Cambridge A Level Computer Science 9608 — 2017 Oct/Nov Paper 3 · Variant 1

1 A Local Area Network (LAN) consists of three computers, one server and a switch. The LAN uses a star topology. (a) Complete the following diagram to show how the computers, the server and the switch could be connected. Switch Server Computer Computer Computer A B C [1] (b) There are four statements in the following table. For each statement, place a tick (✓) in the appropriate column to indicate whether it is true or false. Statement True False The server can send packets to Computer B and Computer C at the same time. The network software on each computer needs to include collision detection and avoidance. Computer B can read a packet sent from the server to Computer C. Computer A can send a packet to Computer B and at the same time the server can be sending a packet to Computer C. [4] (c) The LAN shown in part (a) will be connected to the Internet. (i) A router will be attached to one of the devices on the LAN. State the device used. Give a reason for your choice. Device ............................................................................................................................... Reason .............................................................................................................................. ........................................................................................................................................... ...................................................................................................................................... [2] (ii) Explain why a router is required. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2] (iii) After the router has been connected, Computer A sends several packets to an internet web server. Explain how the packets are transmitted from the router to the web server. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [3]

Mark scheme: Question Answer Marks 1(a) 1 Computer Computer Computer Switch Server A B C Three lines with arrows – one from each device to switch 1(b) 4 Statement True False The server can send packets to Computer B and 9 1 Computer C at the same time. The network software on each computer needs to 9 1 include collision detection and avoidance. Computer B can read the packet sent from the 9 1 server to Computer C. Computer A can send a packet to Computer B and at the same time the server can be sending a packet 9 1 to Computer C. 1(c)(i) Device: Server 1 2 The server can provide a (software) firewall // The server can check all internet traffic // Server acts as proxy 1 Device: Switch 1 Internet traffic by passes the server // Server not overloaded with internet traffic // connected to all computers 1 1 mark for device, 1 mark for suitable reason 1(c)(ii) • Router acts as gateway 2 • Router acts as a firewall • The LAN and the Internet are two different networks • (may) operate on different protocols • Router forwards packets between networks • Router has a public IP address • Router holds a list of local addresses • Router translates local addresses to Internet (IP) addresses (and vice versa) 1 mark for each point, max 2 1(c)(iii) • Each packet has the IP address of the web server / destination 3 address • The routers use routing tables • Routers on the Internet forward packets towards destination • Packets can take different routes from source to destination • Packets are reassembled in order at the web server 1 mark for each point, max 3

2 (a) The following diagram shows four descriptions and four types of computer architecture. Draw lines to connect each description to the appropriate computer architecture. Description Computer architecture Most parallel computer systems use this architecture. SIMD Widely used to process 3D graphics in video games. MIMD A microprocessor is used to control a washing machine. MISD There are a number of processing units. Each processing unit executes SISD the same instruction but on different data. (b) A computer has a single processor that contains four processing units. Explain why this is not an example of a massively parallel computer. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2] (c) An application has previously executed on a single computer. The application will be transferred onto a massively parallel computer. The program code used in the application will need to be updated to ensure that the power of the massively parallel computer is fully used. Explain what changes will be required to the program code. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2] (d) Explain one of the hardware issues that will have to be overcome if a massively parallel computer is to function successfully. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2]

Mark scheme: 2(a) 4 Description Computer architecture Most parallel computer systems use this architecture. SIMD Widely used to process 3D graphics in video games. MIMD A microprocessor is used to control a washing machine. MISD There are a number of processing units. Each processing unit executes the SISD same instruction but on different data 1 mark for each correct line 2(b) • Only one (separate) processor / not many separate processors 2 (is not massively parallel) 1 • Quad core computer system // processing units share the same bus 1 1 mark for each point, max 2 2(c) • Split into blocks of code «. 2 • « that can be processed simultaneously « • « instead of sequentially • Each block is processed by a different processor • which allows each of the many processors to simultaneously process the different blocks of code independently • Requires both parallelism and co-ordination 1 mark for each point, max 2 2(d) 1 mark for identification of hardware issue, for example: 2 • Communication between the different processors is the issue 1 mark for further explanation from: • Each processor needs a link to every other processor • Many processors require many of these links • Challenging topology

3 The following syntax diagrams for a particular programming language show the syntax of: • an assignment statement • a variable • an unsigned integer • a letter • an operator • a digit. Assignment statement Unsigned Variable := Variable Operator integer Variable Digit Letter Digit 1 Unsigned integer 2 Digit 3 4 Letter Operator 5 A + B – 6 C * 7 ^ 8 9 0 (a) The following assignment statements are invalid. Give the reason in each case. (i) C2 = C3 + 123 Reason: ............................................................................................................................. ...................................................................................................................................... [1] (ii) A3 := B1 – B2 Reason: ............................................................................................................................. ...................................................................................................................................... [1] (iii) A32 := A2 * 7 Reason: ............................................................................................................................. ...................................................................................................................................... [1] (b) Complete the Backus-Naur Form (BNF) for the syntax diagrams shown. <digit> has been done for you. <assignment_statement> ::= ................................................................................................................................................... <variable> ::= ................................................................................................................................................... <unsigned_integer> ::= ................................................................................................................................................... <digit> ::= 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 <letter> ::= ................................................................................................................................................... <operator> ::= ................................................................................................................................................... [6] (c) The definition of <variable> is changed to allow: • one or two letters and • zero, one or two digits. Draw an updated version of the syntax diagram for <variable>. Variable Letter [2] (d) The definition of <assignment_statement> is altered so that its syntax has <unsigned_integer> replaced by <real>. A real is defined to be: • at least one digit before a decimal point • a decimal point • at least one digit after a decimal point. Give the BNF for the revised <assignment_statement> and <real>. <assignment_statement> ::= ........................................................................................ ................................................................................................................................................... <real> ::= ........................................................................................................................... ................................................................................................................................................... [2]

