t is that there might be unintended consequences downstream of the PTP time clocks that are very hard to predict. 9.2 Factors Impacting Timing Performance Chapter 6 covers many aspects of timing performance for the case where frequency is carried by a physical signal, and Chapter 5 covers many of the parameters and metrics that apply to the qualific- ation of synchronization performance. On the other hand, Chapter 8 goes into a lot of detail about the various recommendations defining performance, both for clocks/nodes and end-to-end networks. This section examines performance from the aspect of deploying packet timing for two different scenarios (both based on PTP): Packet-based frequency distribution Packet-based phase distribution Separate from the packet performance of PTP, you also need to look at the time error and wander values in the signal after it is recovered by PTP. Not surprisingly, all the metrics used to measure the quality of frequency using physical methods also apply to packet-based methods. And because most of the needs for frequency in a packet network come from interworking with TDM networks, the metrics between physical and packet methods have a lot in common. But the same goes for phase/time synchronization as well. The performance of the PTP packets and the quality of the frequency signal can have an impact on the time metrics for phase. Of course, it also depends on other things: the quality of the oscillator; the characteristics of the PLL; the band- width of the filtering; and the quality of the PTP servo. More details on the design of a PTP clock are provided in Chapter 12, but the next sections deal with the factors influencing timing performance. 9.2.1 Packet-Based Frequency Distribution Performance As mentioned previously, one of the major differences between a physical frequency distribution and a packet-based one is the amount of data available to recover the frequency. When using PTP for frequency, a packet rate of about 64 packets per second is the commonly accepted rate (at leas