AQA A-Level Physics: Time-Division Multiplexing — mark scheme explained

The question

Explain how a multiplexer (MUX) works in time-division multiplexing. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  1. A multiplexer (MUX) is a device that combines multiple input signals into a single output signal.

• S2

  2. The MUX divides the time into equal intervals called time slots.

• S3

  3. Each input signal is assigned a specific time slot during which it can transmit its data.

• S4

  4. The MUX ensures that each signal is transmitted in its designated time slot, preventing overlap and interference.

• S5

  5. The combined signal, containing data from multiple sources in their respective time slots, is then transmitted over the communication channel.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   1. A multiplexer (MUX) is a device that combines multiple input signals into a single output signal.

2. S2

   2. The MUX divides the time into equal intervals called time slots.

3. S3

   3. Each input signal is assigned a specific time slot during which it can transmit its data.

4. S4

   4. The MUX ensures that each signal is transmitted in its designated time slot, preventing overlap and interference.

5. S5

   5. The combined signal, containing data from multiple sources in their respective time slots, is then transmitted over the communication channel.

6. 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.

Common mistakes

• Confusing time slots with frequency bands — 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 — 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 — 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 — 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 — 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 — 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

• Full worked solution (all marking points)5 marks

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