Time-division multiplexing (A-level only) — AQA A-Level Physics

Time-division multiplexing (TDM) is a method of transmitting multiple signals over a single communication channel by dividing time into repeating frames, with each signal assigned a specific time slot within each frame. It is widely used in digital telephony, where many phone calls are combined onto one high-speed link, and in optical fibre networks where data from different users is interleaved. TDM is a core topic in the AQA A-Level Physics Communications option and builds on ideas of sampling, digital signals, and bandwidth.

Understanding TDM requires knowledge of how analogue signals are converted to digital form using sampling and quantisation, and how the resulting bit stream is organised into frames. It contrasts with frequency-division multiplexing (FDM), where signals share the same time but occupy different frequency bands. TDM is essential for efficient use of transmission media and is a key example of how physics principles underpin modern telecommunications technology.

• →TDM allocates each input signal a specific time slot within a repeating frame, so all signals share the same frequency band but are separated in time.
• →A multiplexer (MUX) combines multiple input signals into a single output stream by switching rapidly between inputs; a demultiplexer (DEMUX) reverses this process using synchronisation pulses.
• →The frame rate is determined by the sampling rate of the original signals, and the total bit rate depends on the number of channels, sampling rate, and bits per sample.
• →Synchronisation bits are often added to each frame to allow the receiver to identify the start of each frame and correctly route time slots.
• →TDM is used in digital telephony (e.g. T1 and E1 carriers) and in synchronous digital hierarchy (SDH) optical networks.

• TDM allows multiple signals to share a single communication channel.
• Each signal is allocated the whole channel for short, repeating time slots.
• A multiplexer samples the input signals in a fixed sequence.
• A demultiplexer at the receiver separates the interleaved samples using the same timing.
• The sampling rate must be high enough to represent each signal accurately.
• Transmitter and receiver must remain synchronised so samples reach the correct output.
• TDM is suited to digital signals and is used in telephone and data networks.

• 💡Sketch a simple timeline showing slots labelled A, B, C, A, B, C to make the interleaving visible.
• 💡State clearly that each input gets the full channel bandwidth during its slot, unlike frequency sharing.
• 💡Link the sampling rate to the need to reconstruct each original signal without loss of detail.
• 💡Always use the correct terminology: 'time slot', 'frame', 'multiplexer', 'demultiplexer', and 'synchronisation'. Avoid vague phrases like 'taking turns' without explaining the mechanism.
• 💡When performing calculations, show all steps clearly and convert units (kHz to Hz) before multiplying. Include synchronisation bits if mentioned in the question.
• 💡For explanation questions, structure your answer logically: describe the role of the multiplexer, the frame structure, the synchronisation process, and the demultiplexer. Use bullet points or numbered steps if it helps clarity.

• Thinking that all signals are transmitted at the same time on the same channel; correction: each signal uses the channel only during its own time slot.
• Confusing TDM with frequency-division multiplexing, where signals share the channel simultaneously at different frequencies; correction: TDM separates signals in time, not frequency.
• Ignoring the need for synchronisation between multiplexer and demultiplexer; correction: without common timing, samples are routed to the wrong outputs.
• Students often think TDM separates signals by frequency, confusing it with FDM. Correction: TDM separates signals in time, while all signals use the same frequency band.
• Students may assume that each channel in TDM transmits continuously. Correction: each channel transmits only during its allocated time slot, so the data is sent in bursts.
• Students sometimes forget to include synchronisation bits when calculating total bit rate. Correction: synchronisation bits add overhead, so the total line rate is higher than the sum of the channel data rates.

1. 1Day 1-2: Learn the definition of TDM and how it differs from FDM. Draw a labelled diagram showing a multiplexer, frames, and time slots.
2. 2Day 3-4: Study the role of synchronisation pulses and how the demultiplexer uses them. Practice explaining the process in your own words.
3. 3Day 5-6: Work through calculations of bit rate, frame rate, and time slot duration. Include examples with and without synchronisation bits.
4. 4Day 7-8: Complete past paper questions on TDM, focusing on 4-6 mark explanation questions. Mark your answers using the mark scheme and note recurring mistakes.
5. 5Day 9-10: Create a summary sheet with key formulas, definitions, and a comparison table of TDM vs FDM. Test yourself using active recall prompts.

• 📋Definition and comparison questions: 'State what is meant by time-division multiplexing and explain one difference from frequency-division multiplexing.' Advice: Give a precise definition and highlight the time vs frequency distinction.
• 📋Calculation questions: 'Calculate the total bit rate of a TDM system with N channels, sampling rate fs, and b bits per sample.' Advice: Show formula, substitute values, and include units. Watch for synchronisation bits.
• 📋Explanation questions: 'Explain how a demultiplexer separates the channels in a TDM system.' Advice: Describe the role of synchronisation pulses and the sequential routing of time slots.
• 📋Diagram questions: 'Draw a labelled diagram showing two frames of a TDM signal with three channels.' Advice: Clearly label time slots, frame boundaries, and synchronisation bits if present.