The Bell System

The Bell System encompasses the system of local and long-distance telephone lines established by the Bell company, American Telephone & Telegraph (AT&T), and, post-monopoly, by a long list of local and national companies. I am especially interested in the secure, hardened military communications systems that the Bell system developed and operated for the United States Government.

A particularly useful thing I have been gathering is every map I can find of historic AT&T Long Lines long distance connections, which can be very useful in making sense of what you see scattered around the country today.

AUTOVON, the Automatic Voice Network, was a dedicated telephone system for the Department of Defense which was operated by the Bell system and shared Bell infrastructure. It was a precursor to the later Defense Switched Network (DSN), which is still in use today. AUTOVON is particularly interesting because it was implemented during the crossover from electromechanical to electronic switching and thus incorporated both crossbar and 1ESS switches. Wikipedia has a general overview, but I try to include more depth here, including links to resources.

One item of local interest is the Socorro long lines site, which housed a hardened AUTOVON switch.

• Map of AUTOVON switches, 1973

• Info on Albert LaFrance's website
• Contingency network management plan, a document with a lot of interesting operational details, courtesy of telephonecollectors.info

L-carriers were a set of coaxial-cable based analogue carriers widely used for early long-distance lines.

• Field guide to L-carrier

L-3I, or L-3 Improved, was a version of the L-carrier widely used in the AT&T network to multiplex calls onto coaxial cables. L-carrier was used prior to the introduction of fiber optics for this purpose, and a relatively small number of L routes were built across the country, many of them still easy to pick out today because of the frequent repeaters with above-ground structures and unusually wide trenching scars. What was improved about L-3I specifically was its durability: L-3I was used to carry AUTOVON traffic, and so L-3I infrastructure was hardened against attack, including nuclear attack.

• See the maps page for several maps of the L-3 network - unfortunately none of them great. I would very much like to obtain any higher-quality maps, should you find any.
• About L-carrier infrastructure

• One of the best resources I've found on the L-3I system is Mark Foster's webpage on the subject which has since gone offline. Fortunately, archive.org has an archived copy.