Cambridge A Level Computer Science 9608 — 2021 Oct/Nov Paper 3 · Variant 3

1 Data types can be defined using pseudocode. The data type, ComputerRecord, is defined by the following pseudocode: TYPE ComputerRecord DECLARE ComputerID : INTEGER DECLARE ComputerType : (Laptop, Desktop, Tablet) DECLARE ComputerLocation : (Lab1, Lab2, Lab3, Mobile) DECLARE DateTested : DATE ENDTYPE A variable, SchoolComputer, is declared in pseudocode as: DECLARE SchoolComputer : ComputerRecord (a) Write pseudocode statements to assign 1234 to the ComputerID of SchoolComputer and Lab2 to the ComputerLocation of SchoolComputer. ................................................................................................................................................... ............................................................................................................................................. [2] (b) The type definition for ComputerRecord is changed. (i) The definition has been extended to include the student identification numbers, StudentID, for up to 20 students who can use that computer. Each student identification number is an integer. Write the extra line of pseudocode needed in the type definition for ComputerRecord. ........................................................................................................................................... ..................................................................................................................................... [1] (ii) The values for the field ComputerID must be between 1000 and 1999 inclusive. Rewrite one pseudocode line from the type definition of ComputerRecord to implement the change. ........................................................................................................................................... ..................................................................................................................................... [1] (c) Data about all the computers are stored in a file that uses random file organisation. ComputerID is used as the key field. Explain how a program could search for a record stored in this file. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: Question Answer Marks 1(a) SchoolComputer.ComputerID ← 1234 2 SchoolComputer.ComputerLocation ← Lab2 1(b)(i) DECLARE StudentID : ARRAY[1:20] OF INTEGER 1 1(b)(ii) DECLARE ComputerID : 1000 .. 1999 // 1 DECLARE ComputerID : INTEGER 1000 .. 1999 1(c) Any three from 3 • Computer ID hashed to give address / home location • Compared to ID stored at address / home location • Nothing stored, output message ‘record not found’ • Record IDs equal, record is found • Record IDs not equal, search overflow area / next record • Until record found or whole area searched • If no record found error message

2 The TCP/IP protocol suite can be viewed as a stack with four layers. (a) Complete the diagram by writing the names of the three missing layers. Layer Transport [3] (b) State the purpose of each of the following protocols. HTTP ........................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... FTP ........................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... POP3 ........................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... SMTP ........................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... [4]

Mark scheme: 2(a) One mark for correct layer in correct position 3 Application Transport Internet / Network Network Access // Network Interface // (Data) Link // Physical 2(b) HTTP – used by browsers to fetch web pages 4 // transmission of HTML files FTP – used to transfer files (between computers) over a network(s) POP3 – used to receive / download emails SMTP – used to send emails

3 Hamish is constructing a Local Area Network (LAN) using Ethernet with CSMA/CD. (a) Identify and draw a diagram of the most appropriate topology for this LAN. Topology ................................................................................................................................... Diagram: [3] (b) Explain how devices on the LAN use CSMA/CD. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4]

Mark scheme: 3(a) Bus … 3 … diagram showing bus topology … diagram with correct labels (terminators, terminals etc) 3(b) Any four from 4 • carrier sense multiple access with collision detection • before transmission • … devices listen to see if line is idle • If line idle begins transmission • If a collision is detected • ... device halts transmission/ send a jam signal • … device waits a random wait time • … before attempting retransmission

4 (a) The truth table for a logic circuit with four inputs is shown. INPUT OUTPUT P Q R S X 0 0 0 0 1 0 0 0 1 1 0 0 1 0 0 0 0 1 1 0 0 1 0 0 0 0 1 0 1 0 0 1 1 0 0 0 1 1 1 0 1 0 0 0 0 1 0 0 1 0 1 0 1 0 0 1 0 1 1 0 1 1 0 0 0 1 1 0 1 0 1 1 1 0 1 1 1 1 1 1 (i) Write the Boolean algebraic expression for the truth table as a sum-of-products. X = ............................................................................................................................... [2] (ii) Complete the following Karnaugh Map (K-map) for the truth table. PQ 00 01 11 10 00 01 RS 11 10 [2] (iii) The K-map can be used to simplify the expression in part (a)(i). Draw loop(s) around appropriate groups in the K-map to produce an optimal sum-of-products. [2] (iv) Write the simplified sum-of-products from the K-map. X = ............................................................................................................................... [2] (b) Simplify the expression for X, as represented by the truth table in part (a), using Boolean Algebra. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2]

