Autovon
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AUTOVON, short for Automatic Voice Network, was an American military phone system built in 1963 to survive nuclear attacks. AUTOVON was first established in the United States, using the Army's SCAN (Switch Communications Automated Network) system. Around the mid-1970s AUTOVON expanded to England, Asia, the Middle East, and Panama. It was a major part of the Defense Communications System (DCS), providing non-secure switched voice services. Today the system is unused, replaced in the early 1980s by the Defense Switched Network.
AUTOVON used a combination of its own constructed lines and other lines operated by AT&T and smaller non-bell companies, connected to exchanges located far from other civilian or military targets. In the US the cables were built by AT&T, who also used them to carry about 1/3rd of all civilian long-distance lines as well, as they were much bigger than the military needed. Although unused, the cables remain today and are easily visible on satellite photos. The system was transported over many media other than underground cable, including microwave, coaxial, open wire and, near the end of the system's life, fiber optic. Exaggerated stories of underground concrete cable ducts are in circulation but most of the cable was directly buried without any added concrete, relying instead on the natural protection of dirt.
Most of the cable repeater huts have been sold to private interests, to round out existing parcels, or as possible build to suit tower sites, etc. AT&T is now filling the small underground portion before sale, unless they sell to a major company. The junctions for AUTOVON are also being sold into private ownership, with a few exceptions. Most are stripped of all the equipment, although the AUTOVON junction in Mounds, Oklahoma was sold with all the old equipment still in place.
One interesting feature of the AUTOVON system was the inclusion of precedence for calls. In the civilian networks if there are no free lines the caller is unable to reach the called party, with the system typically sending the reorder tone ("fast-busy" signal) to indicate the problem. Such an event was not acceptable for a military network where some messages absolutely have to get through.
To address this problem, AUTOVON included four precedence levels: Routine, Priority, Immediate and Flash, plus an additional capability: Flash Override. A normal call was equivalent to Routine, and calls of increasing precedence could knock down calls of lower priority (giving them a special tone) if need be, which was called preemption. For instance, if the call was placed with Flash precedence and was switched to a route where all trunks were in use, the switch would then preempt a Routine call if there was one, and then Priority and Immediate. Only in the case where a switch's lines were all being used by Flash or Flash Override would the user receive a reorder signal.
When a user wanted to place a call normally, they would simply dial the number. However, if the call was to be placed with precedence, a key in the rarely used and rarely seen 1633 Hz column of DTMF tones would be pressed before dialing to assign this:
- A (697/1633, FO): Flash Override
- B (770/1633, F): Flash
- C (852/1633, I): Immediate
- D (941/1633, P): Priority
There were complex regulations as to who was allowed these precedence levels. Flash Override was not technically a precedence level, but rather a capability designed to allow the President of the United States or other National Command Authority to preempt any other traffic in the network in an emergency.
AUTOVON used a 3-3-4 numbering scheme similar to the North American commercial telephone system. The network had its own three-digit "area codes" for various geographic regions around the world. The area codes each included several three digit exchange codes, usually corresponding to the central office telephone switches serving each installation. Thus, almost any telephone on a military base could be direct-dialed via AUTOVON. A selected set of telephones were four wire AUTOVON phones, wired directly into the AUTOVON network. Others could initiate AUTOVON calls with operator assistance.
Though the numbering plan was similar to the U.S. civilian scheme, the routing structure was a very complex polygrid system unlike the civilian office classification scheme. It was barely within the information processing capabilities of the 5XB switch which implemented it. The non-hierarchical routing structure was intended to get around any number of nodes destroyed in war. This system inspired similarly survivable ones for message networks, including in future decades the Internet.
Local base switches would be connected to a few AUTOVON trunks, which the user would access by dialing 8 (or in some cases, 88) as the first digit. To dial locally a user would dial 9, and to dial using commercial long-distance, 1 (where this was supported). The United States Department of Defense (DoD) drew up a complex billing system in order to charge for access to AUTOVON, and each base budgeted as they saw fit.
