nput phase and time references, then it enters phase/time holdover, and the clock starts to drift. To attempt to keep the phase/time aligned, the PRTC must rely on the hold- over performance of its local oscillator, which may be good but not great. Many vendors selling dedicated PRTC clocks offer an optional high-quality oscillator (frequently rubidium) to improve the holdover performance. Another option is to connect an optional external input frequency reference obtained from a PRC/PRS. So, many PRTC devices can accept an input 10-MHz signal from a PRC/PRS, which can be used to improve holdover. The other level of performance available for time sources is the enhanced version, the ePRTC. The level of accuracy required to meet this level of performance is defined as +30 ns, considerably bet- ter than PRTC-B. But unlike the other two cases, the ePRTC has a strict holdover requirement in ITU-TG.8272.1. To meet the ePRTC levels of performance in a GNSS receiver typically requires the addition of a very good PRC (such as a cesium clock) as a frequency source for the receiver. In this case, when the source of phase/time is lost, the PRC can provide the stability necessary to meet the holdover that the ePRTC is required to meet. In fact, G.8272.1 defines two classes of holdover, with ePRTC becoming ePRTC-A and a new ePRTC-B class of holdover performance (at time of writing, the val- ues are not yet decided). Table 3-4 summarizes the different classes. Table 3-4 Classes of PRTC Performance Class of Clock Accuracy Holdover PRTC-A +100 ns PRTC-B +40 ns ePRTC-A +30 ns 100 ns / 14 days ePRTC-B +30 ns For further study Beyond the frequency output (see the preceding section "Sources of Frequency") the PRTC is also required to supply timing signals for phase and time. This is supplied as two additional physical sig- nals in addition to the frequency output, although some implementations multiplex these two signals together. Other implementations use different pins on a single cable (such as V.11 and RJ-45) to run more than on