Skip to content
Cambridge IGCSE0478

Types and methods of data transmission

Computer Science 0478 Chapter Notes

What this chapter covers

Types and methods of data transmissionMethods of error detectionEncryption
ShareWhatsAppPost
Types and methods of data transmission notes

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Types and methods of data transmission notes as text: skim, search, and jump between subtopics.

~22 min read

In this chapter

  1. 1Data packets and their structure
  2. 2Packet switching
  3. 3Serial and parallel transmission
  4. 4Simplex, half-duplex and full-duplex
  5. 5Choosing a method for a scenario
  6. 6The universal serial bus (USB) interface
  7. 7Why data is checked after transmission
  8. 8Parity check: odd and even
  9. 9Parity byte and parity block check
  10. 10Checksum and echo check
  11. 11Check digit for data entry
  12. 12Automatic repeat query (ARQ)
  13. 13Why data is encrypted
  14. 14Plain text, cypher text and the key
  15. 15Symmetric encryption
  16. 16Asymmetric encryption
  17. 17Encryption in a secure web connection

1. Data packets and their structure

Data is broken down into packets to be transmitted. A packet has three parts: a header, a payload and a trailer.

The header holds the destination address, the packet number and the originator's address. The payload is the data being transferred. The trailer holds, for example, error detection data or the end of the packet.

Examiner insight

“Some answers did not clearly identify the data being stored, for example 'the sequence' is not enough for the packet number because the whole sequence will not be stored.”

Worked example 14 marks

Describe the structure of a data packet.

  1. 1

    A packet has a header, a payload and a trailer. [1]

  2. 2

    The header holds, for example, the destination address, packet number and originator's address. [1]

  3. 3

    The payload is the data being transferred. [1]

  4. 4

    The trailer holds, for example, error detection data or the end of the packet. [1]

2. Packet switching

In packet switching, data is split into packets. A router controls the route each packet takes, directing it towards its destination on the most efficient route.

Each packet could take a different route, so packets may arrive out of order. Once the last packet has arrived, the packets are put back in order. They cannot be put in order while some packets are still missing.

Examiner insight

“Some candidates identified a router choosing the path for the packets without identifying it chose the most efficient route.” “Some responses did identify that the packets are rearranged when they arrived, but did not state that this only happens once all the packets have arrived as they cannot be put in order when some packets are still missing.”

Worked example 12 marks

The bits of data are divided into sections that are each transmitted independently across the network to their destination.

Identify the name given to these sections of data.

A network component is used to control the path taken across the network for each section of data.

Identify the name of this network component.

  1. 1

    packets [1]

  2. 2

    router [1]

3. Serial and parallel transmission

Serial transmission sends data one bit at a time down a single wire. Parallel transmission sends several bits at the same time.

In serial, bits are less likely to be skewed, data arrives in the order it was sent, and there is less chance of interference or crosstalk. So serial can be sent safely across a longer distance. It is less likely to have errors, not free of them. Parallel has faster transmission than serial, which suits a large amount of data over a short distance, where the bits will not be skewed.

Examiner insight

“It would be beneficial for candidates to show greater accuracy in their response, for example some stated that multiple bits are sent, but did not state this happens at the same time.” “Some candidates stated that parallel was faster, but not what it was faster at doing i.e. the transmission rate.”

Worked example 12 marks

Data is transmitted between a computer and a printer using a wired connection. The computer transmits the data to the printer. The printer transmits data and interrupts to the computer.

State whether the method of transmission should be serial or parallel. Justify your choice.

  1. 1

    No mark for the choice. Any two justification points for the method chosen, here serial.

  2. 2

    There could be a long distance between the devices. [1]

  3. 3

    Less likely to have errors, interference or crosstalk. [1]

4. Simplex, half-duplex and full-duplex

Simplex transmits data in one direction only. Half-duplex transmits data in both directions, but only one direction at a time. Full-duplex transmits data in both directions at the same time. Name the whole method: serial half-duplex, not just serial.

