the quality and freshness of holdover data. Quality/stability of the reference signals received before holdover begins (PTP only, SyncE, eSyncE). This is a similar effect to that of the quality of receiver. Environmental factors, most especially temperature, sunlight/shadow, and airflow. Diurnal factors, although some of these are environmental, such as temperature cycles. Whether the clock in holdover has any assistance from a physical source of frequency (espe- cially SyncE or eSyncE). Looking beyond the design of an individual clock or network element, the topology and design of the network can have a significant impact on the quality and length of the holdover. For most de- ployments the last bullet point is the most crucial. It has already been mentioned that SyncE is an important contributor to holdover performance, but now it needs an explanation. In hybrid mode deployments, the physical frequency is driving the "tick" of the clock (refer to Fig- ure 9-18 in the section "Dynamic Time Error"). If this signal continues to be available after the clock enters holdover, then the rate of progress of the clock continues to track the input reference. Given that the phase is taken from these oscillations, the phase does not have much opportunity to drift. This is how SyncE helps achieve much improved holdover times. Once the slave is phase aligned, if the phase/time reference signal (meaning PTP) disappears but the SyncE is still available, then the holdover performance will be orders of magnitudes better than without the signal-because without it, the clock can only rely on the physics of its own oscillator combined with the holdover data learned during its locked time. There is a similar advantage to using eSyncE over SyncE because of the improved performance of the physical signal that the holdover clock will receive. It has two benefits: Because the physical signal is better, the oscillator on the slave clock results ends up being more stable. When using eSyncE for assisted holdover, the reduced wander of the p