Plan 55-A was one in a series of

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 ...

message switching In telecommunications, message switching involves messages routed in their entirety, one hop at a time. It evolved from circuit switching and was the precursor of packet switching. History Western Union operated a message switching system, Plan ...

systems developed by

Western Union The Western Union Company is an American multinational financial services company, headquartered in Denver, Colorado. Founded in 1851 as the New York and Mississippi Valley Printing Telegraph Company in Rochester, New York, the company cha ...

and used from 1948 to 1976 for processing

telegrams Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas p ...

. It is an automated successor to Plan 51, which commenced service in 1951 in a nationwide network of the

U.S. Air Force The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Sign ...

, but required semi-automatic operation. 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 Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" mean ...

message transfer agent Within the Internet email system, a message transfer agent (MTA), or mail transfer agent, or mail relay is software that transfers electronic mail messages from one computer to another using SMTP. The terms mail server, mail exchanger, and MX host ...

(email server), used much later in the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...

, but it used electro-mechanical technology, which preceded the use of

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

circuitry and computers. A reperforator switching center received messages via serial communication lines from

teleprinter 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. Init ...

s, 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. 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. 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 Leonard Kleinrock (born June 13, 1934) is an American computer scientist and a long-tenured professor at UCLA's Henry Samueli School of Engineering and Applied Science. In the early 1960s, Kleinrock pioneered the application of queueing theor ...

formed part of the theoretical basis for the development of the

ARPAnet The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical fou ...

References

External links

* {{cite web , url=https://archive.org/details/Telegram1956 , title=Telegram for America , author=Western Union Telegraph Company , year=1956 A non-technical

industrial film Sponsored film, or ephemeral film, as defined by film archivist Rick Prelinger, is a film made by a particular sponsor for a specific purpose other than as a work of art: the films were designed to serve a specific pragmatic purpose for a limited t ...

showing telegram handling in the 1950s. Plan 55-A switching centers are shown in some detail. History of telecommunications in the United States Western Union

See also

References

External links