Examiner insight

“Some annotations lacked precision, for example identifying that full-duplex can go both ways, without clarifying that this can happen at the same time.” “Some candidates only gave part of the transmission method used. For example, they gave serial data transmission instead of stating it was serial simplex transmission.”

Worked example 14 marks

An optical mouse is connected to a computer using a universal serial bus (USB) cable. The USB cable transmits data using the serial half-duplex data transmission method.

Describe how the serial half-duplex data transmission method transmits the data.

  1. 1

    (Serial) transmits the data one bit at a time [1]

  2. 2

    … down a single wire [1]

  3. 3

    (Half-duplex) transmits the data in both directions … [1]

  4. 4

    … but only one direction at a time [1]

5. Choosing a method for a scenario

To justify a choice, use the facts in the scenario. Ask which way the data must travel, whether both devices need to transmit at the same time, and how far apart they are.

Simplex does not allow a device to send data back. Half-duplex suits a device that must send data back, when both are not required to transmit at the same time. Full-duplex suits devices that may both be transmitting at the same time, so there is no delay between sending and receiving.

Examiner insight

“Some candidates selected serial and justified it that data only needed to go to the printer, without identifying the question stated data also went from the printer to the computer.” “Some candidates required additional context in their answer to show an understanding that data could travel in both directions.”

Worked example 12 marks

Data is transmitted between a computer and a printer using a wired connection. The computer transmits the data to the printer. The printer transmits data and interrupts to the computer.

State whether the method of transmission should be simplex, half-duplex or full-duplex. Justify your answer.

  1. 1

    No mark for the choice: half-duplex or full-duplex.

  2. 2

    The printer needs to send data back to the computer. [1]

  3. 3

    Half-duplex: both are not required to transmit at the same time. Full-duplex: both may need to transmit at the same time. [1]

6. The universal serial bus (USB) interface

The universal serial bus (USB) is an interface used to connect a device, such as an optical mouse, to a computer and transmit data between them. A USB cable transmits data serially, so bits are less likely to arrive out of order.

Benefits: the cable cannot be plugged in the wrong way round, different transmission speeds are supported, the connection is a universal standard and backwards compatible, it can power the device, and the device is automatically detected.

Examiner insight

“It would be beneficial for candidates to understand the difference between a USB connection and a USB device.” “Some candidates described a USB memory stick, that the interface is the storage of data on a device that can be removed from the computer.”

Worked example 12 marks

An optical mouse is connected to a computer using a universal serial bus (USB) cable.

One benefit of using the USB cable to connect the optical mouse to the computer is that bits of data are less likely to arrive out of order.

Give two other benefits of using the USB cable to connect the optical mouse to the computer.

  1. 1

    Cannot plug the cable in the wrong way round [1]

  2. 2

    Different data transmission speeds are supported (or: backwards compatible; powers the mouse; mouse automatically detected) [1]

Recap

  • A packet has a header, a payload and a trailer. The header holds the destination address, packet number and originator's address.
  • Packet switching: data is split into packets, a router controls each route, packets may arrive out of order, and they are reordered once the last one arrives.
  • Serial: one bit at a time down a single wire. Parallel: several bits at the same time.
  • Serial is safer over a long distance. Parallel has faster transmission, and suits a short distance where bits will not be skewed.
  • Simplex: one direction. Half-duplex: both directions, one at a time. Full-duplex: both directions at the same time.
  • Justify a choice with the facts in the scenario, not a description of the method.
  • USB benefits: cannot be plugged in the wrong way round, different speeds, backwards compatible, powers the device, detected automatically.

7. Why data is checked after transmission

Errors can occur during data transmission due to interference. Data can be lost, gained or changed.

So the receiver checks for errors after the data arrives. The next blocks give the methods the syllabus names.

Examiner insight

“Some candidates stated that data could be corrupted; this required more precision as to what the actual error is that the corruption causes because corruption is a general term for an error occurring.”

Worked example 11 mark

The transmitted data may contain an error. Give one way in which an error can occur during data transmission.

