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X.25

X.25 is a computer 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 X.25 rather than just read about it. In short: X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book in 1976.

X.25 — main illustration
X.25 — illustration

Key takeaways

  • X.25 belongs to computer science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect X.25 to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of X.25 from memory before moving on to harder problems.

Reference excerpt

X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book in 1976. The protocol suite is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer OSI Reference Model, although it was developed several years before the OSI model (1984). It also supports functionality not found in the OSI network layer. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, plain old telephone service connections, or ISDN connections as physical links. X.25 was popular with telecommunications companies for their public data networks from the late 1970s to 1990s, which provided worldwide coverage. It was also used in financial transaction systems, such as automated teller machines, and by the credit card payment industry. However, most users have since moved to the Internet Protocol Suite (TCP/IP). X.25 is still used, for example by the aviation industry.

History

The CCITT (later ITU-T), the organization responsible for international standardization of telecom services, began developing a standard for packet-switched data communication in the mid-1970s based upon a number of emerging data network projects. Participants in the design of X.25 included engineers from Canada, France, Japan, the UK, and the USA representing a mix of national PTTs (France, Japan, UK) and private operators (Canada, USA). In particular, the work of Rémi Després, contributed significantly to the standard, which was based on a virtual circuit service. A few minor changes, which complemented the proposed specification, were accommodated to enable Larry Roberts to join the agreement. Various updates and additions were worked into the standard, eventually recorded in the ITU series of technical books describing the telecommunication systems. These books were published every fourth year with different-colored covers. The X.25 specification is part of the larger set of X-Series.

Standardization of virtual circuits by the CCITT

… excerpt ends here. Continue reading the full article.

Illustrations

X.25 illustration
X.25: Representatives of PTTs and private companies who championed the development of X.25-based networks and services in Europe, North America and Japan. Photographed at the CCITT Rapporteur-group meeting of March 1975 in Ottawa, where they drafted the first X.25 proposal.
Representatives of PTTs and private companies who championed the development of X.25-based networks and services in Europe, North America and Japan. Photographed at the CCITT Rapporteur-group meeting of March 1975 in Ottawa, where they drafted the first X.25 proposal.
X.25: Major contributors to CCITT X.25, photographed just after its approval in March 1976.
Major contributors to CCITT X.25, photographed just after its approval in March 1976.
X.25: A Televideo terminal model 925 made around 1982
A Televideo terminal model 925 made around 1982
X.25: An X.25 modem once used to connect to the German Datex-P network
An X.25 modem once used to connect to the German Datex-P network

Worked examples

Example 1 — a first encounter with X.25

Start with the simplest possible case. Write down what X.25 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer 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 X.25 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 X.25 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 X.25

In research
X.25 appears in computer 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 X.25 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
X.25 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer-related introductions in 1976, History of computer networks, ITU-T X Series Recommendations, so understanding it makes those chapters shorter.
In everyday life
Look for X.25 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 X.25 in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what X.25 means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain X.25 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is X.25 in simple terms?

X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book…

Why does X.25 matter?

Because it connects several computer 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 X.25?

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 X.25.

Tags

  • Computer-related introductions in 1976
  • History of computer networks
  • ITU-T X Series Recommendations
  • ITU-T recommendations
  • Network layer protocols
  • OSI protocols
  • Telecommunication protocols
  • Wide area networks
  • X.25

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