 9-39 Providing Resilient Backup Timing for Electricity Substations What is the requirement for phase/time accuracy in the power application? That is a difficult ques- tion to provide a single answer, because a lot depends on the specific application. The explanation from customers is that the closer the alignment the better, but certainly requiring accuracy to the level of microseconds is a common request. So, once again, it is important to understand the end-to- end budget. A similar approach to providing some resilience to the local GNSS primary source is an increas- ingly common project under way in many industries, although the power industry is especially act- ive. Every industry has several of their own special issues or requirements to deal with, but there is usually a vibrant vendor community who supports those needs and understands those trade-offs. What they are looking for is help from people like you to bring the expertise in carrying a timing signal over the wide area using packet-based techniques. The tricky bit is usually where the two parts come together. 9.10 Summary This chapter covered a lot of ground, focused on real-world considerations when designing and de- ploying a timing synchronization network using PTP and SyncE/eSyncE. It starts out with the be- ginning of the timing chain, the sources of time and the physical signals and connectors used. In- creasingly, the use of the time signal is done by an application with an embedded slave clock, SO there is decreasing need for physical cabling at the edge of the network. As you will see in Chapter 12, testing and accuracy verification using BITS or 1PPS ports is the exception. The chapter then covered several methods to carry frequency signals across the packet network, both using packet techniques (including PTP) and using physical methods in a packet network (SyncE). Some of the packet techniques are especially suitable for specialized use cases such as cir- cuit emulation of TDM circuits. The next section covered the transport of phase/time