  1. 1

    Data can be lost // data could be gained // data can be changed [1]

8. Parity check: odd and even

A parity bit is added to each byte. The sender counts the 1s in the byte. The parity bit is set to 0 or 1 so that the total number of 1s is odd (odd parity) or even (even parity).

After transmission the receiver counts the 1s again. If the count does not meet the parity set, an error has occurred.

Examiner insight

“A common misunderstanding is that the parity bit indicates if the byte should be odd or even. For example, if the parity bit is a 1 then the parity is even. This is incorrect, the parity bit is set according to the number of 1 bits and is a 1 or a 0 depending on which is needed to make that number odd or even.” “Many candidates just gave a generic description of a parity check and did not relate this specifically to it being an even parity check.”

Worked example 13 marks

An odd parity byte check is used to check for errors in data that is transmitted from a computer to a printer. Three bytes of data need to be transmitted. Give the correct parity bit that would be added to each byte of data for an odd parity byte check.

parity bitbyte of data
?0101110
?1110111
?1000100
  1. 1

    `0101110` has four 1s, which is even, so the parity bit is 1 [1]

  2. 2

    `1110111` has six 1s, which is even, so the parity bit is 1 [1]

  3. 3

    `1000100` has two 1s, which is even, so the parity bit is 1 [1]

9. Parity byte and parity block check

A parity check on one byte misses an error when an even number of bits change, or when bits are transposed. The count of 1s is still odd (or still even), so the byte still meets the parity set.

A parity block check checks the bytes of a block horizontally and vertically. The byte and the column that fail locate the error.

Examiner insight

“Some candidates were able to identify when a parity byte check will not detect an error, although some responses required more precision – for example, multiple bits changing can still be detected because multiple can be an odd number. The identification of the even number, or transposition of bits, was important.” “The most common correct answers described the need to check horizontally and vertically.”

Worked example 12 marks

An odd parity byte check is used to check for errors in data that is transmitted from a computer to a printer. There is an error in the data after transmission. This error is not detected by the parity byte check. Explain why the error may not be detected by a parity byte check.

  1. 1

    A transposition error has occurred // an even number of bits has flipped [1]

  2. 2

    so the count of 1s in the byte is still odd, so it still meets the parity set [1]

10. Checksum and echo check

A checksum value is calculated from the data using an algorithm before it is transmitted. It is sent with the data. The receiver recalculates it and compares the two values.

In an echo check the sender compares two sets of data. It is a different method from ARQ.

Examiner insight

“A common error was describing the checksum as counting the number of bits, this would generate the file size and not any value based upon the data and could be mistaken for parity bits.” “A number of candidates incorrectly described a detailed process of how to calculate a checksum. The question did not require this, it required candidates to show how it was used in error detection, so this could not be awarded marks.”

Worked example 13 marks

A checksum is used to check for errors in the data. A checksum value is calculated from the data before it is transmitted. Describe how this checksum value is used to check for errors in the data after transmission.

  1. 1

    The checksum value is sent with the data [1]

  2. 2

    The receiving device uses the same algorithm to recalculate the checksum value [1]

  3. 3

    The checksum values are compared: if the same, no error has occurred; if different, an error has occurred [1]

11. Check digit for data entry

A check digit is calculated from the data using an algorithm and input with the data. The computer recalculates the check digit after input.

It detects errors in data entry, not in transmission. It is used in international standard book numbers (ISBN) and in bar codes.

Examiner insight

“Some candidates required more clarity for what was being compared, that the read-in check digit, and the calculated check digit are compared.”

Worked example 13 marks

The program will take data from bar codes as input. A bar code includes a check digit. Explain how the check digit is used to check for errors in data entry.

  1. 1

    The check digit is calculated from the data using an algorithm and input with the data [1]

  2. 2

    The computer recalculates the check digit using the same algorithm after input [1]

  3. 3

    If the check digits match it has been input correctly // if they do not match it has been input incorrectly [1]

12. Automatic repeat query (ARQ)

ARQ establishes that data is received without error. The receiver checks the data for errors.

