Autovon
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Autovon, the Automatic Voice Network, was an American military phone system built to survive nuclear attacks. AUTOVON was first built in the United States, but spread to England, Asia, the Middle East, and Panama. It was a major part of the Defense Communications System (DCS), providing the non-secure switched voice services. Today the system is unused, replaced by the Defense Switched Network.
AUTOVON used its own set of trunk lines which used a combination of their own 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 remains today and is easily visible on satellite photos. The system was transported over many mediums other than underground cable, including microwave, coaxial, open wire, and fiber optic. Fiber of course was near the end of the life of Autovon. The stories of underground concrete cable ducts is highly overrated, most of the cable was put underground without any added concrete, relying instead on the natural protection from the 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 Mounds OK Autovon junction 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 user is unable to dial the phone, with the system typically sending the "reorder" signal (the "fast-busy") to indicate the problem. However this sort of solution was not acceptable for military networks, where some messages absolutely have to get through.
To support this 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 hang up 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 an exchange where all the lines 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 USA or other National Command Authority to be able to preempt any other trafic 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 coresponding to the central office telephone switches on each installation. Thus, almost any telephone on a military installation could be direct-dialed via AUTOVON. A selected set of the telephones would have the capability to dial directly into the AUTOVON network. Others could initiate AUTOVON calls with operator assistance.
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 user would dial 9, and to dial using commercial long-distance, 1 (if 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.