Plesiochronous (pronounced plee-see-AH-krun-us, from Greek plesos, meaning close, and chronos, meaning time) is an adjective that describes operations that are almost, but not quite, in synchronization - in other words, almost synchronous. The term is used in the Plesiochronous Digital Hierarchy (PDH), the widely-used system in which the transmissions from one continent (such as North America) are internetworked with transmissions in other continents (such as Europe) by making small adjustments in the differing data rates between the systems.
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A plesiochronous system could arise as the result of two systems having slightly different clock readings over time. In such a system, one of the systems or a third system would need to notice the mismatch and make some compensating adjustment, such as repeating or deleting a data packet or frame.