section "Field Testing" later in this chapter for details on how to achieve this. For frequency synchronization, depending on the application, external information might quickly give an indication that the frequency is wandering out of range. A good example would be for cir- cuit emulation, where a frequency mismatch would quickly lead to errors (such as slips) on the TDM attachment circuits or underruns or overruns in the dejitter buffers. Standard methods of mon- itoring circuit and interface statistics would quickly detect these anomalies. For many other cases, or for pre-deployment compliance testing, the engineer might like to gain confidence in the quality of the delivered frequency through some tangible metrics. Figure 12-13 shows the basic topologies for testing using two different topologies with an appropriate test device. Relative Network Testing Absolute Network Testing Reference Reference Tester input Tester input EEC/eEEC EEC/eEEC input output SyncE SyncE SyncE EEC/eEEC 10 MHz output EEC/eEEC Could also be SyncE input BITS or SyncE 10 MHz SyncE network SyncE network Figure 12-13 Testing Frequency Wander in a Network The left-hand case, relative network testing, compares an output signal at the end of the network chain against the input signal produced by the tester, but with a reference frequency (PRC) as the ultimate source. The right-hand case, absolute network testing, measures the frequency signal at the end of a network chain after having been supplied by a separate source at the beginning of the chain. Absolute network testing reflects a more realistic case because the end of the chain is normally loc- ated away from the source of frequency, such as would be the case when field testing. Remember that these constraints on the testing arise because of considerations around the maximum length of the cables. Reference inputs (such as 10 MHz) can run only over very short cable lengths, SO the rel- ative locations of the tester and the PRC need to be considered. If they are not co-located, then an a