SC with full timing support, a transient at the input results from a rearrangement of either the physical frequency signal (such as SyncE) or the PTP message flow. A rearrangement of the PTP packet timing signal occurs when a T-BC/T-TSC loses its current PTP message stream and switches to an alternate flow. This switchover likely results in a transient at the PTP timing input of T-BC/T-TSC. Similarly, a rearrangement of the physical frequency transport occurs when the source reference changes (for example, triggered by changes in the QL values received). Note that the rearrangement does not mean the input signal is lost; it is simply changed to a differ- ent path or source. The signal disappearing completely would not be a transient but would force the clock into holdover. When using two different reference signals (one each for phase and frequency) on a T-BC/T-TSC with full timing support, there are four transient scenarios: Rearrangement of both the PTP timing signal and the physical frequency signal Rearrangement of only the PTP timing signal Rearrangement of only the physical frequency signal Rearrangement of physical frequency signal-for the "long term," the definition of which is that the PRC/PRS traceable frequency source is lost for more than 15 seconds Currently, the ITU-T (in G.8273.2) has only defined a performance metric for point 3 (the re- arrangement of physical frequency), and the expected response for all other scenarios is for further study. For physical rearrangement, the recommendation defines an acceptable response mask only for T-BC/T-TSC Classes A and B. Figure 9-30 shows this mask and indicates the maximum limits of the allowed response. Time after start of SyncE rearrangement (s) Figure 9-30 Phase Error Limit for Physical Frequency Transport Rearrangement for T-BC/T- TSC Classes A and B-Based on Figure B.1 of ITU-T G.8273.2 Holdover Holdover defines the maximum deviation in the PTP and 1PPS output signal during the loss of the PTP packet timing signal and/or physical layer frequency input