at same topology (on the right), the tester can also measure the frequency wander that has accu- mulated in the transit across the network from the PRC/PRS. This requires only a 10-MHz reference signal on the tester. On the DUT, the tester measures either a SyncE signal or a E1/T1/2048-kHz signal from a BITS or TDM traffic port. This test signal is then compared to the wander require- ment from the application (and the relevant ITU-T recommendation). Because a reference signal is needed to measure phase alignment and frequency wander, some test- ers include a GNSS receiver inside the device. This is especially the case with field testers because they typically do not have any reference source available at a remote location. Engineers use these devices by first taking the tester outside and allowing the GNSS to align an accurate built-in oscil- lator. Then the engineers take the tester back inside to test the equipment, and the oscillator goes into holdover when it loses the GNSS signals. If the test device spends only a reasonably short in- terval in holdover, then it can accurately measure the phase (and frequency). When testing with a topology using two different GNSS receivers, where one is connected to the input of the test device(s) and the other is connected to (or integrated with) the tester, there is an additional error between the two receivers that must be considered. This is discussed further in Chapter 11 when considering GNSS timing sources for the front-haul network. One important detail that is often overlooked when trying to accurately measure phase (it does not matter for frequency) is cable length compensation. When the tester is measuring a PTP signal, it is not trying to recover the clock by solving the PTP equations and estimating the mean path delay. The tester just reads the timestamps in the event messages and applies the correctionField (CF). Therefore, the time taken for the PTP signal to cross the cable between the DUT and the tester needs to be compensated for to get an accurate readin