters  by  continuing  or  stopping transmission and displaying action information to the operator. A magnetic tape terminal is an example of terminal equipment using mode I. Mode  II  —A   duplex   operation   normally associated with TTY or teleprinter equipments with  independent  and  simultaneous  two-way operation  capability.  There  are  no  automatic error and charnel controls in mode II operation. Message accountability is maintained through channel sequence numbers and service message actions. Mode III —A duplex operation with automatic error  and  channel  controls  but  only  one-way transmission capability. The return is used only for  error  control  and  channel  coordination response. The mode III channel is reversible on a message basis. Control characters are used in the same manner as in mode I. Mode IV —A  unidirectional  operation  (send only or receive only) without error control and channel coordination. The mode IV channel is nonreversible and is equivalent to half-duplex operation of mode II. Mode V  —A   duplex   operation,   normally associated  with  TTY  or  teleprinter  equipment, with  independent  and  simultaneous  two-way transmission. Control characters acknowledge receipt   of   messages   and   display   limited information   to   the   operator.   Message accountability is maintained through the use of channel sequence numbers. Input and output (I/O) devices, such as teleprinters, provide the central AUTODIN computer with the necessary  means  to  communicate  with  the  user.  Output devices provide the means for changing the computer- processed data into a form specified by or intelligible to the users. The selection of I/O devices depends on the specific use for which a computer is intended. Generally,  I/O  devices  must  meet  several  basic requirements. First, they must be able to modify all data so that it is acceptable to the computer during the input phase of the operation. The devices must also be able to present data in usable form during the output phase and operate  quickly  and  efficiently   with  the  computer. 1-5