Autovon
Autovon, the Automatic Voice Network, was a US military phone system built to survive nuclear attacks. AUTOVON was first built in the US, but spread to England, Asia, the Middle East, and Panama. It was a major part of the Defence Communications System (DCS), providing the non-secure common user switched voice network of the DCS. Today the system is unused, replaced by the Defense Switched Network.
Autovon used its own set of trunk lines which were buried 35 feet underground in concrete conduits, connected to exchanges located far from other civilian or military targets. In the US the conduits 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 conduit remains today and is easily visible on satelite photos.
One interesting feature of the Autovon system was the inclusion of priority 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 five different priorities, Routine, Priority, Immediate, Flash and Flash Override. A normal call was equivalent to Routine, and calls of increasing priority could hang up calls of lower priority if need be. For instance, if the call was placed with Flash priority and was switched to an exchange where all the lines were in use, the switch would then hang up 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.
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