Turn on thread page Beta

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

    • Thread Starter
    Online

    14
    ReputationRep:
    I'll post it below
    • Thread Starter
    Online

    14
    ReputationRep:
    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]

    30A (the hexadecimal number)

    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]

    (Adapted from AS 2016 paper)

    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]

    Full-adder

    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 wired, upgrade cabling
    • 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 (A+B)\cdot (A+C \cdot (D+ \overline{D}))=A+B \cdot C (I think) [4 marks]

    Using De Morgan's law to prove the identity D+\overline{D} = 1:

    Using De Morgan's law, D+\overline{D} = \overline{\overline{D} \cdot D}

    So D+\overline{D} = \overline{0} = 1

    Using this identity to simplify the expression in the question:

    (A+B)\cdot (A+C \cdot 1)

    (A+B)\cdot (A+C)

    A \cdot A + A \cdot C + B \cdot A + B \cdot C

    A \cdot A + A \cdot C + A \cdot B + B \cdot C

    A + A \cdot C + A \cdot B + B \cdot C

    A \cdot(1 + C + B) + B \cdot C

    A \cdot 1 + B \cdot C

    A + B \cdot C

    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
    Offline

    6
    ReputationRep:
    11.4 had a highest frequency of 1200
    Offline

    15
    ReputationRep:
    (Original post by Cam145264)
    11.4 had a highest frequency of 1200
    I agree it was 1200
    Posted on the TSR App. Download from Apple or Google Play
    Offline

    6
    ReputationRep:
    1.3 worked out to be 1.011010 01010
    Offline

    6
    ReputationRep:
    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
    Offline

    2
    ReputationRep:
    (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
    Offline

    13
    ReputationRep:
    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
    Offline

    1
    ReputationRep:
    My teacher sent me this to calculate what grade you got. It uses the old grade boundaries (2017).
    Attached Files
  1. File Type: xlsx AQA GradeCal OldGradeBoundaries.xlsx (11.1 KB, 6 views)
    Offline

    5
    ReputationRep:
    How did people find it? Found it so hard i cant stop crying
    Offline

    13
    ReputationRep:
    (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.
    Offline

    15
    ReputationRep:
    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?
    Offline

    13
    ReputationRep:
    i thought the minimum sampling thing was 2400
    Offline

    5
    ReputationRep:
    I wrote down that fz sales returned the total income for 1 day - is that what you mean by sales?
    Offline

    5
    ReputationRep:
    (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!
    Offline

    5
    ReputationRep:
    (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.
    Offline

    5
    ReputationRep:
    (Original post by loginrunner)
    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!
    Offline

    13
    ReputationRep:
    (Original post by loginrunner)
    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.
    Offline

    13
    ReputationRep:
    (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
    Offline

    8
    ReputationRep:
    I'm pretty sure there were only 2 higher order functions no?

    I found that paper awful.
 
 
 

University open days

  1. University of Bradford
    University-wide Postgraduate
    Wed, 25 Jul '18
  2. University of Buckingham
    Psychology Taster Tutorial Undergraduate
    Wed, 25 Jul '18
  3. Bournemouth University
    Clearing Campus Visit Undergraduate
    Wed, 1 Aug '18
Poll
How are you feeling in the run-up to Results Day 2018?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.