lesson
Agreed rules let systems cooperate
1 · Learn
A protocol specifies rules for communication, such as message structure and how participants respond. Different layers handle different responsibilities, including moving data and requesting a web resource.
A message may be delayed, duplicated or missing, so a design may need acknowledgements, identifiers and retries. Retrying an action without considering duplicates can accidentally repeat an operation.
2 · Worked example
In a model order system, a retry with the same request identifier should not create a second identical order merely because the first response was lost.
3 · Your turn
Why can 'just send it again' be unsafe for an action that creates a record?
Check your answer
The first request may already have succeeded, so the retry could create a duplicate unless the system recognises it.
4 · Apply your learning
Simulate a lost acknowledgement with paper messages and design a rule that avoids duplicate results.
Adult guidance and safety
Use an approved private interpreter, simulator or authoring tool with fictional data. Trace and predict before running. Never execute unknown downloaded code, inspect real credentials or use other people's private data. Working examples still need saved-project and assessment integration before release.
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Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
Powers, roots and trigonometry · Year 7 onward
Use only when the task allows a calculator. Choose one operation at a time and record your working. Trigonometry here always uses degrees (DEG), not radians. Inverse sine, cosine and tangent return an angle, not a reciprocal. Keep exact fractions and π in answers when requested.
Approximate numerical results use up to 12 significant digits. Inverse sine returns −90° to 90°, inverse cosine 0° to 180°, and inverse tangent −90° to 90°; these are principal values, not every possible solution of a trigonometric equation.
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