Cambridge O Level2210

Types and methods of data transmission

Computer Science 2210 Chapter Notes

What this chapter covers

Types and methods of data transmissionMethods of error detectionEncryption
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In this chapter

  1. 1Serial Data Transmission
  2. 2Parallel Data Transmission
  3. 3Simplex Data Transmission
  4. 4Half-Duplex Data Transmission
  5. 5Full-Duplex Data Transmission
  6. 6Applying Transmission Methods

1. Serial Data Transmission

In serial data transmission, data is sent one bit at a time down a single wire or channel. Think of it like people queuing to go through a single door one by one. This method is slower than parallel but is much more reliable over long distances because the bits stay in order and are less prone to interference. A 'start bit' and 'stop bit' are often sent before and after the data byte to help the receiving device synchronise.

Key term

Serial Transmission: A method of data transmission where data is sent one bit at a time, sequentially, over a single communication channel.

Examiner insight

Examiners award marks for correctly linking serial transmission's reliability over distance to the reduced risk of data skew and external interference.

Common pitfall

Assuming serial is always inferior because it's slower. Its key advantage is reliability over long distances, which is often the most important factor.

Fun fact

The 'S' in USB (Universal Serial Bus) stands for Serial. Almost all modern peripherals, from your mouse to your external hard drive, use this highly successful form of serial transmission!

Worked example 14 marks

A university campus has two buildings 500 metres apart. Data needs to be sent from a server in one building to computers in the other. Explain why serial transmission would be a suitable method.

  1. 1

    Step 1: Identify the key factor in the scenario. The key factor is the long distance (500 metres) between the buildings.

  2. 2

    Step 2: Relate serial transmission's advantages to this factor. Serial transmission uses a single wire, which makes it less susceptible to interference over long distances compared to parallel.

  3. 3

    Step 3: Explain the problem with the alternative. Over 500m, parallel transmission would suffer from 'data skew', where bits travelling on different wires arrive at different times, corrupting the data. Serial transmission avoids this as all bits follow the same path.

  4. 4

    Step 4: Conclude with a summary statement. Therefore, serial transmission is suitable because it is reliable and ensures data integrity over long distances, despite being slower than parallel.

Recap

  • Data is sent one bit at a time.
  • It uses a single wire or channel for transmission.
  • Serial transmission is reliable over long distances.
  • It is less prone to errors like data skew and interference.
  • It is generally cheaper to implement than parallel due to fewer wires.
  • The Universal Serial Bus (USB) is a common example of serial transmission.

Quick check

  1. State two advantages of serial data transmission.2 marks

2. Parallel Data Transmission

Parallel data transmission sends multiple bits of data simultaneously over several parallel wires. For example, to send an 8-bit byte, eight wires would be used, with each wire carrying one bit. This is like having eight checkout lanes open instead of one, making it much faster than serial transmission. However, this method is only effective over very short distances.

Key term

Data Skew: An error in parallel transmission where bits sent simultaneously on different wires arrive at the destination at slightly different times, corrupting the data.

Examiner insight

Marks are often given for explaining *why* parallel transmission is unsuitable for long distances, specifically by defining or describing data skew.

Common pitfall

Forgetting to mention data skew as the specific reason why parallel transmission is not used for long distances. Simply saying 'it doesn't work' is not a sufficient explanation.

Fun fact

The wide, flat ribbon cables you might see inside an old desktop PC were used for parallel data transmission between the motherboard and devices like hard drives and CD-ROMs.

Worked example 14 marks

A marketing company uses a large, high-speed printer located 2 metres away from a design computer. Explain why parallel data transmission would be a good choice for connecting the computer to the printer.

  1. 1

    Step 1: Identify the key requirements from the scenario: high speed and short distance (2 metres).

  2. 2

    Step 2: Link parallel transmission's main advantage to the requirements. Parallel transmission is much faster than serial because it sends multiple bits at once. This is ideal for sending large amounts of print data quickly.

  3. 3

    Step 3: Address the main disadvantage of parallel and explain why it is not a problem here. The main issue with parallel is data skew over long distances. Since the printer is only 2 metres away, the risk of data skew is negligible.

  4. 4

    Step 4: Conclude the justification. Therefore, parallel transmission is the best choice because its high speed meets the needs of the printer, and the short distance negates its primary disadvantage.

Recap

  • Data is sent multiple bits at a time.
  • It uses multiple wires, one for each bit being sent simultaneously.
  • Parallel transmission is very fast over short distances.
  • It is not suitable for long distances due to the risk of data skew.
  • Crosstalk (interference between wires) can also be an issue.
  • Older internal computer components like Integrated Circuits (ICs) often used parallel transmission.

