TDM

Send multiple digital data streams over a single fiber optic link. E1 and T1 carrier systems use this exact method for long distance routing.

Core Concept

Time division multiplexing lets multiple channels share the same frequency band but transmit at different times. The system allocates a specific time slot to each user. This sequential arrangement forms a continuous data stream.

A rotating switch or a digital multiplexer IC like the 74HC4051 samples each input one after another. One complete rotation collects a sample from every channel and forms a frame.

The receiver must perfectly synchronize with the transmitter to separate the data correctly. If the clocks drift the receiver routes data to the wrong destination. Frame alignment bits help maintain this critical timing.

Key Equations

The multiplexed bit rate dictates the required bandwidth. Rb = N * fs * n defines the total data speed.

N is the number of channels, fs is the sampling frequency, and n is the number of bits per sample. Frame time is simply Tf = 1 / fs for standard systems.

Given:\nNumber of channels (N) = 24\nSampling rate (fs) = 8000 Hz\nBits per sample (n) = 8 bits\n\nWhy this formula:\nThe total bit rate is the product of channels, sampling rate, and bits per sample.\n\nFormula:\nRb = N * fs * n\n\nSubstitution:\nRb = 24 * 8000 * 8\n\nCalculation:\nRb = 192000 * 8\nRb = 1536000\n\nFinal Answer:\n1.536 Mbps

Exam Tip: TDM systems usually require extra bits for synchronization. Read carefully if the question adds one sync bit per frame, which changes the bit rate to Rb = (N * n + 1) * fs.

Key Properties

• The American T1 system multiplexes 24 voice channels at 1.544 Mbps.
• The European E1 system multiplexes 30 voice channels at 2.048 Mbps.
• TDM prevents analog intermodulation distortion because only one signal travels at any exact moment.
• It integrates perfectly with modern digital computer networks.
• Pulse stuffing resolves slight timing differences between incoming asynchronous data streams.

Quick Revision

• Channels are separated entirely in the time domain.
• All channels use the full bandwidth of the transmission line.
• A commutator acts as the fundamental sampling mechanism.
• Synchronization is the hardest design challenge.
• Digital ICs handle TDM much better than complex analog filters.
• Exam trap: Confusing the channel sampling frequency with the multiplexed system bit rate.