Exchanging Data

What Exchanging Data covers

Exchanging Data is examined on component 01 of OCR Computer Science, Computer systems. 122 recall questions across 9 subtopics.

The 9 subtopics

Each one has its own page.

• CompressionRecall questions on lossy and lossless compression and when each is used, run length encoding and dictionary coding with worked examples, the data each suits, and comparing the two methods.Open →
• EncryptionRecall questions on symmetric and asymmetric encryption, the key distribution problem, which key encrypts, signs and verifies, combining both in secure connections, digital signatures and certificates, and why encryption alone does not guarantee integrity.Open →
• HashingRecall questions on hash functions and why hashing is not encryption, collisions, hash tables and collision handling, storing passwords as salted hashes, checksums for downloads, and hashing a message before signing it.Open →
• Flat file and relational databases, keys and indexingRecall questions on flat file and relational databases, entities and attributes, primary, composite, foreign and secondary keys, choosing a primary key, and indexing and why not every field is indexed.Open →
• Referential integrity, entity relationship diagrams and database management systemsRecall questions on referential integrity and what it prevents, one-to-one, one-to-many and many-to-many relationships in entity relationship diagrams, resolving many-to-many relationships with a linking table, and what a database management system does.Open →
• Normalisation to 3NFRecall questions on normalisation and its three normal forms, repeating groups and atomic values, partial and transitive dependencies, converting tables to 2NF and 3NF, derived fields, and the benefits and costs of normalising.Open →
• SQL: Interpreting and Modifying QueriesRecall questions on SQL SELECT, FROM and WHERE, the wildcard and LIKE, AND and OR, JOIN, nested SELECT, INSERT INTO, DELETE and DROP, and interpreting, writing and correcting queries.Open →
• Capturing, Selecting, Managing and Exchanging DataRecall questions on methods of capturing data and automatic data capture, validation at the point of capture, selecting data, managing and archiving data, exchanging data in CSV and XML, and protecting personal data in transfer.Open →
• Transaction Processing, ACID, Record Locking and RedundancyRecall questions on transactions, the ACID properties of atomicity, consistency, isolation and durability, record locking, lost updates and deadlock, redundancy for availability, and how these keep a high-volume system correct.Open →

What you need to know

Answer each one before you reveal it.

Take Exchanging Data further

The same content, four other ways.

Commonly confused

Pairs that lose marks when they are blurred together.

• A primary key vs A foreign key

  A primary key: the attribute that uniquely identifies each record in its own table. A foreign key: an attribute that refers to the identifying attribute of another table.

• Lossy compression vs Lossless compression

  Lossy compression: reducing file size by discarding detail that cannot then be recovered. Lossless compression: reducing file size in a way that allows the original to be restored exactly.

• Atomicity vs Durability

  Atomicity: the requirement that a transaction either completes in full or has no effect at all. Durability: the requirement that a committed transaction survives a subsequent failure.

Questions from this topic

Each one with the model answer underneath.

1. 1

   What is compression, and what is it actually for?

   Model answer

   Compression is the process of reducing the number of bits needed to store or transmit a file. It is used to save storage space, to reduce the bandwidth a transfer consumes, and so to shorten download, upload and streaming times. It can also reduce cost where storage or data transfer is charged by volume.

2. 2

   Explain the difference between lossy and lossless compression.

   Model answer

   Lossy compression permanently discards some of the original data, usually detail a human is unlikely to notice, so the decompressed file is only an approximation of the original and the discarded data can never be recovered. Lossless compression removes only redundancy by recording the data more efficiently, so the original file can be reconstructed bit for bit exactly when it is decompressed.

3. 3

   Lossy compression generally produces much smaller files than lossless. Why, then, is lossless ever used?

   Model answer

   Because for many kinds of data any loss at all destroys the file's meaning. Lossless is used whenever the exact original bits are needed, and the price of that guarantee is a smaller reduction in size. Lossy is only acceptable where an approximation is good enough for the human viewer or listener.

4. 4

   Why would you never apply lossy compression to an executable program or a spreadsheet?

   Model answer

   Because every bit in them is significant. In an executable, changing or discarding bits alters machine code instructions or addresses, so the program will crash, misbehave or fail to run at all. In a spreadsheet, altered bits change the actual figures and formulae, producing silently wrong numbers. Neither file has any perceptually redundant detail that could be thrown away safely, so only lossless compression is appropriate.

5. 5

   Give a typical use for lossy compression and explain why the loss is acceptable there.

   Model answer

   Photographs on a website, streamed music and streamed video. In a photograph, fine differences between adjacent shades are discarded; in audio, frequencies outside normal hearing or masked by louder sounds are removed. Human perception cannot detect most of what is thrown away, so the file looks or sounds close enough to the original while being far smaller, which matters greatly for fast page loading and smooth streaming.

6. 6

   Name three kinds of file for which lossless compression is essential.

   Model answer

   Any three of: executable programs and other machine code, text documents and source code, spreadsheets and databases, archived files such as ZIP archives, medical or scientific images used for diagnosis or analysis, and any file whose hash or digital signature must still match after decompression. In each case an exact reconstruction of the original is required.

What is inside Levo

Six ways to work the same material.

Exchanging Data revision questions

What does Exchanging Data cover in OCR A-Level Computer Science?

9 subtopics: compression, encryption, hashing, flat file and relational databases, keys and indexing, referential integrity, entity relationship diagrams and database management systems, normalisation to 3nf, sql: interpreting and modifying queries, capturing, selecting, managing and exchanging data and transaction processing, acid, record locking and redundancy. Each one has its own page with questions and model answers.

How is Exchanging Data organised?

By the OCR H446 specification, subtopic by subtopic, so it matches the order you are taught it in.