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# AQA A Level Computer Science Paper 2 Unofficial Markscheme, 7517/2, 15 June 2018 watch

1. I'll post it below
2. AQA A Level Computer Science Paper 2 Unofficial Markscheme, Friday 15 June 2018
I could be wrong, and there may be other possible answers - please feel free to help

01

01.1 Which of the following numbers is the largest? [1 mark]

01.2 Convert the mantissa and exponent into decimal. [2 marks]

0.0859375

01.3 Express -608 in normalised mantissa-exponent form. [3 marks]

Mantissa = 1.011010
Exponent = 01010

02

02.1 Explain how a vector graphic is represented, including reference to how the black rectangle in the picture is represented. [3 marks]

A vector graphic is represented by using a list to store the properties of objects and instructions on how to draw them

The black rectangle would be represented by a list item with a black fill and line colour, and a position and size which is suitable

02.2 Calculate the file size of the image, in bytes - excluding any metadata that may be stored. [2 marks]

Colour depth is 2 bits, so 2 * 50 * 50 = 5000 bits
5000/8 = 625 bytes

02.3 Explain how runtime length encoding is used as lossless compression. [2 marks]

Store the colour of a pixel and a count / frequency;

The count indicates the number of pixels of that colour there are before a pixel of a different colour is used in the image / the count indicates the total number of pixels of that colour there are in a run / the count indicates the number of consecutive pixels of the same colour

02.4 Explain why the given picture is compressed less than the picture at the start of the question. [2 marks]

It has fewer patterns / repeated pixels;
As it has many colours / is less 'blocky';
Therefore RLE will reduce the file size less

03

03.1 Complete the truth table. [3 marks]

03.2 Write a Boolean expression for G, in terms of A, B, and C. [3 marks]

03.3 Explain the purpose of the circuit. [1 mark]

04 Explain how the system could be improved, making reference to: the hardware of the server, the organisation of the network, and the software used on the server and the database management. [12 marks]

Some potential points:

Server hardware:
• Improving the processing ability of the server, e.g. more cores / more RAM etc.
• Use of SSD instead of hard disk, with explanations as to benefits, e.g. running at a lower temperature and so less processor overheating and so faster processing / lower latency / any other valid explanation

Network organisation:
• Use of subnetting
• Use of star topology
• Have dedicated wired network, if clients are to be used as thin-clients
• If the data is transferred over Wi-Fi, use a wired network instead
• If server off-site, upgrade to fibre-optic connection, or move server onsite to be part of wired network

Server software and database management:
• Normalise the database
• Use of WebSocket
• Use of bespoke software on database / update software / optimise
• Use database server with an optimised OS, e.g. no wasted processing on a GUI
• Use server as restful API allowing HTTP packet requests which have less overhead than other TCP packets which may be used

05

05.1 Write down the subnet mask used. [1 mark]

A sequence of twenty-seven 1s followed by five 0s.

05.2 How many computers can connect at maximum to the subnet? [1 mark]

There are 32 possible host IDs, but this means there are 30 possible computers able to connect due to address 0 being network and address 32 being broadcast (32 may be accepted for the mark)

05.3 Explain the DHCP server. [4 marks]

used to assign a dynamic IP address from a pool of available addresses;
to the computer attempting to operate on subnet Z;
used as IP addresses are in short supply / to allow computer to communicate on subnet;
communication is discover-offer-request-acknowledge;
IP address released back to pool of available addresses when device disconnects

05.4 Explain how the external router operates. [2 marks]

NAT is used to modifying network / IP addresses (and port numbers) as they pass through a router / convert IP addresses passing between a public address space and a LAN with a private address space;
port forwarding is used to forward all incoming requests for a particular server to a particular IP address and port;

06

06.1 Explain how an optical disk reader reads the data from an optical disk. [6 marks]

lower wattage (power) laser than that used to create the disk used to read the disk;
laser shone onto the surface of the disk;
sensor / photodiode is used to measure the amount of light reflected
photodiode induces a voltage when a high intensity light is reflected onto it, representing a 1 in binary;
this voltage is induced when the laser shines on a land, which is more reflective;
photodiode does not induce (as big of) a voltage when low intensity light is reflected onto it, representing a 0 in binary;
this (smaller) voltage is (not) induced when the laser shines on a pit, which scatters the light / is less reflective;
constant scanning produces a binary string of data from the disc;
binary string stream sent to a DAC to allow the data to be output to the user