With negative acknowledgement, an error makes the receiver send a negative acknowledgement and the sender re-transmits the data. If none arrives before the timeout, successful receipt is assumed. With positive acknowledgement, the receiver acknowledges data that arrived correctly.

Examiner insight

“Some candidates described the user being involved in the process, that the user sends an acknowledgement or that the user resends the data. This is an automated process that happens between devices without interaction with the user.” “Many candidates had a misconceived idea that an echo check is the same as an automatic repeat request and described an echo check instead for the answer relating to automatic repeat request.”

Worked example 13 marks

The computer and printer agree to use automatic repeat query (ARQ). Explain how ARQ using negative acknowledgement is used to establish that data is received without error.

  1. 1

    The receiver checks for errors, for example with a parity check [1]

  2. 2

    If there is an error the receiver sends a negative acknowledgement to the sender, and the sender re-transmits the data [1]

  3. 3

    If no negative acknowledgement is received before the timeout, successful receipt is assumed [1]

Recap

  • Errors in transmission: interference can cause data loss, data gain or data change.
  • Parity: the parity bit makes the number of 1s odd or even. The receiver counts again.
  • Parity misses an even number of bits changing and transposed bits. A parity block check goes horizontally and vertically.
  • Checksum: calculated by an algorithm, sent with the data, recalculated by the receiver and compared.
  • Check digit: checks data entry (ISBN, bar codes), not transmission.
  • ARQ: negative acknowledgement means re-transmit; no negative acknowledgement before timeout means success.
  • The checks are automatic: the user does not send an acknowledgement or resend data.

13. Why data is encrypted

Data that is transmitted can be stolen. Encryption is used so that any data sent is meaningless to the person who steals it.

Encryption does not stop the data from being stolen. The data can still be read: it is scrambled, so it does not mean anything.

Examiner insight

“The data can still be read; it will just be scrambled, so it will not mean anything to a person who attempts to read it.” “Many candidates understood the data would be meaningless if stolen. Candidates had a common misconception that it stops the data from being stolen.”

Worked example 12 marks

A hospital sends patient records to a clinic over the internet. The records are encrypted. Explain why the records are encrypted.

  1. 1

    The records could be stolen while they are transmitted. [1]

  2. 2

    Encryption makes any data sent meaningless to the person who steals it. [1]

14. Plain text, cypher text and the key

Data before encryption is plain text. An algorithm, or cipher, turns it into cypher text.

An encryption key is a string of characters or numbers used to encrypt data. The key is used by an algorithm to encrypt and decrypt the data.

Increasing the length of the key improves the security of the encryption.

Examiner insight

“Some candidates did not provide a level of response that was of a high enough standard and gave answers such as encrypted text.” “Some candidates were able to recognise increasing the length of the key would be a suitable option. However, many candidates focused on adding another security method, such as a firewall.”

Worked example 13 marks

The data that is transmitted is encrypted.

(i) Give one method of encryption that could be used.

(ii) State what is meant by an encryption key.

(iii) State the name given to the encrypted data.

  1. 1

    symmetric // asymmetric [1]

  2. 2

    a string of characters or numbers used to encrypt data // used by an algorithm or cipher [1]

  3. 3

    cypher text [1]

15. Symmetric encryption

Symmetric encryption is one method of encryption. Asymmetric encryption is the other.

In symmetric encryption, plain text is encrypted into cypher text. An algorithm uses an encryption key to do this.

The key is also used by the algorithm to decrypt the data.

Examiner insight

“Few candidates demonstrated a detailed understanding of the operation of symmetric encryption. The most frequent marks awarded were for understanding that plain text is encrypted into cypher text.” “Some candidates were able to provide a detailed response about the encryption process, but many candidates gave a vague and inaccurate description. It would be beneficial for candidates to understand the terminology involved in the encryption process.”

Worked example 12 marks

A school encrypts a file using symmetric encryption before it is transmitted. Describe how the file is encrypted.

  1. 1

    Plain text is encrypted into cypher text. [1]

  2. 2

    An algorithm (cipher) uses an encryption key to encrypt the data. [1]

16. Asymmetric encryption

Asymmetric encryption uses a public key and a private key.

