k summary of the differences between the physical and packet methods to carry frequency and the impact of the choice on the quality of the frequency recovered at the end of the timing chain. Although the metrics have a lot in common, they have different characteristics. They fall into three major categories: Bandwidth of the signal: The packet rate used by frequency over packet is in the range of 1- 128 messages per second or 128 Hz (for PTP). For an E1 timing signal, it would be 2 MHz, whereas for SyncE it could be many megahertz. Therefore, the rate at which timing data can be transferred varies widely between the two methods. Regularity: The physical signal is very regular at short time scales, whereas the packet rate may vary considerably over short intervals. Yes, a packet stream may have a very consistent av- erage rate over time, but PDV will cause the intervals between messages to be irregular. Noise characteristics: For packet timing, the main cause of timing noise is PDV, which has a larger amplitude and distribution than the jitter and wander in a physical signal. Packet timing relies on not just the network elements but also the transport and the topology. All these factors make the noise more course-grained, complex, and difficult to predict and counteract. Traffic insensitivity: Changing traffic patterns will affect the PDV generated in the packet tim- ing signal, whereas the SyncE signal is unaffected in any way by the flow of packets. The timing signals from the physical layer are usually mostly static with a Gaussian (bell curve) shaped distribution (G.8260); therefore, it makes sense to use every significant instant (data point) available to filter out as much noise as possible and achieve the best stability. In contrast, packet-based timing signals are much more dynamic and do not have a Gaussian distri- bution (for an example, see Figure 9-16 later in the chapter, in the section "Packet-Based Frequency Distribution Performance"), meaning that there are far more outlying data points. Therefore,