et cables. But perhaps the biggest issue is with the format of the ToD. There is now a standardized format for ToD available from the ITU-T (G.8271 Annex A), but the adoption across the industry is not yet universal. There are a couple of de facto standards (NTP, UBX, NMEA), but nothing is supported by 100% of the industry. There is also a format known as "Cisco" format (see Chapter 3 for more information on the formats). Virtual Port (1PPS, ToD, and Frequency) The concept of a virtual port was introduced in the ITU-T G.8275.1 recommendation as Annex C, SO the feature is sometimes referred to as Annex C support. Because it applies to both G.8275.1 and G.8275.2, the specification was moved into Annex B of ITU-T G.8275, SO the original name is now confusing. It is better to refer to it as virtual port support. The idea of a virtual port is that the timing ports (1PPS, ToD, and 10 MHz) or GNSS inputs are made to look like a "virtual" PTP slave port. The normal PTP slave port has its port dataset and real transfer of PTP messages. On the other hand, the virtual port connects to an external PRTC source (no PTP) and has a virtual dataset SO that it looks like other slave ports to the state machine. Most of the parameters normally associated with a port, such as the values in the Announce mes- sage received on that port, are either left as default or configured by the operator. As there is no ac- tual PTP Announce message coming into the virtual port, the engineer must select and configure values that apply to that network and do not conflict with the real GM. These might include priority2 and the clock quality parameters, including clockClass, from the Announce message. One thing to note is that the virtual port can only be configured on a Telecom Boundary Clock (T- BC) because this extra port causes it to exceed the port count (of one) normally allowed with any other ordinary clock such as the Telecom Grandmaster (T-GM) or Telecom Time Slave Clock (T- TSC). Figure 9-4 illustrates the T-BC with a virtual port and the d