plesiochronous

Plesiochronous describes a telecommunications timing environment in which multiple clocks run at nominally the same rate but are not perfectly synchronized. The term comes from the Greek roots plesios meaning near and chronos meaning time. In a plesiochronous network, each node has its own local oscillator that generates timing for its equipment, and timing references are distributed but not centrally synchronized to a single clock.

As clocks drift due to manufacturing tolerances and environmental factors, the rates of the interconnected data streams can diverge slightly. To maintain data integrity, network elements rely on buffering and occasional adjustments, such as frame slips or pointer adjustments, to compensate for drift. This arrangement is characteristic of the plesiochronous digital hierarchy (PDH), used historically before the widespread adoption of synchronous networking.

In PDH, while the nominal data rates are intended to be identical across nodes (for instance, DS1/DS3 in North America or E1/E3 in Europe), the mismatches between clocks mean frames may slowly drift relative to each other. The receiver must synchronize to the incoming framing, and adjustments may be required to preserve alignment of payloads. The lack of a single, shared timing reference distinguishes plesiochronous systems from fully synchronous networks.

With the introduction of SDH/SONET, networks moved toward tighter timing synchronization across nodes via common reference clocks, reducing inter-element drift. Consequently, plesiochronous operation is largely historical for modern telecom, though the concept remains relevant to legacy PDH equipment and certain timing practices.