Time-Division Multiplexing (TDM)

TDM: Frames and Timeslots

Time-Division Multiplexing (TDM) carries multiple digital signals over one transmission medium. The channel is divided into recurring timeslots, and each slot is assigned to a particular stream. The signals use the same line in a fixed sequence. In synchronous TDM, a slot position repeats for the same channel even when that source has no payload. The receiver identifies the channel from its frame position.

Consider a single-lane highway with several on-ramps. A controller lets one vehicle from each ramp enter for a short, fixed period and then moves to the next ramp. Repeating the order creates a continuous flow of vehicles. In telecommunications, the vehicles represent data streams such as phone calls, and the highway represents a physical channel such as copper wire or optical fiber.

TDM Components and Structure

TDM uses a multiplexer, a demultiplexer, and a defined timing structure:

• Multiplexer (MUX): At the sending end, the MUX collects data fragments from the input channels in a fixed order and places them in successive timeslots.
• Demultiplexer (DEMUX): At the receiving end, the DEMUX identifies the timeslots, separates the aggregate stream, and sends each data stream to its intended destination.

Transmission uses a repeating structure of frames and timeslots.

• 1. Timeslot

  A timeslot is a short, fixed period assigned to one input channel. During the slot, that channel uses the transmission medium to send its data fragment, such as an 8-bit voice sample.

• 2. Frame

  A frame contains one timeslot for each multiplexed input channel. When 30 voice channels are multiplexed, the frame contains 30 timeslots plus any control or synchronization fields required by the format. Frames are transmitted cyclically.

TDM in the E1 and T1 systems

E1 and T1 are examples of digital telephone systems based on TDM. E1 is the European primary-rate system at 2.048 Mbit/s, while T1 is the North American system at 1.544 Mbit/s. The E1 system is the detailed example below.

The E1 System (PCM 30/32)

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

• Frame duration: Each frame carries one sample from every channel, so it repeats 8,000 times per second. 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): Used for frame synchronization (FAS) and alarm or service bits.
• Timeslots 1-15: Carry the first 15 user voice or data channels.
• Timeslot 16 (TS16): In the CAS variant, this slot carries channel-associated signaling (CAS). The signaling includes control information such as call setup and teardown.
• Timeslots 17-31: Carry the remaining 15 user voice or data channels.