Data Transmission, Error Checking and Encryption: Computer Science 0478 (Cambridge O Level / IGCSE)
Syllabus 2.1, 2.2, 2.3 · Strand 2 Data transmission
- Questions
- 10
- Total marks
- 56
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 2.1 3 questions completed
- 2.2 5 questions completed
- 2.3 2 questions completed
Before data leaves one device for another it is broken into packets, each carrying a header (destination address, packet number and originator’s address), a payload and a trailer (syllabus 2.1). A router directs each packet along its own route in a process called packet switching, so packets can arrive out of order and must be reassembled once the last one lands. The connection itself can be serial or parallel, and simplex, half-duplex or full-duplex depending on whether data travels one way, one way at a time, or both ways at once, with USB as a common serial interface.
Interference during transmission can flip bits, so receivers run checks such as parity (odd or even), checksum, echo check or a check digit on data like ISBN codes and barcodes to catch the error (syllabus 2.2). An automatic repeat query then uses acknowledgements and a timeout to request a resend when needed. Because intercepted data can otherwise be read by anyone, encryption scrambles it using a shared symmetric key or a public/private asymmetric key pair (syllabus 2.3), so only the intended recipient can restore the original message.
The exam-style questions below are original, written to match this syllabus objective, and each is followed by a full worked solution so you can check your method step by step.
Question 1
An online tax-filing service lets any user download a public encryption key from its website and use it to scramble their tax return before sending it to the server. Only the tax service holds the second, secret key needed to unscramble the file, and this secret key is never published or transmitted anywhere.
Which type of encryption is being used to protect the tax return?
Question 2
A photography studio uses several devices that transmit data in different ways.
Device 1: A digital camera is connected to a laptop with a USB cable. Photo files are copied from the camera to the laptop's hard drive, one bit at a time, along the connection.
Device 2: A wireless intercom lets two assistants talk to each other during a shoot. Both intercoms share a single radio channel, so only one assistant can be heard at any moment; if the other starts speaking at the same time, that message is lost.
Device 3: A security camera continuously streams live video to a monitor in the studio office. The monitor never sends any data back to the camera.
(a) State whether device 1 transmits data using serial or parallel transmission. Give one reason why a USB connection is designed to work this way, even though the cable itself contains more than one wire. [2]
(b) Identify the duplex mode used by device 2, and explain, using the scenario, why this mode allows only one assistant to be heard at a time. [2]
(c) Identify the duplex mode used by device 3, and explain why this mode is appropriate for a continuous, one-way video feed. [2]
Question 3
A university research group in London emails a 400 MB dataset file to a collaborator's laptop in Singapore over the internet.
(a) Describe how the dataset file is prepared before it can be sent across the network, referring to packets, headers, payload and trailer. [3]
(b) The packets that make up the file are sent using packet switching rather than as one continuous stream of data. Describe what happens to each packet during packet switching, referring to the role played by routers. [3]
(c) State why packets belonging to the same file might arrive at the collaborator's laptop in a different order from the order in which they were sent, and explain how the laptop is still able to reconstruct the original file correctly. [3]
Question 4
An automated irrigation system has a soil-moisture sensor node buried in a field. Once every hour, the node measures moisture at four different probe depths and sends the four readings to a base station in the farm office. Each reading is a denary (base 10) value between 0 and 255.
One hour, the four readings measured by the node are: 58, 175, 12, 201.
(a) Calculate the checksum value that the sensor node transmits alongside these four readings, given that the checksum is the sum of the four readings. [2]
(b) Describe what the base station does with the received readings and the received checksum value in order to check for a transmission error. [2]
(c) Due to interference, the base station actually receives the readings 58, 143, 12 and 201, but the checksum value it receives is still 446. Show, with a calculation, how the base station is able to tell that an error has occurred. [2]
(d) Describe how automatic repeat query (ARQ) allows the base station to obtain a correct copy of the readings after detecting this error. [3]
Question 5
A wireless keyboard sends the code for each key pressed to a USB receiver dongle as an 8-bit byte, with one extra parity bit added to each byte. The keyboard uses odd parity, so the total number of 1 bits sent for each keystroke (the 8 data bits plus the parity bit) must always be odd.