Quick check

  1. What is the main disadvantage of parallel data transmission?1 mark
  2. In which situation is parallel transmission faster than serial?1 mark

3. Simplex Data Transmission

Simplex transmission is a one-way street for data. Data can only travel in a single, pre-defined direction and never the other way. The device on the receiving end cannot send a reply or acknowledgement back along the same channel. It is the simplest mode of transmission.

Key term

Simplex: A mode of transmission where data can only be sent in a single, pre-defined direction.

Examiner insight

Candidates must provide a clear, one-way example like a TV remote or radio broadcast. Using a two-way device as an example will not score marks for simplex.

Common pitfall

Confusing simplex with half-duplex. A TV remote is simplex; a walkie-talkie, which allows for a reply (just not at the same time), is half-duplex.

Worked example 13 marks

A computer keyboard sends data to the computer, but the computer does not send data back to the keyboard. Identify and describe the mode of data transmission being used.

  1. 1

    Step 1: Identify the mode. The mode of transmission is Simplex.

  2. 2

    Step 2: Describe the mode in context. Simplex transmission allows data to flow in one direction only.

  3. 3

    Step 3: Apply to the scenario. In this case, data flows from the keyboard (sender) to the computer (receiver). The computer cannot send data back to the keyboard via this connection, making it a one-way communication path.

Recap

  • Data flows in one direction only.
  • The receiver cannot send data back to the sender on the same channel.
  • It is suitable for devices that only need to send or receive, not both.
  • Examples include a TV remote control, radio broadcast, and a computer keyboard.
  • This mode uses the full capacity of the communication link for one-way traffic.

Quick check

  1. Is a baby monitor (the part in the baby's room) an example of simplex transmission? Explain your answer.2 marks

4. Half-Duplex Data Transmission

In half-duplex transmission, data can travel in both directions, but not at the same time. It's like a single-lane road with a traffic controller; cars can go both ways, but they have to take turns. When one device is sending, the other must be listening, and vice versa.

Key term

Half-Duplex: A mode of transmission where data can be sent in both directions, but only one direction at a time.

Examiner insight

The key phrase examiners look for is 'both directions, but not simultaneously'. Using a walkie-talkie as an example is a classic way to secure marks.

Common pitfall

Failing to state that communication cannot happen at the same time. Just saying 'it goes both ways' is an incomplete answer and may not receive full marks.

Worked example 13 marks

Two people are communicating using walkie-talkies. They have to say 'over' when they finish speaking to let the other person know it is their turn to talk. Which mode of transmission is this, and why?

  1. 1

    Step 1: Identify the mode of transmission. This is half-duplex transmission.

  2. 2

    Step 2: Explain why it fits the definition. Data (their voices) can be transmitted in both directions, from person A to B and from B to A.

  3. 3

    Step 3: Explain the limitation. However, they cannot transmit at the same time. One person must stop transmitting before the other can start. The word 'over' is a protocol to manage this turn-based communication.

  4. 4

    Step 4: Conclude. Because communication is possible in both directions but not simultaneously, it is a clear example of half-duplex transmission.

Recap

  • Data flows in both directions, but not at the same time.
  • It is like a conversation where you must take turns to speak.
  • The classic example is a walkie-talkie system.
  • A short delay, called the turnaround time, is needed to switch between sending and receiving.
  • It is more efficient than simplex for tasks that require a response.

Quick check

  1. What is the key difference between half-duplex and full-duplex transmission?1 mark

5. Full-Duplex Data Transmission

Full-duplex transmission allows data to be sent and received in both directions at the same time. This is like a multi-lane highway where traffic can flow in both directions simultaneously without interfering with each other. This is the most efficient mode for interactive communication.

Key term

Full-Duplex: A mode of transmission where data can be sent in both directions simultaneously.

Examiner insight

Clear examples like a telephone call or video conference, where simultaneous two-way communication is essential, score highly. Candidates should explicitly use the word 'simultaneously' or 'at the same time'.

Common pitfall

Simply stating 'two-way communication' without adding the crucial detail 'at the same time' or 'simultaneously'. This distinction from half-duplex is vital.

Fun fact

The Ethernet cable connecting your computer to a network router is a common example of full-duplex transmission, allowing you to download a file while your game is sending data online.

Worked example 13 marks

A telephone conversation allows both people to speak and be heard at the same time. Identify and justify the mode of transmission used.

  1. 1

    Step 1: Identify the mode. The mode of transmission is full-duplex.

  2. 2

    Step 2: Justify the choice based on the scenario. A telephone conversation requires both users to be able to send data (speak) and receive data (listen) simultaneously.

