presenting the connection to a GNSS antenna (the antennas are frequently shaped somewhat like a mushroom). This icon indicates the presence of a GNSS receiver as either a source of frequency or a source of frequency, phase, and time-a PRC or a PRTC, respectively. Because 5G mobile most often requires phase synchronization, you can assume this source of syn- chronization is most often a PRTC. The antenna icon indicates that the PRTC is a GNSS receiver (being the most common source), but it could also be some other device-such as an atomic clock. Just view it as a symbol of a valid source of time or frequency. One option for deployment is that the GM has an external GNSS receiver (or atomic clock) and the GM receives frequency, phase, and time by way of three input signals, namely 10 MHz, 1PPS, and ToD. The GM (being a PTP entity) is therefore a separate device from the PRTC. Chapter 3 provides more details on the PRTC and input signals from sources of synchronization. However, another deployment option is that the GM could have a built-in GNSS receiver and the device recovers the sources of synchronization internally. In this case, the grandmaster is a com- bined GM+PRTC, and the only connector required is one to attach the external GNSS antenna (be- cause the antenna is nearly always external to the device). In the specifications for the telecom profiles for phase synchronization, the grandmaster is known as the Telecom Grandmaster (T-GM). 7.4.1 Slave (Ordinary) Clocks Besides the grandmaster, the other form of ordinary clock is the slave clock. Again, the slave clock has only a single PTP port with a PTP connection to an upstream master port. The slave may either listen for the upstream PTP flows (via some layer 2 or layer 3 multicast mech- anism) or request unicast service of PTP messages via the negotiation process. In the case of uni- cast negotiation (over PTPoIP), the slave is configured with the IP address of the possible masters. Specifically, for the telecom profiles, the slave clocks have two allowed tr