AT&T DS1/DMi/ISDN-PRI manual Network Synchronization and Engineering

5-18SYNCHRONIZATION OF DIGITAL FACILITIES

NETWORK SYNCHRONIZATION AND ENGINEERING

The primary goals of network synchronization are:

●To keep each digital network node reliable

●To make sure that each digital termination can meet the network objectives

The procedures to achieve these goals include:

●Ensure that all nodes and facilities are synchronized to a single source of timing; or at the worst, to two or more stratum-2 timing sources

●Select the most reliable digital facilities to serve as synchronization references at each node

●Choose facilities with the greatest availability and least outage. For example, facilities that are located in hostile environments or that have a history of service disruptions should not be used.

●Ensure that no timing loops can be created even under failure conditions, that is, timing from one node can never serve as a source of timing back to that node, even if it is looped through several other nodes

Selecting a Timing Source for the Switch

Before selecting a timing source, you should draw or obtain a copy of the network topology (which indicates each node and the interconnecting digital trunk facilities). The nodes in the private network should be stratified (divided) into two levels known as externally referenced and internally referenced (sometimes called unreferenced). Externally referenced nodes are those within the private network that connect directly to public network timing sources. Internally referenced nodes have digital facilities that do not connect directly to public network timing sources. Figure 5-11,External and Internal Reference Levels, shows a typical network topology with externally and internally referenced nodes.