The term Plesiochronous is derived from the Greek "plesio", meaning near, and "chronos", time, and refers to the fact that plesiochronous systems run in a state where different parts of the system are almost, but not quite perfectly, synchronised.
ITU-Tstandards, corresponding signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. In general, plesiochronous systems behave similarly to synchronous systems, except that they must have some means to cope with "sync slips", which will happen at intervals due to the plesiochronous nature of the system.
The most common example of a plesiochronous system design is the
Plesiochronous Digital Hierarchynetworking standard. The well known Asynchronous serial communicationis asynchronous only on byte level, but plesiochronous on the bit level.
The modern tendency in
systems engineeringis towards using systems that are either fundamentally asynchronous(such as Ethernet), or fundamentally synchronous(such as SDH), and layering these where necessary, rather than using a mixture between the two in a single technology.
A sender and receiver operate plesiosynchronous if they operate at the same nominal frequency but may have a slight frequency mismatch, which leads to a drifting phase [P. Teehan, M. Greenstreet, G. Lemieux: A Survey and Taxonomy of GALS Design Styles, IEEE Design & Test of Computers September-October 2007, p.419] [S. Johnson, S. Scott: A Supercomputer System Interconnect and Scaleble IOS, 14th IEEE Symposium on Mass Storage Systems, 1995, Footnote on p.358]
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