Mark scheme: 3(a)(i) There should be a colon before the '=' sign 1 3(a)(ii) The second operand should be an unsigned integer and not a variable 1 3(a)(iii) A32 is not a variable, as a variable should be a letter followed by a single digit 1 3(b) <assignment_statement> ::= <variable> := 1 6 <variable> <operator> <unsigned_integer> 1 <variable> ::= <letter> <digit> 1 <unsigned_integer> ::= <digit> | 1 <digit> <unsigned_integer> 1 <letter> ::= A | B | C 1 <operator> ::= + | - | * | ^ 3(c) 2 Variable Letter Letter Digit Digit < one mark > < one mark > Syntax diagram shows one or two letters 1 Syntax diagram shows zero, one or two digits 1 3(d) <assignment_statement> ::= 2 <variable> := <variable> <operator> <real> 1 <real> ::= <unsigned_integer> . <unsigned_integer> 1

4 The Secure Socket Layer (SSL) protocol and its successor, the Transport Layer Security (TLS) protocol, are used in Internet communications between clients and servers. (a) (i) Define the term protocol. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2] (ii) Explain the purpose of the TLS protocol. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [3] (b) A handshake process has to take place before any exchange of data using the TLS protocol. The handshake process establishes details about how the exchange of data will occur. Digital certificates and keys are used. The handshake process starts with: • the client sending some communication data to the server • the client asking the server to identify itself • the server sending its digital certificate including the public key. Describe, in outline, the other steps in the handshake process. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [3] (c) Give two applications where it would be appropriate to use the TLS protocol. 1 ............................................................................................................................................... ................................................................................................................................................... 2 ............................................................................................................................................... ................................................................................................................................................... [2]

Mark scheme: 4(a)(i) A (known) set of rules 1 2 Agreed/standard method for data transmission // governs how two devices communicate 1 4(a)(ii) Max 2 marks for purpose: Max 3 • Purpose of TLS is to provide for secure communication (over a network) • maintain data integrity • additional layer of security Max 2 marks for further explanation from: • TLS provides improved security over SSL • TLS is composed of two layers / record protocol and handshake protocol • TLS protects this information by using encryption • Also allows for authentication of servers and clients 4(b) • The client validates (the server’s) TLS Certificate 3 • The client sends its digital certificate (to the server if requested) • Client sends an encrypted message to the server using the server’s public key • The server can use its private key to decrypt the message « • « and get data needed for generating symmetric key • Both server and client compute symmetric key (to be used for encrypting messages) // session key established • The client sends back a digitally signed acknowledgement to start an encrypted session • The server sends back a digitally signed acknowledgement to start an encrypted session 1 mark for each point, max 3 points 4(c) Applications, for example: 2 • online banking • private email • online shopping • online messaging etc. 1 mark for each point, Max 2

5 (a) (i) Complete the truth table for this 2-input NAND gate: A B X A 0 0 X 0 1 1 0 B 1 1 [1] (ii) Complete the truth table for this 3-input NAND gate: A B C X 0 0 0 0 0 1 A 0 1 0 B X 0 1 1 1 0 0 C 1 0 1 1 1 0 1 1 1 [1] (b) A SR flip-flop is constructed using two NAND gates. S Q Q R (i) Complete the truth table for the SR flip-flop: S R Q Q Initially 1 0 0 1 R changed to 1 1 1 S changed to 0 0 1 S changed to 1 1 1 S and R changed to 0 0 0 1 1 [3] (ii) The final row in the table in part b(i) shows that the output for both Q and Q is 1. Explain why this is a problem. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2] (c) Another type of flip-flop is the JK flip-flop. A JK flip-flop is constructed as follows: J Q Clock Q K (i) Complete this truth table for the JK flip-flop. Initial Final values values Working space J K Clock Q Q Q Q 0 0 1 1 0 1 0 0 0 1 0 1 0 1 0 1 1 1 0 0 1 0 1 1 0 1 0 1 1 0 1 1 0 1 0 1 0 1 1 1 1 1 0 1 1 1 0 1 [4] (ii) Explain why the JK flip-flop is an improvement on the SR flip-flop. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ...................................................................................................................................... [2] (d) Explain the role of flip-flops in a computer. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... .............................................................................................................................................. [2]