When a web browser sends data to a web server, the web browser uses the public key to encrypt the data. The web server uses the private key to decrypt it.

Examiners reported that asymmetric encryption is the more secure of the two.

Examiner insight

“Most candidates were able to recognise that asymmetric would be more secure.”

Worked example 14 marks

A web browser sends data to a web server using asymmetric encryption. Describe how the data is encrypted and decrypted.

  1. 1

    Asymmetric encryption uses public and private keys. [1]

  2. 2

    The web browser uses the public key to encrypt the data. [1]

  3. 3

    The web server uses the private key to decrypt the data. [1]

  4. 4

    Any data sent is meaningless if it is intercepted. [1]

17. Encryption in a secure web connection

The secure socket layer (SSL) protocol uses encryption. Before any data is sent, the web browser asks the web server for its digital certificate, and the web server sends it.

The web browser authenticates the certificate. If it is valid, the data is encrypted and sent. If it is not valid, the data is not sent.

Examiner insight

“Some responses inaccurately described the browser as sending the certificate to the server instead of the other way around.” “Another common error was describing the web server as asking the browser to identify itself, for example reversing the actions.”

Worked example 14 marks

A customer's web browser connects to a shop's web server using the secure socket layer (SSL) protocol. Describe how the data is sent securely.

  1. 1

    The web browser sends a request to the web server for its (digital) certificate. [1]

  2. 2

    The web server sends the digital certificate to the web browser. [1]

  3. 3

    The web browser authenticates the digital certificate. [1]

  4. 4

    If it is valid, the data is encrypted and sent. [1]

