Chapter 5. The Stratum HierarchyA Stratum 1 clock may control strata 2, 3E, 3, 4E, or 4 clocks. A Stratum 2 clock may drive strata 2, 3E, 3, 4E, or 4 clocks. A Stratum 3E clock may drive strata 3E, 3, 4E or 4 clocks. A Stratum 3 clock may drive strata 3, 4E or 4 clocks. A Stratum 4E or 4 clock is not recommended as a source of timing for any other clock system. This hierarchy is shown in Figure 3. Because of the narrower capture and adjustment range of the higher strata clock systems (2 is higher than 3, and so on), driving a Stratum 2 clock from a Stratum 3E or 3 clock is not recommended. In fact, it will not work under some transmission impaired conditions. Also, extreme care must be taken in network applications where more than one Stratum 1 source is used to ensure that these sources are accurate and traceable to some other standard. Another standard commonly used to check on a Stratum 1 clock source's accuracy is the GPS System. A GPS receiver can also be used directly as a source of Stratum 1 quality. Stratum 1 clock administration, operation, and maintenance can be a costly effort. Atomic sources may not have long maintenance-free operating intervals, and may experience failures without giving an indication that the source is off frequency. In addition, if a Stratum 1 source of timing is shown to be inaccurate, the network must be able to accept another network's timing until the problem is corrected. Thus, GPS is attractive in order to assure accuracy and minimize cost.
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