Mark scheme: 4(a)(i) One mark for 2 or 3 correct, two marks for 4 correct 2 𝑿= 𝑷ሜ. 𝑸ሜ. 𝑹ሜ. 𝑺ሜ+ 𝑷ሜ. 𝑸ሜ. 𝑹ሜ. 𝑺+ 𝑷. 𝑸. 𝑹. 𝑺ሜ+ 𝑷. 𝑸. 𝑹. 𝑺 4(a)(ii) One mark for each correct pair of rows/columns to max 2 2 PQ 00 01 11 10 00 1 0 0 0 01 1 0 0 0 RS 11 0 0 1 0 10 0 0 1 0 4(a)(iii) One mark for each correct loop max 2 2 PQ 00 01 11 10 00 1 0 0 0 01 1 0 0 0 RS 11 0 0 1 0 10 0 0 1 0 4(a)(iv) One mark per bullet point 2 • 𝑷. 𝑸ሜ. 𝑹ሜ • +𝑷. 𝑸. 𝑹 𝑿= 𝑷. 𝑸ሜ. 𝑹ሜ + 𝑷. 𝑸. 𝑹 // 𝑿= 𝑷. 𝑸. 𝑹 + 𝑷. 𝑸ሜ. 𝑹ሜ 4(b) One mark for correct use of distributive law 2 One mark for correct use of complement law 𝑿= 𝑷ሜ. 𝑸ሜ. 𝑹ሜሺ𝑺ሜ+ 𝑺ሻ+ 𝑷. 𝑸. 𝑹ሺ𝑺ሜ+ 𝑺ሻ 𝑿= 𝑷ሜ. 𝑸ሜ. 𝑹ሜሺ1ሻ+ 𝑷. 𝑸. 𝑹ሺ1ሻ or Two marks for correct use of redundancy law 𝑿= ሺ𝑷ሜ. 𝑸ሜ. 𝑹ሜሻ. 𝑺ሜ+ ሺ𝑷ሜ. 𝑸ሜ. 𝑹ሜሻ. 𝑺+ ሺ𝑷. 𝑸. 𝑹ሻ. 𝑺ሜ+ ሺ𝑷. 𝑸. 𝑹ሻ. 𝑺 𝑿= 𝑷ሜ. 𝑸ሜ. 𝑹ሜ+ 𝑷. 𝑸. 𝑹

5 (a) Flora has written a program that uses the variables a, b, c and d. Part of the program contains the following calculations: a = 2 b = 5 c = 7 d = a * b – (a + b + c) (i) Write the Reverse Polish Notation (RPN) for the expression: a * b – (a + b + c) ..................................................................................................................................... [2] (ii) Show the changing contents of the stack as the value for variable d is calculated from the RPN expression. [4] (b) Convert the following RPN expression back to its original infix form. d b * b c d + - + a / ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (c) Explain why expressions are evaluated using RPN. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: 5(a)(i) a b * a b + c + - 2 One mark for a b * One mark for a b + c + - 5(a)(ii) One mark per ring max 4 4 5(b) Two marks all 3 elements of the expression are seen 3 One mark if 2 elements of the expression are seen (d * b) // d * b +(b – (c + d)) // + b – (c + d) / a One mark for fully correct expression ((d * b) + (b – (c + d))) / a // (d * b + b – (c + d)) / a 5(c) Any three from 3 • Evaluation does not need to use rules of precedence for operators • No need for brackets // infix may require the use of brackets • Enables evaluation in the sequence read / left to right • … no need to backtrack

6 Mohammad is working away from his company’s head office. He wants to send a secure message over a computer network to the head office. (a) (i) Explain the way in which a digital signature for the message would be produced. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [4] (ii) State two reasons why a digital signature for the message is required. 1 ........................................................................................................................................ ........................................................................................................................................... 2 ........................................................................................................................................ ........................................................................................................................................... [2] (b) The message is encrypted using asymmetric key cryptography before it is sent and decrypted when it arrives at the head office. (i) Describe this process of encrypting and decrypting the message. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [5] (ii) State one reason for using asymmetric key cryptography. ........................................................................................................................................... ..................................................................................................................................... [1]

Mark scheme: 6(a)(i) Any four from 4 • The message in plain text Is hashed … • …using an agreed algorithm • to produce a message digest • this is then encrypted • … using the private key of the sender 6(a)(ii) Any two from 2 • To make sure the message is from Mohammed // Authentication • Message has not been tampered with during transmission • Mohammed cannot deny that he sent the message // Non-repudiation 6(b)(i) Any five from 5 • Two matching keys are used … • … one public and one private • obtain the public key of head office • Before the message is sent • The message Is encrypted (by the sender’s computer) using the public key of the receiver • When the message is received at head office • The message Is decrypted (by the receiver’s computer) using the private key of the receiver 6(b)(ii) Only the receiver has the key to decrypt the message // private key does not 1 need to be transmitted

