AUTOVON not Autobahn

The warning on the Autovon directory was "DO NOT DISCUSS CLASSIFIED INFORMATION ON NONSECURE  TELEPHONES. OFFICIAL DOD TELEPHONES ARE SUBJECT TO  MONITORING FOR COMMUNICATIONS SECURITY PURPOSES  AT ALL TIMES." The Automatic Voice Network (AUTOVON) was a world-wide American military telephone system. Note the past tense. It was not secure. It was never intended to be secure and there was no pretense about that: A description in their own telephone directory is as follows:

"The Global AUTOVON is the principal long-haul, nonsecure, common user voice communications network for the Department of Defense (DoD). It provides worldwide direct distance dialing station to station service through a system of government owned and leased automatic switching and transmission facilities."

For the record not just anyone was allowed to use AUTOVON. AUTOVON included four precedence levels: Routine, Priority, Immediate and Flash, and had an additional capability called Flash Override. Beside the normal number pad was an additional column on the keypad labeled FO, F, I, and P to activate the precedence levels. They each had their own DTMF codes. If you are familiar with DTMF you'd be interested to know that column of buttons produced a tone at 1633 Hz. If you're not familiar that made no sense. More here and here.

In April of 1964, the U.S. Army’s Switched Circuit Automated Network (SCAN) and the U.S. Air Force’s North American Air Defense network were combined to form AUTOVON. (Some sources list this as 1963) In 1966 Air Defense Command switched over to AUTOVON. the system and it's 70 switching centers, 8,000 interswitch trunks, and 16,000 access lines were maintained by the the Defense Communications Agency (DCA) with the Computer Sciences Corporation acting as it's primary design contractor. In 1970 the newly formed Defense Communications Agency Systems Engineering Facility (DCASEF) in Reston, Virginia took over it's operation.

It is interesting to note that the management of AUTOVON required some of the earliest network management software ever written. With a minimum call volume and 9 connections per node even modeling the new network was difficult. Few engineers outside AT&T even had an idea how to design such things. The model they used, known as the Circuit Switched Network Design and Analysis Model (CSNDAM), was useful but far from perfect.  It was based on the work of Steven S. Katz at Bell Labs who described it in his white paper Statistical Performance Analysis of a Switched Communications Network. Testing was done using Wilkinson’s Equivalent Random Technique (ERT), which was developed by Roger I. Wilkinson and S.R. Neal at Bell labs in 1955 for managing toll traffic volume.

Engineers Harry Barker and David Calabrese continued to manage the AUTOVON network into the late 70s on the traffic modeling that got them through the 1960s. Still in use in the late 1980s, the AUTOVON network reached from the USA to Asia, Middle  East, and from Alaska to Panama. At it's peak AUTOVON had about 18,000 subscribers. The system was replaced in the early 1990s by the Defense Switched Network. More here.