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Edexcel GCSE CS · 2.3

Data Storage and Compression

Section A: 10 multiple choice questions, 10 marks. Section B: 4 written questions, 12 marks. Total 22 marks.

Name

Class

Date

Mark  / 22

Section A

Circle the letter of the correct answer. One mark each.

  1. 1How many bytes are in one mebibyte?1
    1. A1024
    2. B8192
    3. C1000000
    4. D1048576
  2. 2Which storage type has no moving parts?1
    1. AOptical
    2. BSolid state
    3. CMagnetic
    4. DAll three have moving parts
  3. 3Why is magnetic storage often chosen for large backups?1
    1. AIt offers the lowest cost per gigabyte at large capacities
    2. BIt has no moving parts
    3. CIt is the fastest available
    4. DIt cannot be overwritten
  4. 4What is the defining feature of lossless compression?1
    1. AIt removes unnecessary colours
    2. BThe original file can be reconstructed exactly
    3. CIt only works on images
    4. DThe file always becomes half the size
  5. 5How would B B B W W B be written using run length encoding?1
    1. A6 values unchanged
    2. B3B 2W 1B
    3. C3B 2W
    4. DB3 W2 B
  6. 6Why would run length encoding be a poor choice for a photograph?1
    1. ARLE only works on text
    2. BPhotographs have no metadata
    3. CPhotographs are already lossless
    4. DPhotographs contain very few runs of identical pixels, so the file could grow
  7. 7Which is the correct term for storage whose contents are lost when power is removed?1
    1. ANon volatile
    2. BOptical
    3. CVolatile
    4. DSecondary
  8. 8Which compression method would be appropriate for a spreadsheet of financial figures?1
    1. ALossless, because losing any value would make the figures wrong
    2. BEither, since the effect is the same
    3. CNeither, spreadsheets cannot be compressed
    4. DLossy, because spreadsheets contain redundant cells
  9. 9A file is 4 GiB. How many mebibytes is that?1
    1. A1024
    2. B4000
    3. C4096
    4. D4194304
  10. 10Why does a solid state drive have a limited lifetime?1
    1. AThe magnetic coating degrades
    2. BThe platters wear out from spinning
    3. CThe laser dims with use
    4. DEach memory cell can only be rewritten a finite number of times

Section B

Answer in the space provided. The number of marks tells you how many separate points to make.

  1. 1StateState one advantage of solid state storage over magnetic storage.1
  2. 2RecommendA film company needs to store 40 TiB of raw footage that will be archived and rarely accessed. Recommend a type of secondary storage and justify your choice.4
  3. 3ExplainExplain the difference between lossy and lossless compression.4
  4. 4ShowA row of pixels contains B B B B B W W B B B. Show how it would be stored using run length encoding, and state how many values are saved.3

Mark scheme

Section A

1D2B3A4B5B6D7C8A9C10D

Section B

1 State one advantage of solid state storage over magnetic storage. 1

One mark for each of the following, to a maximum of 1.

  • Faster access, or no moving parts so more durable, or lower power use

Model answer. Solid state storage has no moving parts, so it is far more durable when the device is carried around, and data is accessed considerably faster because there is no read head that must physically move to the right place.

2 A film company needs to store 40 TiB of raw footage that will be archived and rarely accessed. Recommend a type of secondary storage and justify your choice. 4

One mark for each of the following, to a maximum of 4.

  • Recommends magnetic storage
  • Justifies it on the very large capacity required
  • Justifies it on cost per gigabyte or terabyte
  • Notes that the slower access speed does not matter for an archive

Model answer. The company should use magnetic hard disk storage. The volume involved is very large, and magnetic drives offer by far the lowest cost per terabyte at that scale, so the whole archive can be held for a fraction of what solid state would cost. The main drawback of magnetic storage is that access is slower, because the read head has to move physically to the data, but that is close to irrelevant here since the footage is being archived and will rarely be read. Optical storage is unsuitable because a single disc holds only a few gigabytes, so 40 tebibytes would need thousands of them.

3 Explain the difference between lossy and lossless compression. 4

One mark for each of the following, to a maximum of 4.

  • Lossy compression permanently removes data
  • So the original file cannot be recovered exactly
  • Lossless compression stores the same data more efficiently
  • So the original file can be reconstructed exactly

Model answer. Lossy compression achieves a large reduction in size by permanently discarding data that is judged to be less important, such as colour detail in an image that the eye barely registers or frequencies in audio that most people cannot hear. Because that data is gone, the original file can never be reconstructed exactly. Lossless compression instead finds a more efficient way of storing exactly the same information, for example by replacing runs of repeated values with a value and a count. Nothing is discarded, so decompressing gives back a file identical to the original, but the reduction achieved is more modest than lossy compression can manage.

4 A row of pixels contains B B B B B W W B B B. Show how it would be stored using run length encoding, and state how many values are saved. 3

One mark for each of the following, to a maximum of 3.

  • Identifies the runs as five B, two W, three B
  • Writes them as value and count pairs
  • States that six values replace ten, saving four

Model answer. The row contains five B, then two W, then three B, so under run length encoding it is stored as the pairs 5B, 2W, 3B. That is three pairs, which is six values in total, in place of the ten values in the original row, so four values are saved. Nothing has been lost, since the original row can be written out again exactly from the pairs.