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SYNCHRONIZATION OF DIGITAL FACILITIES B-5
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synchronized together. However, to provide higher quality synchronization performance, stratum-3 clocks
are currently being used with some customer-premises equipment.
Because of recent changes within the synchronization hierarchy, stratum-4 clocks are now defined as
stratum-4E or stratum-4. The specifications for stratum-4E define limits that minimize and control phase
changes that occur while switching from one synchronization source to another. However, stratum-4 clocks
do not comply with this specification and all other stratum-4 specifications remain the same for both clocks.
It is recommended that a stratum-4E clock be used as a synchronization source for public network
connections (see Technical Publication 62411).
Each stratum, from 1 to 4, represents a progressively less stable and less expensive clock. Within AT&T,
there is a system of stratum-1 clocks. These clocks use the AT&T standard reference frequency, formerly
the Bell System reference frequency. The stratum-1 output is transmitted to various public digital network
nodes via either broadband analog facilities or the DATAPHONE Digital Service (DDS).