AT&T 2500 series, 7500 series, 7100 series, 7200 series manual 4SYNCHRONIZATION of Digital Facilities

B-4SYNCHRONIZATION OF DIGITAL FACILITIES

____________________________________________________________________________________________________________________________

_

For the hierarchical method, a node containing a very stable reference frequency is identified as the source or master reference. The master reference is transmitted to another node that is synchronized (slaved) to this master reference. A network implementing this method is hierarchical in nature with nodal clocks supplying the synchronization frequency to certain other nodes, which in turn supply the reference to still other nodes. Figure B-2,Synchronization Hierarchy, shows a hierarchical network configuration.

With the hierarchical method, the existing digital transmission facilities are used to distribute the reference frequency. For example, with a T1-carrier, the slave nodes can derive their reference clock from the 1.544-Mbps rate. Since the clock frequency is derived from the digital bit stream, the traffic-carrying capacity of a carrier system is not diminished (that is, the bandwidth is not used to carry a separate clock signal).

Reliable operation is an important consideration for all parts of a telecommunications network. So, the synchronization network should consist of both primary and secondary synchronization facilities. In addition, each node must be equipped with an internal clock that (with automatic switching) can bridge short disruptions of the primary and secondary synchronization reference. Therefore, if synchronization is disrupted, the internal clock will assume control. The internal clock will drift at a rate determined by its stability (also called free run ability or accuracy).

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 B-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