Plesiochronous
Encyclopedia
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
.
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
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.
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
.
According to ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
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
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
Plesiochronous Digital Hierarchy
The Plesiochronous Digital Hierarchy 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...
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
Systems engineering
Systems engineering is an interdisciplinary field of engineering that focuses on how complex engineering projects should be designed and managed over the life cycle of the project. Issues such as logistics, the coordination of different teams, and automatic control of machinery become more...
is towards using systems that are either fundamentally asynchronous (such as Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
), or fundamentally synchronous
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....
(such as Synchronous optical networking
Synchronous optical networking
Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...
), and layering these where necessary, rather than using a mixture between the two in a single technology.