Plan 55-A

Plan 55-A was one in a series of store and forward message switching systems developed by Western Union and used from 1948 to 1976 for processing telegrams.[1] It is an automated successor to Plan 51, which commenced service in 1951 in a nationwide network of the U.S. Air Force, but required semi-automatic operation.[2][3]

Based on the technology of punched paper tape storage, the systems of the design were called reperforators. A reperforator performed functions similar to an email message transfer agent (email server), used much later in the Internet, but it used electro-mechanical technology, which preceded the use of semiconductor circuitry and computers.

A reperforator switching center received messages via serial communication lines from teleprinters, such as the Teletype Model 28 ASR, or from other switching centers on receiving consoles, each consisting of a paper tape punch feeding tape into a paper tape reader via a storage bin. The reader decoded the message header, and sent the header characters to the director. The director, much like a telephone switch, connected the receiving console to a sending console in the same switching center by a cross-office connection. The message was 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 limit congestion to the edges of the network.

Each sending console also consisted of a paper tape punch and reader. Output from each sending console was transmitted via outgoing lines to other switching centers or to destination teleprinters.[3] Each message typically contained one telegram.

Each received message had up to nine routing indicators, or destinations. For two or three destinations, the messages were sent simultaneously on three cross-office connections to outgoing sending consoles. Additional destinations resulted in a copy of the message being sent to the multiple call spillover unit, which removed the routing indicators for destinations already handled and sent the message through the system again.[3]

Nationwide, Western Union's switching centers were arranged in a hub and spokes architecture involving fifteen locations. The U.S. Air Force used the Plan 55-A system worldwide with ten centers.

An analysis of the queueing delays in Plan 55-A by Leonard Kleinrock[4] formed part of the theoretical basis for the development of the ARPAnet.

A small part of a Plan 55-A message switching center, showing paper tape punches and readers used for buffering.

See also


  1. ^ "The Decline and Fall of the Reperforator". Western Union News. Western Union. 10 (5). May 5, 1977.
  2. ^ Vernam, G.S. (May 1958). "Automatic Telegraph Switching System Plan 55-A". Transactions of the American Institute of Electrical Engineers. IEEE: 239–247. doi:10.1109/TCE.1958.6372793.
  3. ^ a b c Private Wire Services - Plan 55 Switching System - Equipment Description FWS-10. Western Union. June 1, 1957.
  4. ^ Kleinrock, Leonard (December 1962). "Message Delay in Communication Nets with Storage (PhD thesis)" (PDF). Cambridge: Massachusetts Institute of Technology.

External links

  • 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.
Gilbert Vernam

Gilbert Sandford Vernam (3 April 1890 – 7 February 1960) was a Worcester Polytechnic Institute 1914 graduate and AT&T Bell Labs engineer who, in 1917, invented an additive polyalphabetic stream cipher and later co-invented an automated one-time pad cipher. Vernam proposed a teleprinter cipher in which a previously prepared key, kept on paper tape, is combined character by character with the plaintext message to produce the ciphertext. To decipher the ciphertext, the same key would be again combined character by character, producing the plaintext.

Vernam later worked for the Postal Telegraph Company, and became an employee of Western Union when that company acquired Postal in 1943. His later work was largely with automatic switching systems for telegraph networks.

Leonard Kleinrock

Leonard Kleinrock (born June 13, 1934) is an American computer scientist. A professor at UCLA's Henry Samueli School of Engineering and Applied Science, he made several important contributions to the field of computer networking, in particular to the theoretical foundations of computer networking. He played an influential role in the development of the ARPANET, the precursor to the Internet, at UCLA.

Store and forward

Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in a networking context, verifies the integrity of the message before forwarding it. In general, this technique is used in networks with intermittent connectivity, especially in the wilderness or environments requiring high mobility. It may also be preferable in situations when there are long delays in transmission and variable and high error rates, or if a direct, end-to-end connection is not available.


A teleprinter (teletypewriter, Teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initially they were used in telegraphy, which developed in the late 1830s and 1840s as the first use of electrical engineering. The machines were adapted to provide a user interface to early mainframe computers and minicomputers, sending typed data to the computer and printing the response. Some models could also be used to create punched tape for data storage (either from typed input or from data received from a remote source) and to read back such tape for local printing or transmission.

Teleprinters could use a variety of different communication media. These included a simple pair of wires; dedicated non-switched telephone circuits (leased lines); switched networks that operated similarly to the public telephone network (telex); and radio and microwave links (telex-on-radio, or TOR). A teleprinter attached to a modem could also communicate through standard switched public telephone lines. This latter configuration was often used to connect teleprinters to remote computers, particularly in time-sharing environments.

Teleprinters have largely been replaced by fully electronic computer terminals which typically have a computer monitor instead of a printer (though the term "TTY" is still occasionally used to refer to them, such as in Unix systems). Teleprinters are still widely used in the aviation industry (see AFTN and airline teletype system), and variations called Telecommunications Devices for the Deaf (TDDs) are used by the hearing impaired for typed communications over ordinary telephone lines.

Western Union

The Western Union Company is an American worldwide financial services and communications company. Its headquarters is in Meridian, Colorado, although the postal designation of nearby Englewood is used in its mailing address. Up until it discontinued the service in 2006, Western Union was globally the best-known American company in the business of exchanging telegrams.Western Union has several divisions, with products such as person-to-person money transfer, money orders, business payments, and commercial services. They offered standard "Cablegrams", as well as Candygrams, Dollygrams, and Melodygrams.

Western Union, as an industrialized monopoly, dominated the American telegraph industry in the late 19th century. It was the first communications empire and set a pattern for American-style communications businesses as they are known today.

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