eaning that no intentional impairment is applied to the signal. For other tests, the signal can be altered (for example, to test noise tolerance or transfer). As this is a back to back test, a 1PPS reference is not really needed because the tester generates its own phase and then reads it back from the DUT to de- termine a relative result. The method of testing of the two clocks (T-BC versus T-TSC) is quite similar except for the follow- ing basic differences: There is no PTP master port on a T-TSC (by definition); therefore, it cannot send the tester a PTP output signal. 1PPS is the only option available to measure phase on a T-TSC, whereas it is optional on the T-BC. The T-BC is the most important (and numerous) component of the timing delivery network, and the function of the T-BCs is to transport PTP, SO it makes sense to meas- ure their output time signal using PTP. On a T-BC, the frequency is usually measured on the same port as PTP. If you can configure SyncE output on the T-TSC, you can measure frequency signals like any other EEC. Otherwise, some TDM interface such as a BITS port could be used to measure frequency (on both the T- BC and T-TSC). No PTP output limits are defined for the T-TSC because it has no role in passing on PTP timing signals. For this reason, it makes little sense for most operators to test a network element as a T- TSC because the end application is not hosted in the T-TSC router. (In the real network, the T- TSC would not be in a router.) In the real network, the application device (for example, a radio transmitter) would contain the final T-TSC in the chain, and all the network elements would be configured as T-BCs. One drawback is that the radio unit (containing the T-TSC) might not be equipped with a 1PPS out- put port to measure its phase. In cases of general-purpose compute servers, getting a 1PPS sig- nal output is difficult (it is available only on a NIC in the server). For radios, another option might be to measure the radio signals output from the transmitter to determine 