A user presses a key whose 8-bit data byte is 1 0 1 1 0 0 1 0 (bit 1 to bit 8, reading left to right).
(a) Calculate the parity bit that the keyboard must transmit with this byte, and give the full 9-bit sequence (the 8 data bits followed by the parity bit) that is actually sent. [2]
(b) A burst of interference during transmission flips bit 6 of the data (changing it from 0 to 1), but does not affect the parity bit. State the 8 data bits as received, and show, with a calculation, how the dongle's parity check detects that an error has occurred. [2]
(c) On a different occasion, the same interference flips both bit 6 and bit 8 of the data byte (changing both from 0 to 1), again without affecting the parity bit. Explain, with reference to this example, why a simple parity check is unable to detect this particular error. [3]
Question 6
A contactless smart-card reader at a train station barrier reads an 8-bit ticket-type code from a card and transmits it to the barrier's controller. The system uses even parity, so the total number of 1 bits sent (the 8 data bits plus one parity bit) must always be even.
The 8-bit code read from a particular card is 0 1 1 0 1 0 1 1 (bit 1 to bit 8, reading left to right).
Which 9-bit sequence (the 8 data bits followed by the parity bit) does the reader transmit?
Question 7
A library gives every book a 6-digit ID code. A 7th digit, the check digit, is added to the end of the code and is calculated as follows: add together all six digits of the ID, then find the remainder when this total is divided by 10. This remainder is the check digit.
A barcode scanner at the front desk scans a book's 6-digit ID and transmits it, followed by the check digit, to the library's server over a wireless connection. The 6-digit ID of a particular book is 5 2 8 1 6 3.
(a) Calculate the check digit for this ID, showing your working. [2]
(b) On one occasion, interference on the wireless connection changes the third digit of the ID from 8 to 3 during transmission, but does not affect the check digit. State the 6 digits as received, and show, with a calculation, how the server is able to tell that an error has occurred. [2]
(c) On a different occasion, interference instead swaps the order of the third and fourth digits of the ID (8 and 1) during transmission, without changing the value of any digit or the check digit. Explain, with reference to a calculation, why this check digit method is unable to detect that an error has occurred. [3]
Question 8
A smart thermostat sends short text commands to a boiler control unit over a wireless link so that a homeowner can adjust the heating remotely. The system uses an echo check to detect transmission errors.
(a) Describe the steps carried out in an echo check when the thermostat sends the command "SET25" to the boiler control unit. [3]
(b) Due to interference on the wireless link, the boiler control unit actually receives the command as "SEQ25", and it echoes back exactly what it received. State what the thermostat compares this echoed command with, and explain how the thermostat is able to tell that an error has occurred. [2]
(c) Explain one disadvantage of using an echo check, rather than a method such as a checksum, to detect errors in this system. [2]
Question 9
A small accounting firm, BrightBooks, uses encryption in two different situations.
Situation 1: Two accountants who both work in the same office regularly back up files from a shared laptop onto a USB drive. Before doing this, they agreed a single secret password with each other in person, and this same password is used to both encrypt and decrypt the backup files.
Situation 2: BrightBooks emails encrypted invoices to external clients over the internet. Many of these clients are new customers who BrightBooks has never met in person and has no existing secure way of contacting privately.
(a) State which type of encryption, symmetric or asymmetric, is more appropriate for situation 1, and justify your answer with reference to the scenario. [2]
(b) State which type of encryption is more appropriate for situation 2, and justify your answer with reference to the fact that BrightBooks has never met these clients. [2]
(c) Describe the key-distribution weakness of symmetric encryption that makes it unsuitable for situation 2, and explain how asymmetric encryption avoids this weakness. [4]
Question 10
Two employees use a video-calling app during a meeting. Their laptops are connected to the office network by Ethernet cables that support full-duplex transmission. Throughout the call, each laptop continuously sends its own webcam video and audio to the other laptop, while at the same time continuously receiving the other laptop's webcam video and audio, for the whole duration of the call.
Which duplex mode is being used for this video call?