 |
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| What is
Network Synchronization? |
|
| Network Synchronization
is the means of ensuring that the signals of an entire digital network are
effectively clocked or synchronized by a single, well-defined set of related
frequencies.
In practice,
this involves the planning, designing and operation of each network and
sub-network to have all their systems/ subsystems clocked at the same
average rates and with an accuracy that is traceable to Universal Time
Coordinate (UTC).
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| Why a Synchronous
Network? |
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| Digital services
require real time end-to-end bit integrity, without any unnecessary signal
delays. The cost effective method of meeting these demands is to make all
interconnected networks operate as one synchronous network, by limiting
sub-networks from operating at the same nominal frequency and within a well
defined tolerance. |
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| What is
the Interconnection Hierarchy? |
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| The interconnection
hierarchy arranges the clocks in a master-slave order with the clocks of
highest performance at the top of the hierarchy and clocks at lesser levels,
called Stratum Levels, of performance at the lower levels of the hierarchy.
The master clock at the top of the hierarchy is called a Stratum 1 or G.811
clock and is embedded within a Primary Reference Source. The slave clocks
at the lower levels of the hierarchy are called Stratum 2, 3E, and 3, for
example, and are designed to be connected to an upstream clock so that its
average output frequency is equal to that of the input. |
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| What is
a Clock? |
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| Clocks are
devices used to provide timing and synchronization information to the equipment
elements of a digital transmission system or network. Clocks are embedded
within switching, transport equipment, or in stand-alone synchronization
equipment. |
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| What are
the types of clocks? |
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• Master
clock - Primary Reference
• Source Slave clocks - All others |
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| What are
the Modes of Operation of One Slave Clock? |
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| There are
three modes of operation of these slave clocks:
• normal
(synchronized),
• holdover, and
• free-run,
• plus possibly some special cases of transition between these modes.
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|
| What is
the normal mode? |
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• The
normal mode is when the clock is synchronized with one of the references.
• In the mode, the long-term average accuracy of the clock is equal
to that of its synchronization source, which is normally one part in 10
-11 or better. |
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| What is
the holdover mode? |
|
| The holdover
mode is the operating condition of a clock that has lost its controlling
input and is using stored data acquired, called history, while in normal
operation to control its output frequency.
The accuracy
of a good clock in the holdover mode starts out equal to the normal mode
accuracy, and over time may slowly drift from this frequency, as determined
by the stability of the clock’s internal oscillator.
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| What is
the free-run mode? |
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| The free-run
mode is the operating condition of a clock whose output is totally internally
controlled. The clock has never had, or has lost, all external reference
input, and has lost all data from a previously connected source.
The specifications
for free-run accuracy apply over the lifetime of a clock, typically for
20 years. The need for specifying free-run accuracy is to enable the specification
of pull-in ranges of slave clocks to assure that, under worst-case conditions
of lost reference sources for an extremely long time, the frequency of
a clock cannot drift beyond the pull-in range of the downstream clock
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