Chapter 7. Typical Clock Systems

Clock system arrangements are based on intended use. A long distance carrier may elect to provide and administer Stratum 1 clock sources at all major toll switch locations. Another less costly method is to provide one or two Stratum 1 sources, followed by a distribution of the timing information on dedicated DS1 paths to Stratum 2 systems at each switching center. Some long distance carriers are using the Global Positioning Satellite (GPS) system to discipline rubidium sources. These sources would then distribute timing to Stratum 2 clock systems. In some cases, providing each location with a Stratum 1 GPS system is the most efficient solution.

Local telephone companies may choose one DS1 path from one incoming group as a primary reference, and another DS1 path from another incoming group as a secondary reference. These two references feed a Stratum 2 system at the toll level, and a Stratum 3 system at the local switching system level. Such a system provides reliable timing at the local level. As an alternative, many local telephone companies, because of problems they have experienced while using someone else's timing, are beginning to consider the user of Stratum 1 systems of their own.

Another method of distribution timing is via older analog microwave links. Many of these furnish a source of synchronization, which is transmitted as sub-carriers for synchronizing the single side band (SSB) and frequency division multiplex (FDM) equipment. These pilots may be converted to 1.544 Mb/S DS1 framed ones signals for use in distribution and timing of newer digital equipment, which may be using supergroups on the existing microwave to provide digital services.

A private network may require a Stratum 3E clock at each location to provide a local source of timing. The Stratum 3 clocks are referenced to the telephone company by bridging one of the incoming DS1 signals to extract timing information.