though they are quite similar in many respects, the carriage of phase/time will probably not succeed without significant mitigation and refresh. This is especially the case for asymmetry because it is not a significant factor when recovering only frequency, but it needs to be very tightly controlled for phase/time. The second point from the preceding quote is that using PTPoIP over unaware PTP nodes must be limited to very well-controlled cases with very good packet performance and only a small number of hops. 9.1.1 Hybrid Mode Versus Packet Only At every opportunity, this book proposes that the deployment of PTP (to carry phase/time) should occur in combination with SyncE (to carry frequency). When it comes to deployment, the timing engineer needs to understand whether this is possible, and what are the trade-offs for either approach. There is a somewhat semitechnical way to describe the reasons for the difference in performance when using a physical method for frequency. When setting out to synchronize a slave clock, the timing system needs to perform two separate functions. First, the slave must get the local oscillator running as close to its (correct) nominal frequency as possible (20 MHz has been used as a typical reference oscillator in earlier examples). To achieve this goal with a packet-only method, the slave must watch the clock for a while and fig- ure out if the time is going too quickly or slowly, then adjust the local oscillator, wait for a while, and retest. This is necessary because the data from the master is irregular (non-Gaussian) and of low bandwidth because of the limited message rate. The slave has only the periodic timestamp to guide it and, given the variability in the packet network, that data can move around somewhat. But with a physical frequency signal, such as the 125-MHz signal from gigabit Ethernet, the slave can recover the frequency from the incoming SyncE, and dividing it by the appropriate factor gives the timing subsystem a very clean reference for the phase-locked loop (PLL)