here is also another PTP clock type called a transparent clock (TC), which will be covered later. Chapter 7 provides comprehensive coverage on PTP clock types, including TC clocks. Note that the PTP port is a logical entity on a PTP clock that has a number of possible states, the most important two being whether it performs either as a master or a slave. This has little to do with the network interfaces (sometimes also called ports) on a router or other device. The PTP port is logical entity that participates in a flow of PTP timing messages. With some implementations of PTP, the PTP port can correspond to a physical network interface, but even then, you should treat it as a separate entity entirely. 3.3 Sources of Frequency, Phase, and Time Now that the components and concepts behind timing distribution have been discussed, it is time to see how those components work together to make a functioning synchronization network. The place to start is at the beginning of the chain, where the components of the time signals are sourced. The two sources of synchronization information are the PRC/PRS (used as sources for frequency) and the PRTC (used as a source for frequency, phase, and time). The most deployed and cost-effect- ive versions of both device types are based on a GNSS receiver, SO this section deals with the satel- lite systems behind them. 3.3.1 Satellite Sources of Frequency, Phase, and Time Atomic clocks are very stable and accurate sources of frequency over the long term, which is why they are the basis behind all timing services available from the various GNSS systems. The GNSS systems generally work by sending out specially encoded timing signals from orbiting satellites, as well as orbital information about each of the satellites in the constellation. These signals are radio waves in several specially reserved chunks in the 1.15-GHz to 1.6-GHz frequency range in what is known as the L band (they are predominately one-way signals). With a clear view of the sky, the GNSS receiver can simultaneously receiv