ristics of SDH Equipment Slave Clocks (SEC) ITU-T G.813 outlines the minimum requirements of SDH equipment clocks. From the scope of the recommendation: This Recommendation contains two options for the SEC. The first option, referred to as "Option 1," applies to SDH networks optimized for the 2048 kbit/s hierarchy. These net- works allow the worst-case synchronization reference chain as specified in Figure 8-5 [of] G.803. The second option, referred to as "Option 2," applies to SDH networks optimized for the particular 1544 kbit/s hierarchy that includes the rates 1544 kbit/s, 6312 kbit/s, and 44 736 kbit/s. This recommendation includes the following main topics and appendixes: Frequency accuracy for Option 1 and Option 2 SEC nodes Pull-in, hold-in, and pull-out ranges for both options Noise generation, noise tolerance, and noise transfer Transient response and holdover performance Appendix I, "Guidance on the relationship between network limits and input noise tolerances" Appendix II, "Considerations on bandwidth requirements, noise accumulation and payload wander accumulation" The appendixes are very good reading if the topic is of interest. The main reason for coverage of this recommendation in this book is that it defines the frequency limits for the Option I and Option II SEC nodes (Ethernet, TDM, and optical) when you want to test the timing performance of a clock. G.813 is approaching 20 years old, although two corrigenda (corrections) have been published, in 2005 and 2016. To read the complete recommendation, you will need all three documents, which are accessible at the following link: https://handle.itu.int/11.1002/1000/13084. 8.2.1 Other Documents There is one document that does not quite fit well with the others in this physical frequency section, but it is quite important. G.703 is a recommendation that specifies the physical and electrical char- acteristics of interfaces. This is an important recommendation to help achieve interoperability between vendors and implementations. Of course, many of the 