In
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
, a plesiochronous system is one where different parts of the system are almost, but not quite, perfectly
synchronised. According to
ITU-T
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
standards, a pair of signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. A sender and receiver operate plesiosynchronously if they operate at the same nominal clock frequency but may have a slight clock frequency mismatch, which leads to a drifting phase.
[S. Johnson, S. Scott: A Supercomputer System Interconnect and Scalable IOS, 14th IEEE Symposium on Mass Storage Systems, 1995, Footnote on p.358] The mismatch between the two systems' clocks is known as the plesiochronous difference.
In general, plesiochronous systems behave similarly to
synchronous system
In digital electronics, a synchronous circuit is a digital circuit in which the changes in the state of memory elements are synchronized by a clock signal. In a sequential digital logic circuit, data are stored in memory devices called flip-fl ...
s, except they must employ some means in order 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 hierarchy
The plesiochronous digital hierarchy (PDH) is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems. The term ''plesiochronous'' is ...
networking standard.
The
asynchronous serial communication
Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Instead of a common synchronization signal, the data stream contai ...
protocol is
asynchronous on the byte level, but plesiochronous on the bit level. The receiver detects the start of a byte by detecting a transition that may occur at a random time after the preceding byte. The indefinite wait and lack of external synchronization signals makes byte detection asynchronous. Then the receiver samples at predefined intervals to determine the values of the bits in the byte; this is plesiochronous since it depends on the transmitter to transmit at roughly the same rate the receiver expects, without coordination of the rate while the bits are being transmitted.
The modern tendency in
systems engineering
Systems engineering is an interdisciplinary field of engineering and engineering management that focuses on how to design, integrate, and manage complex systems over their enterprise life cycle, life cycles. At its core, systems engineering util ...
is towards using systems that are either fundamentally
asynchronous (such as
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
), or fundamentally
synchronous
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
(such as
synchronous optical networking
Synchronous optical networking (SONET) and synchronous digital hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diode ...
), and layering these where necessary, rather than using a mixture between the two in a single technology.
The term ''plesiochronous'' comes from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
πλησίος ''plesios'' ("near") and
χρόνος ''chrónos'' ("time").
See also
*
Buffer (telecommunication)
In computer science, a data buffer (or just buffer) is a region of a memory used to temporarily store data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device (such a ...
*
Clock drift
Clock drift refers to several related phenomena where a clock does not run at exactly the same rate as a reference clock. That is, after some time the clock "drifts apart" or gradually desynchronizes from the other clock. All clocks are subject to ...
*
Isochronous timing
A sequence of events is isochronous if the events occur regularly, or at equal time intervals. The term ''isochronous'' is used in several technical contexts, but usually refers to the primary subject maintaining a constant period or interval ( ...
*
Jitter
In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. Jitter is a significa ...
*
Mesochronous network
In telecommunication, a mesochronous network is a network in which the clocks run with the same frequency but unknown phases. Compare synchronous network.
See also
*Synchronization in telecommunications
*Isochronous signal
*Plesiochronous system
...
*
Synchronization in telecommunications
Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if telephone exchanges are not synchronized, then bit slips will occur and degrade performance. Telecommuni ...
References
{{reflist
Network architecture
Synchronization