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5-4
SYNCHRONIZATION OF DIGITAL FACILITIES
3.
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Minimum costs
Ease of administration
Ease of maintenance
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 5-2, 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 either the 56K-bps data rate or the 8000-frames-per-second 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).