Recap

  • Encryption makes stolen data meaningless. The data can still be read.
  • Plain text is turned into cypher text by an algorithm (cipher) using an encryption key.
  • A longer key makes the encryption more secure. A firewall does not.
  • Symmetric and asymmetric are methods of encryption. Asymmetric uses a public and a private key.
  • The web browser encrypts with the public key. The web server decrypts with the private key.
  • In SSL the web browser requests the certificate and the web server sends it.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State the three parts of a data packet.3 marks
  2. The header of a packet contains the destination address. Give two other items in the packet header.2 marks
  3. Describe the difference between serial and parallel data transmission.2 marks
  4. A walkie-talkie lets two people talk to each other, but only one can speak at a time. State whether this is simplex, half-duplex or full-duplex. Justify your answer.2 marks
  5. A sensor transmits readings to a computer. The computer never transmits anything to the sensor. State whether the method of transmission should be simplex, half-duplex or full-duplex. Justify your answer.2 marks
  6. A message is sent across a network using packet switching. Describe how the message is transmitted and received.4 marks
  7. Two computers are in buildings 150 m apart. One student says serial transmission should be used between them. Justify the student's choice.2 marks
  8. Two circuit boards in the same machine are 10 cm apart. A large amount of data must be passed between them. State whether serial or parallel transmission is better and justify your choice.3 marks
  9. A weather station on a hill is connected to a school computer by a cable 3 km long. The station transmits a reading every minute. Once a day the computer transmits a reset command to the station. The station does not transmit while the command is being sent. A student writes: 'Use parallel simplex. Parallel is faster. Serial would have no errors, but that is not needed. Simplex is right because the data is sent one bit at a time.' (a) Explain why parallel transmission is not suitable here. (b) Rewrite 'Parallel is faster' so that it is accurate enough to gain credit. (c) Correct the statement 'Serial would have no errors'. (d) Explain what is wrong with the student's reason for simplex. State the method that should replace simplex and justify it.7 marks
  10. A scanner in a clinic is connected to a computer using a USB cable. The computer sends each scan across a network to a specialist's computer 200 km away. The specialist's computer sends comments back to the clinic while the scan is still arriving. (a) A technician says: 'The benefit of USB is that a USB memory stick is small and portable.' Explain why this is not a benefit of the USB connection. Give one benefit of the USB connection for the scanner. (b) The network link uses the serial full-duplex method. Justify each part of this method for this link. (c) The scan is sent as 40 packets. Packet 17 is the last packet to arrive. Explain how packet 17 can arrive after packet 40. State when the specialist's computer can put the packets back in order, and why not before.8 marks
  11. Interference affects data during transmission. Give two ways in which the data can be affected.2 marks
  12. A computer sends the seven data bits `1011001` using odd parity. Give the parity bit.1 mark
  13. A receiver uses even parity. Each byte arrives with its parity bit first: `01100110`, `11011010` and `10010001`. State which bytes contain an error. Give a reason for each.3 marks
  14. Two bits in a byte change during transmission. The receiver uses an odd parity check. Explain why the error may not be detected.2 marks
  15. A student says: ‘A checksum is the number of bits in the file.’ Explain why this is wrong.2 marks
  16. Mei says: ‘A check digit checks that data arrived correctly after it was sent across a network.’ Correct her statement.2 marks
  17. A shop scans a bar code that includes a check digit. Describe what the computer compares and what each result means.3 marks
  18. ARQ with negative acknowledgement is used. The receiver finds no error in the data. Describe what happens.2 marks
  19. A greenhouse sensor sends four bytes to a controller, followed by a parity byte. Even parity is used. The first bit of each byte is its parity bit. The parity byte makes the number of 1s in each column even. One bit has changed during transmission. The controller receives byte 1 `01010011`, byte 2 `11100100`, byte 3 `00011001`, byte 4 `11000110` and parity byte `01101100`. (a) Identify the byte that fails the parity check. Give a reason. (b) Identify the column, counted from the left, that fails the parity check. Give a reason. (c) Give the corrected byte. (d) A student says the error is in the parity byte. Explain why the student is wrong.5 marks
  20. A drone sends the seven data bits `1100101` to its controller. An odd parity bit is added at the front. (a) Give the byte that is sent. (b) The byte is sent three times. The controller receives `11010101`, then `10000101`, then `00000101`. For each byte received, state whether the parity check detects an error. Give a reason for each. (c) A student says: ‘A parity check cannot detect an error when multiple bits change.’ Use your answers to (b) to explain why this needs more precision. (d) ARQ with negative acknowledgement is used. Describe what happens after the controller receives `11010101`.8 marks
  21. State the name given to data after it has been encrypted.1 mark
  22. State the name given to data before it is encrypted.1 mark
  23. Name the type of encryption that uses a public key and a private key.1 mark
  24. A company encrypts the data it sends over the internet. State why it does this.2 marks
  25. A student writes: ‘Encryption makes data unreadable.’ Correct the student's statement.2 marks
  26. Describe what is meant by an encryption key.2 marks
  27. A phone's web browser sends a password to a bank's web server using asymmetric encryption. State which key each of them uses, and what each uses it for.2 marks
  28. A company encrypts the data it transmits. The manager wants to improve the security of the encryption itself and suggests adding a firewall. (a) State why adding a firewall does not do this. (b) State one change that does.2 marks
  29. A farmer uses a web browser to send milk yield data to a dairy's web server. The data is sent using asymmetric encryption. A hacker intercepts the data and also has a copy of the public key. The farmer says: ‘The hacker has a key, so the hacker can now understand the data.’ Explain why the farmer is wrong.4 marks
  30. A ticket machine runs a web browser. It sends card payments to a bank's web server using the secure socket layer (SSL) protocol. A trainee writes this description. Line 1: The web server asks the web browser to identify itself. Line 2: The web browser sends its digital certificate to the web server. Line 3: The web server authenticates the digital certificate. Line 4: The data is sent even if the digital certificate is not valid. Line 5: Encryption makes the data unreadable if it is intercepted. Line 6: The bank's website decrypts the data using the public key. Lines 1 to 5 each contain one error. Line 6 contains two errors. Rewrite all six lines correctly.7 marks

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Number systemsComputer architectureTypes of software and interruptsThe internet and the world wide webAutomated systemsAlgorithm design and problem-solving