 as a primary input, it moves to holdover when both the local source and the backup PTP timing signal from the network are lost. The most likely cause of PTP loss is some network fault or misbehavior. In this case, without any reference signal present, the holdover performance directly reflects the per- formance of the oscillator. This state is referred to as holdover based on oscillator, and good phase holdover cannot be achieved without including at least stratum 2 level (expensive) oscillators. Be- cause network elements at the edge of a network are not normally SO equipped, the phase holdover cannot stay in specification for very long. Network clocks commonly have stratum 3E clocks as a compromise (and may exceed 1 us within a few hours). Because holdover is important, good design ensures that having no PTP source available must only be allowed to occur for a short interval. So, holdover using the local oscillator is only expected to be a short-term temporary measure to maintain phase/time temporarily until the problem is rectified. G.8273.4 specifies a mathematical function that limits the phase error of T-BC-A/T-TSC-A during holdover based on oscillator. Without getting into mathematical details of the function itself, it is sufficient to understand that the permissible phase error is a time-based function that depends upon the following: The initial frequency offset Temperature variation, which has an adverse impact on the oscillator stability and hence on hol- dover performance The drift of the oscillator as the elapsed holdover time progresses A constant offset, which is specified as 22 ns Figure 9-32 illustrates the G.8273.4 limit on phase error that a T-BC-A/T-TSC-A should display during holdover based on oscillator. The graph illustrates the limit for a clock at constant temperat- ure and a maximum observation interval of 1000 S. The graph in Figure 9-32 also illustrates how quickly the phase can degrade-starting from 22 ns at the start of holdover and crossing 1 us within 1000 S. To get a true sense 