A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Time-Division Multiplexing — mark scheme explained
The short answer
Time-division multiplexing (TDM) is a method used to transmit multiple signals over a single communication channel by dividing the time into distinct intervals, or slots. Each signal is assigned a specific time slot during which it can send its data.
The question
Explain how a multiplexer (MUX) works in time-division multiplexing.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For explanation questions, ensure you cover all key components (time slots, synchronization, MUX, DEMUX) and provide clear, concise descriptions. For comparison questions, clearly outline the differences between TDM and other techniques. For application questions, use specific examples to demonstrate your understanding.
What the command words demand
- Explain
- Provide a detailed description of how TDM works, including key components and processes.
- Describe
- Give a clear account of the role of MUX and DEMUX in TDM.
- Compare
- Highlight the differences between TDM and other multiplexing techniques, such as FDM.
- Apply
- Use your understanding of TDM to explain its application in specific scenarios, such as telephone networks or digital television.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes for a 6-mark question on TDM. This allows time to plan your answer, write it out clearly, and check for accuracy.
- 1. A multiplexer (MUX) is a device that combines multiple input signals into a single output signal.1 mark
- 2. The MUX divides the time into equal intervals called time slots.1 mark
- 3. Each input signal is assigned a specific time slot during which it can transmit its data.1 mark
- 4. The MUX ensures that each signal is transmitted in its designated time slot, preventing overlap and interference.1 mark
- 5. The combined signal, containing data from multiple sources in their respective time slots, is then transmitted over the communication channel.1 mark
Final answer: A multiplexer (MUX) works by dividing the time into equal intervals called time slots and assigning each input signal to a specific time slot for transmission. This ensures that multiple signals can be combined into a single output signal without overlap or interference.
Work through every step correctly and you earn all 5 marks.
Another worked example
Describe the role of a demultiplexer (DEMUX) in TDM.
- 1. A demultiplexer (DEMUX) is a device at the receiving end of a communication channel.1 mark
- 2. The DEMUX receives the combined signal that contains data from multiple sources in their respective time slots.1 mark
- 3. The DEMUX uses the same clock signal as the multiplexer to ensure synchronization.1 mark
- 4. The DEMUX extracts the data from the appropriate time slots and separates the combined signal back into individual signals.1 mark
- 5. The separated signals are then output to their respective destinations, such as different telephones or television sets.1 mark
Final answer: A demultiplexer (DEMUX) receives the combined signal containing data from multiple sources in their respective time slots. It uses a clock signal to extract the data from the appropriate time slots and separates the combined signal back into individual signals.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Confusing time slots with frequency bands
Why it happens: Students sometimes confuse TDM, which uses time slots, with FDM, which uses frequency bands. This can lead to incorrect descriptions of how the signals are combined and separated.
Fix: Review the definitions and principles of both TDM and FDM. Understand that TDM divides the time into intervals (time slots) while FDM divides the frequency spectrum into distinct bands.
Forgetting to mention synchronization
Why it happens: Synchronization is a crucial aspect of TDM, but students often overlook it when explaining how TDM works. This can result in incomplete or incorrect answers.
Fix: Always include the role of synchronization in your explanation. Mention that a clock signal ensures all signals are transmitted in their correct time slots to prevent overlap and interference.
Not explaining the roles of MUX and DEMUX clearly
Why it happens: Students may describe the functions of the multiplexer (MUX) and demultiplexer (DEMUX) in vague terms, leading to unclear or incorrect answers.
Fix: Clearly explain that the MUX combines multiple input signals into a single output signal by assigning each input to a specific time slot. The DEMUX separates the combined signal back into individual signals by extracting data from the appropriate time slots.
Failing to mention the efficient use of bandwidth
Why it happens: The efficient use of bandwidth is a key advantage of TDM, but students often forget to include this in their answers. This can result in incomplete or less comprehensive responses.
Fix: Always highlight that TDM allows multiple signals to share the same communication channel, reducing the need for additional infrastructure and improving overall system capacity.
Not providing specific examples of TDM applications
Why it happens: Students may provide general descriptions of TDM without giving concrete examples. This can make their answers less convincing or relevant.
Fix: Include specific examples of TDM applications, such as telephone networks, digital television, and data communication systems. Explain how TDM is used in these contexts to improve efficiency and capacity.
Confusing the roles of MUX and DEMUX
Why it happens: Students sometimes mix up the functions of the multiplexer (MUX) and demultiplexer (DEMUX), leading to incorrect descriptions of how TDM works.
Fix: Clearly differentiate between the MUX and DEMUX. The MUX combines multiple input signals into a single output signal, while the DEMUX separates the combined signal back into individual signals at the receiving end.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Explain MUX Operation | Describe how a multiplexer combines signals using time slots in time-division multiplexing. | 5 |
| Explain DEMUX Role | Describe how a DEMUX separates a combined TDM signal back into individual output signals. | 5 |
| Compare Multiplexing Techniques | Compare TDM and FDM, outlining the key differences in how each shares a channel. | 6 |
| Explain TDM Application | Describe how time-division multiplexing transmits multiple digital TV signals using time slots, MUX and DEMUX. | 6 |
| Total across these question types | 22 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.