06.2 Suggest one reason why USBs are used over optical disks. [1 mark]

more portable as physically smaller in size / less susceptible to damage from scratches and sunlight

07

07.1 Draw three relationships on the entity-relation diagram. [2 marks]

07.2 Explain why the SQL code is wrong. [2 marks]

there is no data type for the primary key / there is no semi-colon at the end / the data types and column names are the opposite way round

Note: a NOT NULL specifier being omitted from the primary key does not result in wrong code

07.3 State two reasons why normalisation is used. [2 marks]

to improve access / query speeds;
easier to maintain and change the database;
data integrity is maintained;
smaller tables with fewer fields;
records in a table on the 'one' side of a one-to-many relationship cannot be deleted

07.4 Write an SQL query to find the Surname, Forename and Date of Birth for athletes participating at the event on date X, with alphabetical surname order. [5 marks]

Example SQL query:
Code:
SELECT Surname, Forename, AthleteID

FROM Athlete, EventEntry, Fixture

WHERE Athlete.AthleteID = EventEntry.AthleteID AND EventEntry.FixtureID = Fixture.FixtureID AND Fixture.FixtureDate = X

ORDER BY Surname
08 Discuss the issues legislators may face in the digital age. [3 marks]

There are probably lots of different answers here, but I'm guessing it may be 1 mark for each point discussed

09

09.1 Complete the table to categorise the numbers. [2 marks]

The first number was natural, integer, rational and real;
The second number was rational and real

09.2 Explain what an ordinal number is, and in context of the array shown. [2 marks]

shows order / position / rank;
in the array, act as pointers

10 Use the rules of Boolean algebra to show that (I think) [4 marks]

Using De Morgan's law to prove the identity :

Using De Morgan's law,

So

Using this identity to simplify the expression in the question:

11

11.1 What is the sampling resolution used? [1 mark]

4 bits

11.2 What is the sampling frequency? [1 mark]

1000 Hz

11.3 Explain the impact of point A and B's difference, and explain how the impact may be reduced with reference to the sampling method. [2 marks]

the digital signal will be quieter at that point / not represent the analogue waveform as accurately;
the sampling resolution can be increased to allow more possible amplitude values to be measured

11.4 The highest frequency in the sound is 1200 Hz. State the minimum sampling frequency to be used to accurately represent the waveform. [1 mark]

By the Nyquist-Shannon theorem, the frequency is 2400 Hz.

12

12.1 For the Caesar cipher program, write down the purpose of each register used. [2 marks]

R1: storing the character to be encrypted using the Caesar cipher

R2: storing the Caesar cipher's encryption key (from the Unicode table)

R3: storing the character which was encrypted using the Caesar cipher

12.2 For the Caesar cipher program, complete the section 1 of assembly code. [1 mark]
Code:
MOV R3, R1
12.3 For the Caesar cipher program, complete the section 2 of assembly code. [3 marks]
Code:
CMP R3, #91
BLT finished
SUB R3, R3, #26
B finished
13

13.1 State and explain how two measured will increase the security of a Wireless access point. [2 marks]

Stop SSID broadcasting so the WAP is harder to find / add MAC address whitelisting so only allowed devices connect to the network via the WAP / add a security key so that users would need to know this before connecting to the network

13.2 Explain how CSMA/CA with RTS/CTS can be used when one device on a network is sending another device on the network data. [6 marks]

The sending device listens for signals to determine whether another device is transmitting;
If a signal is detected, the sending device waits;
A random period of time before listening again;
When a signal is not detected, the sending device sends a RTS signal;
The WAP sends a CTS signal back when the channel is idle;
When the sending device gets a CTS signal, it sends the data

14

14.1 One property of big data is its variety. Describe two other properties of big data. [2 marks]

its volume: it is too big to fit on a single server, and so must be stored on multiple servers;
its velocity: it is generated / received very quickly

