Edexcel GCSE Computer Science (1CP2) Exam Questions

• C Tebibyte (1)
• The only correct answer is C. The correct answer is tebibyte.
• A is not correct because a gibibyte is 1024 mebibytes
• B is not correct because a mebibyte is 1024 kibibytes
• D is not correct because a terabyte is 1000 gigabytes

• C 8192 (1)
• The only correct answer is C. The correct answer is 8192.
• A is not correct because 1024 is the number of bytes in a kibibyte
• B is not correct because 8000 is the number of bits in 1000 bytes
• D is not correct because 8 388 608 is the number of bits in a mebibyte

Award up to two marks for a linked description such as:

• Makes each file smaller (1) so less secondary storage is needed for the videos (1)
• Uses less bandwidth (1) so the video streams without buffering (1)
• Reduces the amount of data sent (1) so users use less of their data allowance (1)
• Can be streamed (1) because the data can be sent as fast as the user watches it (1)

Award one mark for a valid point on its own; the second mark is only awarded when the development is linked to that point.

Do not award a second mark for a development that repeats the point made.

Allow "takes less time to transmit/upload/download" as the development for a smaller file.

Do not accept responses that say compression saves memory/RAM (the videos are stored in secondary storage).

Do not accept compression makes the video more secure or improves its quality.

Award up to two marks for a linked explanation such as:

• The repeated phrase is stored once (1) along with a count of the number of times it occurs (1)
• The repeated phrase is replaced by a shorter code/reference (1) and the original phrase is held in a dictionary/lookup table (1)
• A shorter code is used in place of each repeated phrase (1) so fewer characters/bits are needed to store the file (1)
• Groups of repeated characters are replaced by a count and a single value (1) so the file stores less data (1)

For both marks, the expansion must follow/associate with the statement.

Allow naming of a suitable method, e.g. run-length encoding or dictionary coding, in place of a description of that method.

Award the marks once only where the same method (run-length encoding) is described more than once.

Do not accept 'it makes the file smaller' on its own without a method of compression.

Do not accept answers that describe lossy compression, e.g. that data/detail is removed or lost permanently, because lossless compression keeps all of the original data.

Do not accept 'it removes the repeated words/duplicates' on its own, as this suggests data is lost; credit only where the answer says the phrase is stored once with a count or code.

Indicative content

Why lossless compression is suitable

• No data is removed, so the original file can be restored exactly when it is decompressed.
• Source code must be reproduced exactly, because a single missing or changed character would stop the program working when it is run.
• A text document must keep every character, so that the meaning of the document is not changed.

How lossless compression makes a file smaller

• Run length encoding stores a repeated value once, together with a count of how many times it appears, so long runs of the same character are held in less space.
• Dictionary encoding finds patterns or words that are repeated in the text, stores each one once in a dictionary and replaces every occurrence with a short code.
• The more repetition there is in a file, the greater the reduction in size that can be achieved.

Benefits to the company

• The compressed files take up less space on the file server, so more programs and documents can be stored.
• There is less data to transfer, so backing the files up to another location finishes sooner.
• The company does not need to keep a separate uncompressed copy as well, because the compressed file can be restored exactly, which saves more storage space.
• The files must be decompressed before they are used, which takes time each time a file is opened.

Levels

• Level 0 (0 marks): No rewardable content.
• Level 1 (1–2 marks): Basic, independent points are made, showing elements of understanding of key concepts/principles of computer science. (AO1) The discussion will contain basic information with little linkage between points made or application to the context. (AO2)
• Level 2 (3–4 marks): Demonstrates adequate understanding of key concepts/principles of computer science. (AO1) The discussion shows some linkages and lines of reasoning with some structure and application to the context. (AO2)
• Level 3 (5–6 marks): Demonstrates comprehensive understanding of key concepts/principles of computer science to support the discussion being presented. (AO1) The discussion is well developed, with sustained lines of reasoning that are coherent and logically structured, and which clearly apply to the context. (AO2)

Level 3 responses must cover both why lossless compression is suitable and how it makes a file smaller.

Do not accept the idea that lossless compression removes data that is not needed.