, PTP) Network Time Protocol (NTP) Other packet methods Synchronous Ethernet (SyncE) Global navigation satellite (GNSS) receiver Needs calibration Needs calibration Atomic clock (GNSS) (GNSS) This chapter deals with timing transport using packet methods, SO it concentrates on PTP, although it briefly covers NTP and some other packet methods. An additional resource is Annex A, "Using PTP," of IEEE 1588-2008, which covers the highlights of issues that may arise during PTP deployments. The flow of this chapter starts from the head of the timing network and proceeds down toward the application using the timing signal. Therefore, the place to start is at the head of the timing distribu- tion network, and that is with the source of the timing signal to be carried. 9.1.1 Physical Inputs and Output Signals This section begins with the source of timing signals that are to be transported across the network. "Chapter 3. Synchronization and Timing Concepts", introduces the various forms of primary refer- ence clock (PRC), primary reference source (PRS), and primary reference time clock (PRTC) and their associated signals. This section expands on Chapter 3 with more specific details on their usage and implementation. Frequency Inputs and Outputs (10 MHz, BITS, and so on) Here is a summary of the major factors to consider when examining the source of timing signals as well as some common pitfalls when doing SO: Ensure the cabling and connectors are correct and that they properly coexist (for instance, by matching signal characteristics such as impedance and gain/loss). Choose signal types and understand the differences in how they are configured. Provide quality level (QL) traceability and ensure the configuration matches the type of quality levels being passed. If the signal has no QL values, then configure the receiving node to assume an expected QL to use whenever the signal is present. At the start of a packet-based frequency distribution network, there will be a master packet-based equipment clock (PEC-M) or PTP grandmast