Plesiochronous system: Difference between revisions
Omnipaedista (talk | contribs) rv; the grammar was correct |
The sentence begins ''is derived...'', without the accusative plural form -chronous- gives a mess meaning and needs punctuation |
||
| Line 1: | Line 1: | ||
The term '''Plesiochronous''' is derived from the [[Greek language|Greek]] πλησίον ''plesion'', |
The term '''Plesiochronous''' is derived from the [[Greek language|Greek]] words: πλησίον ''plesion'', "near" and χρόνος ''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, [[synchronization|synchronised]]. |
||
According to [[ITU-T]] standards, corresponding signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. |
According to [[ITU-T]] standards, corresponding signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. |
||
Revision as of 13:02, 11 July 2012
The term Plesiochronous is derived from the Greek words: πλησίον plesion, "near" and χρόνος 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.
According to ITU-T standards, corresponding 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 frequency but may have a slight frequency mismatch, which leads to a drifting phase[1][2]
In general, plesiochronous systems behave similarly to synchronous systems, 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 networking standard.
The Asynchronous serial communication 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 synchronizaton 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 is towards using systems that are either fundamentally asynchronous (such as Ethernet), or fundamentally synchronous (such as Synchronous optical networking), and layering these where necessary, rather than using a mixture between the two in a single technology.
See also
References
Contains material from FOLDOC, used with permission