 clock that distributes the time synchronization signals to other clocks in the network. This master clock is called the grand- master (GM) clock, and it will have a PRTC giving it a stable frequency source and traceability to UTC. The basic rule is that because it is a grandmaster, it cannot take time information from another clock (except, of course, the PRTC reference). Be aware of the distinction between a PRTC and a GM clock. The PRTC is a source of frequency, phase, and time (aligned to UTC) and the GM is a PTP packet-timing function-it provides timestamped PTP packets. These two different roles can be housed in separate devices; combined into a single device (PRTC+GM); or embedded inside a network element such as a switch or router. PTP defines a packet interface-called a port-that under normal operations performs a principal role based on its state: A port in master state: A master port on a clock transmits a packet time signal to a slave port on a downstream clock. A port in slave state: The slave port receives that signal from an upstream master port. PTP defines any clock with a single port, master or slave, as an ordinary clock (OC). So, the GM in a PTP network is an OC, as it has only a single master port. Typically, the slave at the bottom of the hierarchy is also an ordinary clock, because it can only be a slave clock and has a single slave port. However, there are clocks with multiple ports that support both functions-being a master clock to downstream clocks as well as a slave to upstream clocks. Such clocks are known as boundary clocks (BC). The PTP BC has the capability to recover the time received on the slave port, use it to discipline its own timing circuits and distribute it to the connected slave network elements. Figure 3-13 illustrates the various ports and clock types. Frequency, phase, and time Grandmaster (ordinary) clock Master port Packet network Slave port Boundary clock Master port Slave port Slave (ordinary) clock Figure 3-13 GM, BC, and Slave Clocks with Master and Slave Ports T