Computer Science 9618/13 May/June 2023: question paper and mark scheme

Mark scheme

1(a) NOT B 1 1(b) 1 mark for first 4 rows correct; 1 mark for second 4 rows correct 2 A B C X 0 0 0 1 0 0 1 0 0 1 0 1 0 1 1 0 1 0 0 1 1 0 1 0 1 1 0 0 1 1 1 1

Mark scheme

2(a) 1 mark each to max 2 2 WAN covers a large geographical area and LAN covers a small geographical area LAN connections between devices are usually physical, whereas the WAN connections are often virtual A LAN has a high data transfer rate, whereas a WAN has a low data transfer rate The ownership of a LAN is private; the ownership of a WAN can be private or public LAN is usually more secure than a WAN because protection is easier to implement 2(b)(i) 1 mark each to max 2 2 All computers are connected to at least one other device There are multiple routes between devices The computers can act as relays, passing packets on towards the final destination 2(b)(ii) 1 mark each to max 2 2 If one line goes down there are more routes available Improved security as not using one main line No/fewer collisions New nodes can be added without interruption or interfering with other nodes More secure because data is sent over a dedicated connection 2(c) 1 mark each to max 2 2 Server performs all processes required by the task and/or data storage Clients only sends requests to the server and displays the returned results 2(d) 1 mark each to max 4 4 Examples: Some students might only have one sort of connection on their device Wired provides better performance for the student's device ... for example, enabling faster access to university databases There will be less interference if students connect via a cable Students can transmit private/confidential data/work securely ... for example, their final dissertation Wireless connection means that the students can use their devices in different rooms/sites/outside/anywhere more freely // student devices can be portable Wireless connection enables the students to bring multiple devices // bring their own devices // change devices 2(e) 1 mark for identification 3 IP address is made up of a network ID and a host ID 1 mark each to max 2 for description Each device in a subnetwork has the same network ID // Each subnetwork has a different network ID Every device in each subnetwork has a different host ID but the same network ID // the host ID uniquely identifies the device within the subnetwork

Mark scheme

3(a) 1 mark for each term 5 Resistive Circuit Capacitive Change Coordinates A resistive touchscreen has two layers. When the user touches the screen, the layers touch and a circuit is completed. A capacitive touchscreen has several layers. When the top layer is touched there is a change/increase/decrease in the electric current. A microprocessor identifies the coordinates of the touch. 3(b) 1 mark each to max 3 3 Examples: Scans the scene in real time Identifies if there are faces in the image Uses facial recognition ... uses image recognition ... takes each frame individually ... analyses the pixels ... stores pattern for a face ... looks for patterns that match/come close to the pattern for a face Camera focuses on the pattern identified 3(c)(i) 1 mark each 3 The amplitude is recorded a set number of times a second Each (instance of an) amplitude is given a corresponding binary number The binary number (of each amplitude) is saved in sequence 3(c)(ii) 1 mark each; max 2 for rate and max 2 for resolution 4 Sampling rate There are smaller 'gaps' in the sound wave // sound is recorded more often Digital waveform is closer to the analogue waveform The quantisation errors are smaller Sampling resolution There are more bits per sample // a wider range of amplitudes can be stored Each binary amplitude/note (in the digital recording) is closer to the analogue amplitude/note Digital waveform is closer to the analogue waveform The quantisation errors are smaller

