Zeitmultiplex

Zeitmultiplex, literally time multiplexing in German, is a method of transmitting multiple signals over a single communication channel by allocating each signal a distinct time slot within a repeating frame. The English term is time-division multiplexing (TDM). In practice, the slots are synchronized between transmitter and receiver to preserve the sequence of samples.

Operation consists of sampling each input signal in turns and inserting the samples into frame slots. The demultiplexer at the destination uses the frame timing to recover the original signals. In synchronous TDM, a fixed frame with a fixed number of slots is used, yielding predictable latency and simple hardware. In statistical or adaptive TDM, slots are allocated according to demand, improving efficiency when traffic is bursty but requiring more complex control.

Historically, Zeitmultiplex has been central to digital telephony. In PDH systems, fixed-slot TDM carried channels such as E1 at 2.048 Mbps (32 × 64 kbps) and T1 at 1.544 Mbps (24 × 64 kbps). Modern networks employ SDH/SONET, where higher-order multiplexing combines many tributaries into high-rate frames (for example, STM-1 at 155.52 Mbps) and uses virtual containers with synchronous framing. The concept remains a building block in some backhaul and carrier architectures, even as packet-based transport becomes more prevalent.

Advantages of Zeitmultiplex include efficient use of a fixed-capacity channel, straightforward hardware, and predictable per-channel latency. Limitations involve inflexible channel bandwidth, potential idle slots in low-traffic conditions, and the need for precise synchronization. In many contemporary networks, time-division multiplexing coexists with or is supplanted by packet-switching and dynamic bandwidth allocation, though TDM remains foundational in optical transport and hybrid systems.