The packet method for recovering frequency is somewhat slow and cumbersome, mainly because the amount of data it is receiving is limited to 16 sets of PTP timestamps a second (for G.8275.1). Meanwhile, purely from the bandwidth point of view, in the case of the physical signal, there are millions of reference data points (significant instances) per second. See Chapter 12 for details of the timing subsystem and the physical components involved in selecting, down-sampling, and aligning input signals. The second major benefit of using SyncE (or some other physical method) to carry frequency is that it is not affected by the nondeterministic behavior of packet networks. The SyncE signal is always available and stable, no matter what the traffic load on the network. The packet signal, on the other hand, suffers the consequences of whatever else is happening on the path. The other great assistance from SyncE is its ability to help the slave clock maintain very good hold- over performance. This holdover improvement arises because once a slave clock is aligned in phase, if both the master and slave clock are running at the same frequency, then the phase difference between them cannot change. Then when the PTP source of phase/time disappears, there is still a frequency signal to keep the oscillators aligned, SO the phase cannot move out of specification. Without the frequency source, the oscillator is left to cope on its own and is at the mercy of its own physics. These cases (with and without SyncE) are covered in more detail later in the chapter in the section "Holdover." SyncE clearly has real benefits, and for successful deployments, it is critical that the network ele- ments and transport systems can transport SyncE (and ESMC) correctly and that the PTP clocks can operate in hybrid mode (physical frequency plus packet phase/time). 9.1.1 PTP-Aware Nodes Versus PTP-Unaware Nodes The other aspect that makes the job of the network timing engineer easier is when the network, as far as is possible, consists of network elem