science

Link Access Procedure for Modems

Link Access Procedure for Modems 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 Link Access Procedure for Modems rather than just read about it. In short: Link Access Procedure for Modems (LAPM) is part of the V.42 error correction protocol for modems. LAPM is an error control protocol defined in ITU-T recommendations V.42.

Key takeaways

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

Reference excerpt

Link Access Procedure for Modems (LAPM) is part of the V.42 error correction protocol for modems. LAPM is an error control protocol defined in ITU-T recommendations V.42. Like many data link layer protocols, it is a variant of HDLC. Like the Microcom Networking Protocols that preceded it, LAPM uses cyclic redundancy checking (CRC) and retransmission of corrupted data (ARQ) to ensure data reliability. Error control protocols such as LAPM use frames of variable lengths, each beginning with a header and ending with a frame check sequence trailer (a cyclic redundancy check). Correctly received packets are acknowledged, and unacknowledged packets are automatically retransmitted (ARQ). The larger the payload included in each frame, the smaller the relative cost of transmitting the header and trailer bits. Usually LAPM adds only about 5 percent framing overhead. This is a noticeable improvement over the fixed 25% overhead of start and stop bits required by direct asynchronous serial communication, but obviously such large frames impose a cost in the form of communication latency. LAPM has an optional selective reject (SREJ) functionality which allows it to resend only the corrupted frames, providing faster recovery from an error. The ITU-T V.42 LAPM procedure is considered more robust than the Microcom Networking Protocols (MNP) that preceded it, and has a more sophisticated and powerful data compression option in V.42bis, allowing much greater data throughput. ITU-T V.42bis added a string compression algorithm called BTLZ (British Telecom Lempel Ziv), in which frequently occurring strings of characters are replaced by code words. The "dictionary" of strings is dynamically maintained during transmission and keeps track of changes in the data; new strings are added and old strings are deleted. As the data may not always be compressible (for example encrypted or pre-compressed data) the modem has the ability to switch between compressed and uncompressed modes of operation. Compression performance is continually monitored and, if no compression is obtained, the modem transmits the data in uncompressed form. Even while in uncompressed mode, the modem monitors the characteristics of the data and switches compression back on as soon as some benefit can be achieved.

See also Link access procedure Point-to-Point Protocol (PPP)

References

Worked examples

Example 1 — a first encounter with Link Access Procedure for Modems

Start with the simplest possible case. Write down what Link Access Procedure for Modems 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 Link Access Procedure for Modems 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 Link Access Procedure for Modems 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 Link Access Procedure for Modems

In research: Link Access Procedure for Modems 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 Link Access Procedure for Modems 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: Link Access Procedure for Modems is common in secondary-school and first-year university syllabi. It links to neighbouring topics Link access protocols, Modems, so understanding it makes those chapters shorter.
In everyday life: Look for Link Access Procedure for Modems 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 Link Access Procedure for Modems in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Link Access Procedure for Modems 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 Link Access Procedure for Modems out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Link Access Procedure for Modems in simple terms?

Link Access Procedure for Modems (LAPM) is part of the V.42 error correction protocol for modems. LAPM is an error control protocol defined in ITU-T recommendations V.42.

Why does Link Access Procedure for Modems 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 Link Access Procedure for Modems?

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 Link Access Procedure for Modems.