Tektronix CTS 850, User Manual

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Functional Overview
CTS850 SDH/PDH Test Set User Manual
2–27
Synchronous, Plesiochronous, Asynchronous
To understand correctly the concepts and details of SDH, it is
important to be clear about the meaning of Synchronous, Plesiochro-
nous, and Asynchronous.
In a set of Synchronous signals, the digital transitions in the signals
occur at exactly the same rate. There may however be a phase
difference between the transitions of the two signals, and this would
lie within specified limits. These phase differences may be due to
propagation time delays, or wander introduced in the transmission
network. In a synchronous network, all the clocks are traceable to
one Primary Reference Clock (PRC). The accuracy of the PRC is
better than +1 in 10
11
and is derived from a cesium atomic standard.
If two digital signals are Plesiochronous, then their transitions occur
at ”almost” the same rate, with any variation being constrained
within tight limits. These limits are set down in ITU–T recommenda-
tion G.811. For example, if two networks need to interwork, their
clocks may be derived from two different PRCs. Although these
clocks are extremely accurate, there is a small frequency difference
between one clock and the other. This is known as a plesiochronous
difference.
In the case of Asynchronous signals, the transitions of the signals do
not necessarily occur at the same nominal rate. Asynchronous, in this
case, means that the difference between two clocks is much greater
than a plesiochronous difference. For example, if two clocks are
derived from free–running quartz oscillators, they could be described
as asynchronous.