14.2 Complete the graph schema. [3 marks]

directed edge between the trains and the cities they deliver to, with label;
a new circle (object) representing the company;
a new rectangle (property) of the company, representing its offices, connected by a labelled undirected edge

15

15.1 How many of the functions use a high-order function? [1 mark]

3 (the first function was not high-order, the second and third were, and the fourth made use of the third)

15.2 Complete the table to show the application of the functions. [3 marks]

first row: value of the two items in the list being multiplied together;
second row: value of two items in each sub-list being multiplied together;
third row: sum of values in the answer to the second part

15.3 Explain what the f z sales returns. [1 mark]

The total number of sales in 1 day
3. 11.4 had a highest frequency of 1200
4. (Original post by Cam145264)
11.4 had a highest frequency of 1200
I agree it was 1200
5. 1.3 worked out to be 1.011010 01010
6. 7.2. The things that were incorrect was that there was no semi-colon at the end of the statement and the datatype and the column names were the opposite way around
7. (Original post by Cam145264)
7.2. The things that were incorrect was that there was no semi-colon at the end of the statement and the datatype and the column names were the opposite way around
I think another one may've been that the primary key didn't have a type, just the constraint PRIMARY KEY
8. The part about assembly language:

What the registers do:
• R1 is what stores the character to be encrypted,
• R2 is what acts as a key value from the Unicode table,
• R3 is what stores the final encrypted character.

Assembly code part 1:
Code:
CMP R3, #90 - compares R3 with number 90
BGT subtract - acts as the if statement in the pseudocode; branches to new routine if R3 is greater than 90
subtract:
SUB R3, R3, #20 - subtracts 20 from R3 and stores in R3
B finished - branches to finish of code and halts (this line might not be needed)
Assembly code part 2:
Code:
DoNotEncrypt:
MOV R3, R1 - the pseudocode said that the encrypted character is just the original character for this part
9. My teacher sent me this to calculate what grade you got. It uses the old grade boundaries (2017).
Attached Files
11. How did people find it? Found it so hard i cant stop crying
12. (Original post by Strawbecky)
How did people find it? Found it so hard i cant stop crying
It was decent in my opinion - it could've been much worse. I don't think it was 'hard' per se, since it's mostly remembering stuff.

I'm so glad not much networking stuff came up. There was a nice balance between AS and A2 content.
13. I'm grateful operating systems didn't come up (unless i missed it). I have a couple of questions about the paper still:

What does the fold function do in the functional programming question?
Was one of the errors for the SQL, that it didn't have a semi-colon at the end?
14. i thought the minimum sampling thing was 2400
15. I wrote down that fz sales returned the total income for 1 day - is that what you mean by sales?
16. (Original post by Cam145264)
7.2. The things that were incorrect was that there was no semi-colon at the end of the statement and the datatype and the column names were the opposite way around
No semi colon at the end, thank-you so much for saying that. I spent quite a while looking for the second error and just stuck that down in desperation. Hopefully that's it!
17. (Original post by jarjarbinkss)
i thought the minimum sampling thing was 2400
Yes it was, question said 1200 and - using Nyquist's theorem you get 2400.
I'm grateful operating systems didn't come up (unless i missed it). I have a couple of questions about the paper still:

What does the fold function do in the functional programming question?
Was one of the errors for the SQL, that it didn't have a semi-colon at the end?
1. For this particular case, it calculated the total of all the values in the list.

2. I hope so. I added this quickly at the end because I couldn't find anything else!
I'm grateful operating systems didn't come up (unless i missed it). I have a couple of questions about the paper still:

What does the fold function do in the functional programming question?
Was one of the errors for the SQL, that it didn't have a semi-colon at the end?
The fold function adds up every single element in a set. Say for instance I have a set of [1,2,3,4,5], the fold function would return 15, as each element in the set is added together - the set effectively 'folds' into itself.
20. (Original post by lbowes)
I wrote down that fz sales returned the total income for 1 day - is that what you mean by sales?
I would imagine that saying something about the total amount of money the shop earns in a day is worthy of credit.

It was probably the easiest question on the paper
21. I'm pretty sure there were only 2 higher order functions no?

I found that paper awful.

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