science

Plan 55-A

Plan 55-A is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Plan 55-A rather than just read about it. In short: Automatic Telegraph Switching System 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. It is an automated successor to Plan 51, which commenced service in 1951 in a nationwide network of the U.S.

Key takeaways

Plan 55-A belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Plan 55-A to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Plan 55-A from memory before moving on to harder problems.

Reference excerpt

Automatic Telegraph Switching System 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. 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. 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.

Operation 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. 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.

Queuing theory An analysis of the queueing delays in Plan 55-A by Leonard Kleinrock formed part of his doctoral research in the early 1960s.

Illustrations

Worked examples

Example 1 — a first encounter with Plan 55-A

Start with the simplest possible case. Write down what Plan 55-A claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Plan 55-A before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Plan 55-A ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Plan 55-A

In research: Plan 55-A appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Plan 55-A in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Plan 55-A is common in secondary-school and first-year university syllabi. It links to neighbouring topics History of telecommunications in the United States, Western Union, so understanding it makes those chapters shorter.
In everyday life: Look for Plan 55-A outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Plan 55-A in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Plan 55-A means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Plan 55-A out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Plan 55-A in simple terms?

Why does Plan 55-A matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Plan 55-A?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Plan 55-A.