Mark scheme

4(a) 1 mark for each bullet point. Mark in pairs. Max 2 for each description. 4 Reduces data redundancy ... because linked tables mean that each data item is stored only once Reduces program-data dependency ... because the data is separate from the software so changes to the data do not require programs to be re-written Reduces data inconsistency // improves data integrity ... because by only storing data once it only needs to be updated once // changes in one table will automatically update in another // linked data cannot be entered differently in two tables Complex queries are easier to run Can provide different views ….so users can only see specific aspects of the database 4(b) 1 mark each 4 Term Description Entity An object that data is stored about. Tuple A row of data in a table about one instance of an object. Secondary key An additional/alternative key used as well as the primary key to locate specific data // a candidate key that has not been chosen as a primary key. Foreign key A field in one table that is linked to a primary key in another table. 4(c) 1 mark each 4 Only 3 tables with appropriate identifiers (i.e. one table for customer, one for booking and one for car) Appropriate Primary key in each table underlined Booking table includes Primary key from car and Primary key from customer as Foreign keys All original fields are in correct tables Example answer: BOOKING(BookingID, CarRegistration, CustomerID, StartDate, EndDate) CAR(CarRegistration, CarModel, CarColour) CUSTOMER(CustomerID, CustomerFirstName, CustomerLastName, EmailAddress, TelephoneNumber) 4(d)(i) 1 mark each to max 2 2 Length check: the registration number must be 6 characters long Format check: the registration number must be in the format letter- digit-digit-digit-letter-letter Type check: the registration number must be alphanumeric 4(d)(ii) 1 mark each 2 Visual check: Manually compare the registration number entered with the source document Double entry: Enter the registration number twice and the computer compares to check they are the same 4(d)(iii) The registration number on the original document might be in the correct 1 format but may be the incorrect registration number for that car.

Mark scheme

5(a)(i) 1 mark each to max 2 2 Programmer can test sections of the code without every part working / being written Programmer can debug in real time … so that errors can be fixed and the program continued from that point The effect of any changes made by the programmer can be seen immediately To avoid dependent errors 5(a)(ii) 1 mark each to max 3 3 The compiler produces an executable file ... so the user cannot access / edit / sell the code ... and users do not need the translator to run the game The game can be compiled for different hardware specifications ... and then used to generate more income for the programmer The program can be tested multiple times without having to retranslate each time 5(b) 1 mark for appropriate licence; 1 mark for each point to max 3 4 Commercial software licence User has to pay for the product so the programmer can gain an income Enables the program to be copyrighted ... so the user cannot legally edit the program // the programmer retains control over product … and can take legal action against people who attempt to illegally copy it /sell it on Shareware licence Enables the program to be copyrighted The user cannot legally edit the program so the developer retains control over product User can try the program for free and then pay for the full game which allows the programmer to gain an income so more people can experience it and therefore be more likely to buy it

Mark scheme

6(a) 1 mark each to max 5 5 The sender hashes the document ... to produce a digest The sender encrypts the digest to create the digital signature The message and the signature are sent to the receiver The receiver decrypts the signature to reproduce the digest The receiver uses the same hashing algorithm on the document received to produce a second digest The receiver compares this digest with the one from the digital signature If both of the receiver’s digests are the same the document is authentic 6(b) 1 mark each for identification and appropriate description of 2 pieces of 2 software, max 2 Type of Description software Antivirus scans the computer for viruses and checks against a stored database of viruses, that needs to be updated regularly and then deletes / quarantines them compares downloaded files to a database of known viruses and prevents the download continuing Antispyware scans the computer for spyware and checks against a stored database of viruses, that needs to be updated regularly and then deletes / quarantines them compares downloaded files to a database of known spyware and prevents the download continuing. Firewall monitors incoming and outgoing traffic and compares it to criteria that are set by the user such as through a whitelist/blacklist/identifying allowed / blocked IP addresses compares incoming and outgoing traffic to criteria blocks those that do not match criteria Antimalware scans the computer for viruses and checks against a stored database of viruses, that needs to be updated regularly and then deletes / quarantines them compares downloaded files to a database of known viruses and prevents the download continuing

Mark scheme

7(a) 1 mark for each correct line 3 Description Denary value The smallest integer that can −127 be represented in 8-bit two's complement. 127 −255 −128 The largest integer that can be represented in 8-bit two's complement. −256 256 128 The largest unsigned integer that can be represented in 8 bits. 255 7(b) 1 mark each to max 4 4 The system clock gives out timing signals … which are sent on the control bus …to synchronise the other system components The Control Unit initiates data transfer …by generating signals that are sent on the control bus to other components 7(c) 1 mark for each Register transfer notation 2 Stage description Register transfer notation The Program Counter (PC) is PC ← [PC] + 1 incremented The data in the address stored in the MDR ← [[MAR]] Memory Address Register (MAR) is copied to the Memory Data Register (MDR)