  3. 3

    Step 3: Define the mode. Full-duplex is the only mode that allows for simultaneous, two-way data transmission. If it were half-duplex, the users would have to take turns speaking, which is not how a natural phone call works.

Recap

  • Data flows in both directions at the same time.
  • It is the most efficient mode for two-way communication.
  • It is like a normal telephone conversation.
  • Examples include telephone calls, video conferencing, and broadband internet connections.
  • It requires two separate communication channels (or a channel that can be split) to achieve simultaneous flow.

Quick check

  1. Give a real-world example of a device or service that requires full-duplex transmission.1 mark

6. Applying Transmission Methods

In any real-world system, you must consider both the method of bit transmission (serial or parallel) and the directional mode (simplex, half-duplex, or full-duplex). The final choice is a trade-off based on four key factors: speed, distance, cost, and the need for interactivity. For example, a USB connection is 'serial full-duplex' – it sends bits one-by-one but can do so in both directions at once.

Key term

Transmission Method: The combination of bit transmission (serial/parallel) and directional mode (simplex/half/full-duplex) used to send data between devices.

Examiner insight

Top marks are awarded to students who can justify their choice of transmission methods by weighing up the advantages and disadvantages of each in the context of the given scenario (e.g., distance, speed requirements, cost, interactivity).

Common pitfall

Only identifying one type of transmission (e.g., 'serial') when a scenario question asks for a complete description, which requires both the bit and directional methods.

Worked example 16 marks

A business manager's computer is connected to a central file server 100 metres away. The manager needs to send and receive files, often at the same time. The accuracy of the data is critical. Identify the two transmission methods that should be used and justify your choice.

  1. 1

    Step 1: Break the problem into two parts: bit transmission (serial/parallel) and directional mode (simplex/half/full).

  2. 2

    Step 2: Choose the bit transmission method. The distance is 100 metres, which is a long distance for data transmission. Therefore, serial transmission must be used. Justification: Serial is chosen to avoid data skew and interference over the 100m distance, ensuring data accuracy is maintained.

  3. 3

    Step 3: Choose the directional mode. The manager needs to 'send and receive files, often at the same time'. This requires simultaneous two-way communication. Therefore, full-duplex transmission must be used. Justification: Full-duplex allows data to be sent to the server and received from the server at the same time, which matches the user's requirement.

  4. 4

    Step 4: Combine the answers for a final conclusion. The most suitable connection would use Serial and Full-Duplex data transmission. This provides a reliable, error-free connection over the 100m distance while allowing for efficient, simultaneous two-way data transfer.

Recap

  • Every data connection has two types of transmission method: one for bits (serial/parallel) and one for direction (simplex/half/full).
  • Choose serial for long distances (> a few metres) or for cheaper connections (e.g., USB).
  • Choose parallel for very high speed over very short distances (e.g., internal computer components).
  • Choose simplex for one-way 'broadcast' devices (e.g., remote control).
  • Choose half-duplex for turn-based two-way communication (e.g., walkie-talkie).
  • Choose full-duplex for interactive, simultaneous two-way communication (e.g., telephone call, internet connection).

Quick check

  1. A connection sends data multiple bits at a time, but only in one direction. Name the two transmission methods used.2 marks

End-of-chapter exercise

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

  1. Describe how data is transmitted using serial half-duplex data transmission.4 marks
  2. Compare and contrast serial and parallel data transmission, giving one advantage and one disadvantage for each.4 marks
  3. A live video stream is sent from a security camera to a monitoring station. The camera never needs to receive data. Which two methods of data transmission would be most appropriate? Justify your choices.4 marks
  4. Explain what is meant by 'data skew' and identify which method of bit transmission it affects.2 marks
  5. Give a different real-world example for each of the following transmission modes: simplex, half-duplex, and full-duplex.3 marks
  6. A bank is setting up a new ATM (cash machine) 2km away from its central server. The ATM needs to send customer requests and receive authorisations. This communication must be secure and reliable. Which bit transmission method (serial or parallel) should be used? Justify your answer.3 marks
  7. Why is a telephone conversation considered full-duplex, while a conversation using walkie-talkies is considered half-duplex? Refer to the flow of data in your answer.4 marks
  8. A computer sends a large file to an old printer connected by a 1-metre cable. The main priority is to print the file as quickly as possible. Identify and justify the two transmission methods that would be most suitable for this connection.5 marks
  9. A start bit and a stop bit are extra bits sent along with a byte of data. Which type of bit transmission (serial or parallel) uses them and why are they necessary?2 marks
  10. For a connection between two devices, you must choose one method from {serial, parallel} and one from {simplex, half-duplex, full-duplex}. How many possible combinations of transmission methods are there?1 mark

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