Time Division Multiplexing (TDM)
Time-Division Multiplexing (TDM) uses different time slots on the same physical connection to transmit multiple signals simultaneously, effectively enabling multi-channel communication. In TDM, time serves as the key parameter for dividing the signal, so it’s crucial that each signal occupies a distinct time slot without overlapping on the timeline. Essentially, TDM divides the total transmission time allocated to the channel into discrete time slots—known as "time slots"—and assigns these slots exclusively to individual signal sources for use.
Basic principles
Time-division multiplexing is suitable for transmitting digital signals. Since the channel’s bit transmission rate exceeds the data rate of each individual signal, the channel can be divided into several time segments, which are alternately allocated to multiple signals. Each time slot is exclusively occupied by one multiplexed signal, allowing multiple digital signals to be transmitted sequentially within a defined timeframe—and thus enabling a single physical channel to carry several digital signals simultaneously. For instance, if each input signal has a data rate of 9.6 kbit/s and the maximum line bitrate is 76.8 kbit/s, up to 8 signals can be transmitted. At the receiving end, sophisticated decoders use additional information received alongside the signals to accurately distinguish between the different digital streams.

Features
Time-division multiplexing is based on the sampling theorem. The sampling theorem allows continuous (analog) baseband signals to be replaced by discrete samples that appear at specific intervals in time. When these sampling pulses occupy only brief moments, gaps are created between them—gaps that can then be used to transmit the sampled values of other signals. As a result, it becomes possible to simultaneously carry multiple baseband signals over a single channel.
Time-Division Multiplexing Technology
Time-division multiplexing (TDM) is a communication technique that interleaves different signals across distinct time slots, allowing them to be transmitted simultaneously over the same channel. At the receiving end, a specific method is used to extract the signals from each time slot, reconstructing the original signals. This technology enables multiple signals to be carried over a single channel.
App Extension
The TDM method is further divided into the following two types:
Synchronous Time-Division Multiplexing System (divided into two categories):
1. Quasi-synchronous PDH series (for the Public Switched Telephone Network, PSTN)
2. Synchronous Series SDH (used in backbone networks such as fiber-optic communications)
Statistical (asynchronous) time-division multiplexing systems (divided into two categories):
1. Virtual circuit mode (e.g., X.25, Frame Relay, ATM)
2. Datagram mode (e.g., TCP/IP)
The PSTN system employs a combination of PDH and SDH: PCM/PDH is used for small-user access and switching, while SDH is deployed in the core backbone network.
There are two types of PDH standards in the world.
1. A 30/32-channel PCM system based on A-law companding (European standard, used in Europe, China, Russia, and other regions)
2. A 24-channel PCM system based on u-law compression (American standard, used in North America, Japan, Taiwan, and other regions)
Time slice allocation:
Synchronous Time Division Multiplexing (TDM) features pre-allocated, fixed-time slots, ensuring that all signal sources are transmitted in synchronized timing. In contrast, Asynchronous Time Division Multiplexing (ATDM) allows for the dynamic allocation of time slots on the transmission medium.
Development:
While traditional circuit-based time-division multiplexing technology is already well-established, it faces significant limitations in further boosting the transmission rate of a single optical fiber due to the "electronic bottleneck." Although electrical time-division multiplexing can achieve data rates of 10 Gbit/s per fiber, Germany’s SHF 40 Gbit/s electrical TDM device, though commercially available, remains prohibitively expensive because of its complex technology. Therefore, to push the capacity of optical communication systems even higher, researchers are now focusing primarily on two advanced multiplexing techniques: wavelength-division multiplexing (WDM) and optical time-division multiplexing (OTDM).
WDM multiplexes multiple optical signals of different wavelengths onto a single fiber, while at the receiving end, these signals are demultiplexed based on their distinct wavelengths. Thanks to advancements in gain-flattened EDFA technology, WDM has continued to evolve and become increasingly mature. In contrast, OTDM transmits only a single wavelength of light over a single fiber. Importantly, OTDM first requires that the optical pulses be in RZ format, with each signal occupying a different time slot before being combined into a single stream—essentially interleaving several pulses with varying delays relative to the initial pulse—to significantly boost the transmission rate of a single fiber. Both WDM and OTDM offer unique advantages, making it clear that combining the two technologies holds tremendous potential for further enhancing optical communication capacity. This integration is poised to become a key trend driving the future development of optical communications.