Edexcel GCSE Computer Science (1CP2) Exam Questions

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

• The computer needs to tell/distinguish the uppercase A from the lowercase a (1) because each different character has its own unique 7-bit code/pattern (1)
• A and a are different characters (1) so each needs a different 7-bit pattern to be stored and displayed correctly (1)
• If A and a had the same 7-bit pattern (1) the computer could not tell which character to display (1)

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

Do not accept 'because they are different letters/cases' without saying they need different/unique codes or that the computer must tell them apart.

Do not accept 'because A is a capital letter' on its own.

Do not accept 'because the codes are different numbers' or '65 is not 97' without linking to the need to tell the characters apart or to unique codes.

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

• Fewer samples are taken every second (1) so less detail of the sound wave is captured (1)
• Fewer samples are taken every second (1) so the file is smaller/uses less storage space (1)
• The wave is measured less often (1) so parts/changes in the wave between samples are missed (1)

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

The 'fewer samples are taken/measured less often' point can only be credited once; the second mark is for a valid expansion linked to it.

Do not accept restating the question, e.g. 'the time between measurements is longer' or 'the sample interval is longer', as either the point or the expansion.

Do not accept 'the sample rate is lower' as the expansion, as this says the same thing as a longer sample interval.

Do not accept 'the sound is lower quality/less accurate' on its own without saying what is lost, e.g. less detail of the wave captured/parts of the wave missed.

Do not accept answers about using fewer bits per sample or about compression, as these do not follow from the sample interval.

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

• Only 16 different characters can be represented (1) so there are not enough characters codes for every letter of the alphabet (1)
• 4 bits give only 16 different combinations (1) which is not enough for the uppercase letters, lowercase letters, digits and symbols on a keyboard (1)
• More than 16 characters are needed on a keyboard (1) so some letters, digits or symbols cannot be represented (1)
• The number of character codes available is limited to 16 (1) because 4 bits give 2 to the power of 4 combinations (1)

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

Calculating that 4 bits give 2 to the power of 4 = 16 combinations can earn either the point or the linked reason, provided the other half of the answer says what this means (too few characters/codes).

Allow an expansion that names only the letters of the alphabet, e.g. there are not enough character codes for the 26 letters (1).

Do not accept '4 bits can only store 4 characters' (the number of bits is not the number of characters).

Do not accept 'it can only store 16 bits' (16 is the number of character codes/combinations, not the number of bits).

Do not accept 'not enough memory/storage' (the limitation is the number of character codes, not the amount of storage).

Do not accept 'it is limited/smaller' without saying how many characters/codes are available or what cannot be represented.

Indicative content

The number of characters that 7-bit ASCII can represent

• Seven-bit ASCII gives 2 to the power of 7, which is 128 different codes, so it can represent a maximum of 128 different characters.
• This is enough for the English alphabet, digits and common punctuation, but it is a fixed and limited set.
• The website may need more than 128 characters once customer names from different countries are included.

Characters that cannot be represented

• Accented letters such as é do not have a 7-bit ASCII code, so they cannot be stored correctly.
• Characters from any alphabet other than the Latin one, such as Greek or Cyrillic letters, cannot be represented in 7-bit ASCII.
• If a customer enters a character that is not in the character set, the website may store the wrong character or show a replacement symbol.
• The company may have to misspell or simplify a customer's name because the required character cannot be stored.

The effect of using more bits for each character

• Each extra bit doubles the number of available codes, so for n bits there are 2 to the power of n codes; 8 bits give 256 codes and 16 bits give 65 536 codes.
• Using more bits allows a larger character set to represent accented letters and other alphabets.
• More bits per character means each character takes more storage, so the website's text files and pages are larger.
• Larger pages take longer to transmit over the internet to a customer.

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 the limit of 128 codes and at least one type of character that cannot be represented; a point about using more bits on its own is not sufficient for Level 3, and not every bullet needs to be covered for Level 3.

Do not accept the idea that 7-bit ASCII can represent all characters, that extended or 8-bit ASCII can represent every character in every alphabet, or that accented letters are stored as plain letters without the accent.