The Principle of Time-Sharing

Time-Division Multiplexing (TDM) is a fundamental technique for transmitting multiple digital signals over a single communication medium. Instead of giving each signal its own physical line, TDM allows them to share the line by taking turns.

Imagine a single-lane highway where cars want to travel in the same direction. TDM is like a traffic controller who lets one car from each on-ramp onto the highway for a brief, fixed period. The controller cycles through the on-ramps so quickly that from the perspective of the cars, it feels like they are moving continuously. In telecommunications, the "cars" are data streams (e.g., individual phone calls), and the "highway" is the physical channel (e.g., a copper wire or optical fiber).

How TDM Works: Frames and Timeslots

The TDM process is managed by two key components and organized around a precise timing structure:

• Multiplexer (MUX): At the sending end, the MUX collects a piece of data from each input channel, one after another.
• Demultiplexer (DEMUX): At the receiving end, the DEMUX separates the combined stream back into the original data streams and sends them to the correct destinations.

This process is organized into a repeating structure based on Frames and Timeslots.

• 1. Timeslot

  A timeslot is a very short, fixed-length period of time assigned to a single input channel. During its timeslot, a channel gets exclusive use of the entire transmission medium to send its piece of data (e.g., an 8-bit sample of a voice call).

• 2. Frame

  A frame is a structure that contains one timeslot for each of the input channels being multiplexed. For instance, if 30 voice channels are being multiplexed, a frame will contain 30 timeslots (plus some extra slots for control and synchronization). This entire frame is sent repeatedly at a high speed.

Interactive TDM Demonstration

TDM in Action: The E1 and T1 Digital Systems

The concept of TDM is the foundation of the digital telephone hierarchies used worldwide. The two main standards are the European E1 system and the North American T1 system. Let's analyze the E1 system as a detailed example.

The E1 System (PCM 30/32)

The E1 standard was designed to carry 30 digital voice channels over a single link. Its timing is based on the sampling rate of a standard PCM voice signal, which is 8,000 samples per second.

• Frame Duration: Since each frame must carry one sample from each channel, the frame must repeat 8,000 times per second. Therefore, the frame duration is $T_{\text{frame}} = 1 / 8000 \text{ Hz} = 0.000125 \text{ s} = 125 \text{ \mu s}$.
• Timeslots per Frame: An E1 frame is divided into 32 timeslots.
• Bits per Timeslot: Each timeslot carries an 8-bit sample.
• Total Bits per Frame: $32 \text{ slots/frame} \times 8 \text{ bits/slot} = 256 \text{ bits/frame}$.
• Total Bit Rate: $256 \text{ bits/frame} \times 8000 \text{ frames/s} = 2,048,000 \text{ bps} = 2.048 \text{ Mbps}$.

E1 Timeslot Allocation:

• Timeslot 0 (TS0): Reserved for frame synchronization (FAS) and alarms.
• Timeslots 1-15: Carry the first 15 user voice/data channels.
• Timeslot 16 (TS16): Reserved for channel-associated signaling (CAS), which carries control information like call setup and teardown.
• Timeslots 17-31: Carry the remaining 15 user voice/data channels.