Mark scheme: 5(a)(i) 1 A B X 0 0 1 0 1 1 1 0 1 1 1 0 5(a)(ii) 1 A B C X 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 0 5(b)(i) 3 S R Q Q Initially 1 0 0 1 R changed to 1 1 1 0 1 1 S changed to 0 0 1 1 0 1 S changed to 1 1 1 1 0 1 S and R changed to 0 0 0 1 1 5(b)(ii) • Q and Q have same value 2 • Q and Q should be complements of each other • Flip-flop becomes unstable 1 mark for each point, max 2 5(c)(i) Initial Final 4 values values Working space J K Clock Q Q Q Q 0 0 1 1 0 1 0 0 0 1 0 1 0 1 0 1 1 1 0 0 1 0 1 1 0 1 0 1 1 0 1 1 0 1 0 1 0 1 0 1 1 0 1 1 1 1 0 0 1 1 1 1 0 1 1 0 1 mark per shaded row 5(c)(ii) • S-R flip-flop has an invalid combination of S and R // The S_R flip flop 2 allows both Q and Q to have the same value // S-R flip-flop inputs may arrive at different times 1 • The J-K flip-flop does not allow for Q and Q to have the same value // All four combination of values for J and K are valid // J-K flip-flop incorporates a clock pulse for synchronisation 1 5(d) • A flip-flop can store either a 0 or a 1 2 • Computers use bits to store data • Flip-flops can therefore be used to store bits (of data) • Memory can be created from flip-flops 1 mark for valid point, max 2

6 A large warehouse stores goods that must be kept above a temperature of 15 degrees Celsius. The warehouse has six temperature sensors which are each placed at a different location in the warehouse. A computer system is programmed to turn on appropriate heaters when one of the sensors is below the minimum temperature. (a) (i) State the name given to the type of system described. ...................................................................................................................................... [1] (ii) Justify your answer to part (i). ........................................................................................................................................... ...................................................................................................................................... [1] (b) Sensors and heaters are two types of device used in this system. State two other devices that are used. Justify your choice. Device 1 .................................................................................................................................... Justification ............................................................................................................................... ................................................................................................................................................... Device 2 .................................................................................................................................... Justification ............................................................................................................................... ................................................................................................................................................... [4] (c) The computer system stores the temperature readings for the six sensors in six 8-bit memory locations. Six of the bits in an 8-bit register, LOWREG, are used to indicate whether a particular reading is below the minimum temperature. A value of 1 means the reading is below the minimum temperature. For example: This pattern of bits in LOWREG shows that sensor 5, sensor 4 and sensor 1 have readings below the minimum temperature. 6 5 4 3 2 1 Not used Not used 0 1 1 0 0 1 The following table shows part of the instruction set for a processor which has one general purpose register, the Accumulator (ACC), and an Index Register (IX). Instruction Explanation Op code Operand Direct addressing. Load the contents of the given address to LDD <address> ACC. LDR #n Immediate addressing. Load the number n to IX. Indexed addressing. Form the address from <address> + LDX <address> the contents of the index register. Copy the contents of this calculated address to ACC. STO <address> Store the contents of ACC at the given address. INC <register> Add 1 to the contents of the register (ACC or IX). ADD <address> Add the contents of the given address to the ACC. Bitwise OR operation of the contents of ACC with the contents OR <address> of address. CMP #n Compare the contents of ACC with number n. CMP <address> Compare the contents of ACC with the contents of <address>. JMP <address> Jump to the given address. Following a compare instruction, jump to <address> if the JPE <address> compare was True. Following a compare instruction, jump to <address> if the JGE <address> content of ACC is greater than or equal to the number used in the compare instruction. Question 6(c) continues on the next page.

Mark scheme: 6(a)(i) Control system 1 6(a)(ii) System is controlling devices // turns heaters on and off // use of actuators 1 maintain the environment // makes use of feedback 6(b) Computer/microprocessor 4 « to process the sensor readings Analogue to digital convertor « Sensor produces analogue signal but processor requires digital data Digital to analogue convertor « Processor produces digital signal but actuator may require analogue sign Actuator « May be required to turn heater on or off 1 mark for device, 1 mark for justification, max 2 devices 6(c)(i) One mark per column excluding LOWTEMP 4 LOWTEMP LOWREG COUNTER ACC IX 15 B00000000 1 0 17 1 2 2 1 14 B00000000 B00000010 B00000010 2 4 4 2 6(c)(ii) • COUNTER has an initial value of 1 2 • Test for final value is before COUNTER updated • COUNTER is doubled in value each time around loop • six sensors values/bits to check • COUNTER is doubled in value 6 times // 25 • Values of COUNTER at test will therefore be 1 – 2 – 4 – 8 – 16 – 32 1 mark for valid point, max 2 6(c)(iii) • Load the contents of LOWREG into ACC 3 • Check bit position in LOWREG • For each of the least significant 6 bits • Use AND operation / mask to isolate a bit • Jump to code corresponding to bit being looked at • if value of bit is 1 • Send signal to appropriate actuator to turn on the heater 1 mark for valid point, max 3