AT&T 7200 series, 7500 series, 7100 series, 2500 series manual Synchronization of Digital Facilities

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-4Eor 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).