d T-GM combination. Because the PTP servo on that clock has a GNSS reference available, it can measure the difference between the phase de- livered by the GNSS and the phase recovered from the PTP slave port. This measured difference is mostly made up of asymmetry, either dynamic or static (more on that in the following section "Asymmetry"). The amount of asymmetry is constantly measured and retained by the PTP slave. If the local GPS receiver ever becomes unavailable, then the PTP will select that PTP slave port as its source of phase/time. The extra step here is that the servo then applies the latest value for asymmetry as a cor- rection to the incoming signal. This compensates for any asymmetry that was present when that measurement was taken. Of course, any change in that error resulting from any subsequent rearrangements and transients in the network between this clock and the remote T-GM cannot be measured after the GPS reference disappears. It cannot be a long-term solution, but as a short-term emergency measure to add a level of resilience, it has merit. There is more information on this solution in the section "Assisted Partial Timing Support with G.8275.2" in Chapter 11, as it is particularly aimed at the mobile operator. 9.1.1 Leap Seconds and Timescales In talking with customers over the past year or so, most of whom are medium to very large MSPs, along with numerous cable operators, a couple of questions come up repeatedly: How do systems handle the leap second event, and what must the operator do? Why bother with the whole issue of leap seconds? The solution suggested by some is that it would be far simpler to just run some local wall-clock time and forget about all the complexities with different timescales. The following sections deal with both these issues. Leap Seconds Both "Chapter 1. Introduction to Synchronization and Timing", and Chapter 7 covered the issue of leap seconds in some detail from the background point of view. But exactly how is the leap second treated in an operational environme