Automatic Voice Network

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An AUTOVON telephone keypad with the four priority levels

The Automatic Voice Network (AUTOVON) was a worldwide military telephone network in the USA. The construction of the system started in 1963 on the basis of the existing Switch Communications Automated Network (SCAN) of the US Army. In June 1966, the Air Defense Command telephone network was migrated to the new system. In 1969 AUTOVON exchanges were opened in the United Kingdom and later in other European countries, in Asia, the Middle East and Panama. It was a major part of the Defense Communications System (DCS) and was responsible for providing unsecured automatically switched voice services. The system was replaced by the Defense Switched Network in the early 1990s .

cables

AUTOVON used a combination of exclusively laid lines and lines that were rented from AT&T and small, independent telephone companies. The lines were connected to switching centers, which were built apart from other civil or military goals. In the USA mainly L-carrier coaxial cable multiplexers, which were built by AT&T, were used for this. AT&T also used excess capacity to handle civil wide-area connections. Even though they are unused today, some of the cables still exist today and their routes can be seen on satellite photos. In addition to underground cables, the content of the network was also transmitted via other media, including microwave links, overhead lines and - shortly before the system was replaced - fiber optic cables. In contrast to stories about concrete sewers, in reality the cables were mostly laid directly in the ground and were only protected by a layer of oil.

Most of the repeater buildings were sold to private investors in order to increase existing capacities or to convert them into telecommunication rooms etc. AT&T had underground parts replenished before buying, unless they were selling to a large company. With a few exceptions, the nodes of the AUTOVON network were also sold. Most of the time, the technology was completely expanded, even if the hub in Mounds, Oklahoma was sold with all the equipment.

Initially, a 4-wire version of the 5XB switch was used as an exchange , which was replaced by the more versatile 1ESS switch in the early 1970s after it had proven its reliability.

Multilevel presedence and preemption

The AUTOVON system provided a function for interrupting and forcing conversations, which was called multilevel precedence and preemption (MLPP). In the public telephone network it could happen that a line between the caller and the called party was not available and the call was therefore not established. The caller then received a special form of the busy tone ("reorder tone"). Such behavior was unacceptable in a military telephone network, as some telephone calls could not be postponed.

AUTOVON was therefore given four priority levels: Routine , Priority , Immediate and Flash , and there was also a higher level called Flash Override . These levels were activated by keys in an additional column on the number switch, through which the DTMF signals A, B, C and D were generated:

  • A (679 and 1633 Hz): Flash Override (FO)
  • B (770 and 1633 Hz): Flash (F)
  • C (852 and 1633 Hz): Immediate (I)
  • D (941 and 1633 Hz): Priority (P)

Routine was the non-priority level and did not require a special signal - the user simply had to dial the phone number. For calls with priority, the mentioned priority level had to be selected before the telephone number. Conversations with a higher priority could interrupt lower conversations if necessary. If, for example, a call with the priority level Flash was established and there was no free line in a trunk group on the route, the exchange ended a call with the priority level routine or - if none was active - a call with the priority or immediate level. The caller only heard a busy signal if all lines were already busy with Flash or Flash Override calls.

Who was allowed to use which priority levels was the subject of a complex set of rules. Flash Override was not designed as a privilege level, but was designed to allow the US President or other members of the National Command Authority to interrupt all other telephone traffic in an emergency.

The International Telecommunication Union accepted the MLPP specification as recommendation Q.955.3 in March 1993.

Numbering plan

AUTOVON used a numbering plan that was similar to the North American numbering plan . The telephone network had its own three-digit area codes for various geographic regions worldwide. Each area was subdivided into various three-digit "exchange codes", which were usually assigned to an exchange in a military facility. As a result, almost every military facility could be called directly via AUTOVON. A selected number of telephones were four-wire telephones that could establish calls directly to the AUTOVON network. Others were able to set up AUTOVON talks with the help of a mediator.

Although the numbering plan was similar to the civilian plan, the routing structure - in contrast to the hierarchical civilian variant - was a very complex, heavily meshed system. It was at the limit of the 5XB exchanges on which it was implemented. The non-hierarchical routing structure was necessary in order to still be able to reach every goal if a large number of switching centers were destroyed during a war. This system inspired similarly viable systems for the exchange of messages including the later Internet.

Local exchanges were connected to the AUTOVON system via a few switching lines, which could be reached by dialing the number 8 (or in some cases 88). To call locally, the 9 had to be dialed. For commercial wide area connections - if supported - 1 had to be preselected. The United States Department of Defense (DoD) billed connections to the AUTOVON system through a sophisticated billing system, and each facility billed according to local guidelines.

See also

Web links

Individual evidence

  1. ^ AT&T (1974) Events in Telephone History
  2. ^ Copy of a 1971 "Official Global Autovon Telephone Directory"
  3. ^ Clause 3 - Multi-Level Precedence and Preemption (MLPP) International Telecommunication Union Recommendation Q.955.3 . Retrieved December 27, 2013.