AT&T DEFINITY 7200 series Guia do Utilizador descarregar pdf (Página 213)

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).

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AT&T DEFINITY 7200 series Guia do Utilizador Página 213