7 A large apartment block has 20 floors. On each floor there is one security camera and four sensors. The image from each security camera is output to a display screen for that floor. • There are 20 display screens in the reception area on the lowest floor. • The data from the sensors are read and processed by a computer system. • Warning messages can also be displayed on each display screen. (a) (i) Identify the type of system described. ..................................................................................................................................... [1] (ii) Justify your answer to part (a)(i). ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) Identify two types of sensor that could be used by this system. State a reason for the use of each sensor. Sensor 1 ............................................................................................................................ Reason .............................................................................................................................. ........................................................................................................................................... Sensor 2 ............................................................................................................................ Reason .............................................................................................................................. ........................................................................................................................................... [4] (b) A program regularly checks each sensor’s readings. If the value of the reading is out of range, a warning message is displayed on the screen for that floor. A pseudocode algorithm to output the warnings has been written using these identifiers. Identifier Data type Description FloorNumber INTEGER Floor number SensorNumber INTEGER Sensor number Always BOOLEAN Value to ensure continuous loop The pseudocode algorithm uses: • the function CheckSensor(Floor, Sensor) that returns TRUE if the sensor reading is out of range and FALSE otherwise • the procedure ScreenOut(Floor, Sensor) that outputs the warning message "Problem on Floor" to the appropriate screen. (i) Complete the pseudocode algorithm. 01 Always 02 REPEAT 03 FOR FloorNumber 1 TO ………………………………………………………………………… 04 FOR SensorNumber 1 TO ………………………………………………………………………… 05 IF CheckSensor(FloorNumber, SensorNumber) 06 THEN 07 ScreenOut(FloorNumber, SensorNumber) 08 ENDIF 09 ENDFOR 10 ENDFOR 11 12 // delay loop 13 // delay loop 14 UNTIL ………………………………………………………………………… [4] (ii) Write a delay loop in pseudocode for lines 12 and 13 of the pseudocode algorithm. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (c) If a sensor reading is out of range, a bit is set in a memory location allocated to that floor. The addresses for the memory locations are 401 to 420. For example, memory location 401 is used to store the status of the sensors 1 to 4 on floor 1, memory location 402 is used to store the status of the sensors 1 to 4 on floor 2. The table shows data for some of the floors, with sensor 1 on floor 1 set, sensor 2 on floor 2 set and sensors 3 and 4 on floor 20 set. Bits Memory Sensor number location 1 2 3 4 Floor 401 0 0 0 0 1 0 0 0 1 402 0 0 0 0 0 1 0 0 2 … … … … … … … … … … 420 0 0 0 0 0 0 1 1 20 (i) The data in memory location 410 is shown. 410 0 0 0 0 0 1 0 1 State what this data represents. ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Explain the way in which the data from sensor 3 on floor 7 can be checked. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [5]

Mark scheme: 7(a)(i) Monitoring 1 7(a)(ii) Any two from 2 • System does not alter any conditions in the building • No actuators required • No feedback loop used 7(a)(iii) One mark for identifying the sensor, one for reason in context 4 Any two from • Acoustic / sound sensor (1) so the system can check for excessive noise / check for intruders (1) • temperature sensor (1) the system can check for excessive heat/fire (1) • smoke sensor (1) so the system can check for fire (1) • pressure (1) so the system can check for the presence of intruders (1) • motion (1) so the system can check for the presence of intruders (1) • infra-red / microwave (1) so the system can check for the presence of intruders (1) • proximity (1) so the system can check for the presence of intruders (1) • moisture sensor (1) to check for flooding (1) 7(b)(i) FALSE // TRUE 4 20 4 Always = TRUE / Always // NOT Always/Always = FALSE 7(b)(ii) One mark for suitable loop, could be FOR, REPEAT or WHILE 2 One mark for suitable delay must be at least 100 FOR Count ← 1 to 99999 ENDFOR / NEXT 7(c)(i) Floor 10 2 sensors 2 and 4 out of range 7(c)(ii) Any five from 5 • Find the location 407 • … using 4 as first digit of the address • … + floor number for the last two digits • Decide on bit to use (1/2) or (7/6) • AND 00000010 with contents of calculated location (407) • … to mask out that bit • Check if that bit is set / not set