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.

I/O devices use coded languages. These languages are:

ASCII Code -American Standard Code for Information Interchange, eight-level paper tape; and ITA #2 Code -American version of international TTY alphabet, five-level paper tape.

Message Header Programming

At the beginning of each AUTODIN message is a header (format line 2) containing pertinent information on the destination of the message. The originator can address a message either to a single addressee or to multiple addresses. This system saves time and requires fewer communications facilities, since only one message is prepared by the originator and sent to the switching center.

The timing system contained in AUTODIN equipment briefly connects a switching center to each subscriber terminal in turn. Computer memories act as reservoirs for the incoming messages of each subscriber terminal. The computer is programmed to connect each terminal in turn during a cycle. Messages received in their entirety are scheduled for output to the addressees' charnels as their turns arrive in the cycle.

AUTODIN has built-in safeguards that can detect almost any type of hardware or format error. Additionally, a complete (reference) copy of all relayed messages is kept on AUTODIN computer tape. A separate (journal) copy is made of only the addressee(s). Using this journal copy as an index enables the system to locate the reference copy of any message.

AUTODIN Tape Messages

The AUTODIN system is programmed to accept properly cut tapes and route them through the various switching centers and terminals en route to their ultimate destination. The system is then able to produce a tape and hardcopy for the designated addressee(s).

When preparing a message tape for the AUTODIN system, you must adhere to certain tape-cutting procedures. For example, format lines 1, 2, and 4 must not deviate; otherwise, the ASC will reject the message. The next paragraphs discuss the most important points on proper preparation of tape messages for transmission in the AUTODIN system.