AT&T DS1/DMi/ISDN-PRI manual

SYNCHRONIZATION OF DIGITAL FACILITIES 5-5

MASTER REFERENCE

FREQUENCY

NOTE

SLAVE

NODES

DIGITAL TRUNK

PRIMARY REFERENCE

NOTE: The dashed lines indicate which nodes supply reference frequencies and those facilities used to transmit the reference.

Figure 5-2.Synchronization Hierarchy

Switching nodes in digital networks are divided into synchronization layers called strata. There are four strata, 1 to 4, where stratum 1 has the highest accuracy and stratum 4 the lowest. Public digital networks use stratum 1, 2, and 3 synchronization. Historically, private digital networks used stratum-4 clocks all 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-4 type I or stratum-4 type II. The specifications for stratum-4 type I define limits that minimize and control phase changes that occur while switching from one synchronization source to another. However, stratum-4 type-II clocks do not comply with this specification and all other stratum-4 specifications remain the same for both type-I and type-II clocks. Beginning in 1990, public-network connections cannot use a stratum-4 type-II clock as a synchronization source (a stratum-4 type-I clock can be used).

Each stratum from 1 to 4 is 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).