science

Longitudinal redundancy check

Longitudinal redundancy check 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 Longitudinal redundancy check rather than just read about it. In short: In telecommunication, a longitudinal redundancy check (LRC), or horizontal redundancy check, is a form of redundancy check that is applied independently to each of a parallel group of bit streams. The data must be divided into transmission blocks, to which the additional check data is added.

Key takeaways

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

Reference excerpt

In telecommunication, a longitudinal redundancy check (LRC), or horizontal redundancy check, is a form of redundancy check that is applied independently to each of a parallel group of bit streams. The data must be divided into transmission blocks, to which the additional check data is added. The term usually applies to a single parity bit per bit stream, calculated independently of all the other bit streams (BIP-8). This "extra" LRC word at the end of a block of data is very similar to checksum and cyclic redundancy check (CRC).

Optimal rectangular code While simple longitudinal parity can only detect errors, it can be combined with additional error-control coding, such as a transverse redundancy check (TRC), to correct errors. The transverse redundancy check is stored on a dedicated "parity track". Whenever any single-bit error occurs in a transmission block of data, such two-dimensional parity checking, or "two-coordinate parity checking", enables the receiver to use the TRC to detect which byte the error occurred in, and the LRC to detect exactly which track the error occurred in, to discover exactly which bit is in error, and then correct that bit by flipping it.

Pseudocode International standard ISO 1155 states that a longitudinal redundancy check for a sequence of bytes may be computed in software by the following algorithm:

lrc := 0 for each byte b in the buffer do lrc := (lrc + b) and 0xFF lrc := (((lrc XOR 0xFF) + 1) and 0xFF)

which can be expressed as "the 8-bit two's-complement value of the sum of all bytes modulo 28" (x AND 0xFF is equivalent to x MOD 28).

Other forms Many protocols use an XOR-based longitudinal redundancy check byte (often called block check character or BCC), including the serial line interface protocol (SLIP, not to be confused with the later and well-known Serial Line Internet Protocol), the IEC 62056-21 standard for electrical-meter reading, smart cards as defined in ISO/IEC 7816, and the ACCESS.bus protocol. An 8-bit LRC such as this is equivalent to a cyclic redundancy check using the polynomial x8 + 1, but the independence of the bit streams is less clear when looked at in that way.

References

This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).

Worked examples

Example 1 — a first encounter with Longitudinal redundancy check

Start with the simplest possible case. Write down what Longitudinal redundancy check 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 Longitudinal redundancy check 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 Longitudinal redundancy check 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 Longitudinal redundancy check

In research: Longitudinal redundancy check 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 Longitudinal redundancy check 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: Longitudinal redundancy check is common in secondary-school and first-year university syllabi. It links to neighbouring topics Error detection and correction, ISO standards, so understanding it makes those chapters shorter.
In everyday life: Look for Longitudinal redundancy check 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 Longitudinal redundancy check in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Longitudinal redundancy check 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 Longitudinal redundancy check out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Longitudinal redundancy check in simple terms?

Why does Longitudinal redundancy check 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 Longitudinal redundancy check?

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 Longitudinal redundancy check.