Plan 55-A was a store and forward message switching system developed by Western Union and used from 1948 to 1976. Western Union's own system handled telegrams, and a military version of the system was used by the U.S. Department of Defense. Leonard Kleinrock analyzed the queueing delays of Plan 55-A in his PhD thesis, and this work formed part of the theoretical basis for the ARPAnet.
Plan 55-A was a predecessor of E-mail. A Plan 55-A center performed functions similar to an e-mail server, but without computers.
In a Plan 55-A switching center, incoming messages came in over serial communication lines from teleprinters or from other switching centers, and were received by "receiving consoles", each consisting of a paper tape punch feeding tape into a paper tape reader via a storage bin. The reader would read the message header, and send the header characters to a "director". The director, much like a telephone switch, would connect the receiving console to a "sending console" in the same switching center. This was referred to as a "cross-office connection". The message would then be transmitted from the receiving console to the sending console, character by character, punching a second paper tape at the sending console. Cross-office connections, and their readers and punches, were slightly faster than external connections, to keep congestion out at the edges of the network.
Each sending console, like each receiving console, consisted of a paper tape punch and reader. Output from each sending console went out on outgoing lines to other switching centers or to destination teleprinters. Each message typically contained one telegram.
- "The Decline and Fall of the Reperforator". Western Union News (Western Union) 10 (5). May 5, 1977.
- Private Wire Services - Plan 55 Switching System - Equipment Description FWS-10. Western Union. June 1, 1957.
- Kleinrock, Leonard (December 1962). "Message Delay in Communication Nets with Storage (PhD thesis)" (PDF). Cambridge: Massachusetts Institute of Technology.
- Western Union Telegraph Company (1956). "Telegram for America". A non-technical industrial film showing telegram handling in the 1950s. Plan 55-A switching centers are shown in some detail.
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