y the protocol features they want and design an industry- specific sub-version-known as a profile. These profiles allow implementations of PTP that only support the features needed for a specific use case, which eases interoperability and simplifies deployment. Finally, although PTPv2 claims to be capable of delivering microsecond to sub-microsecond accur- acy, the achievable accuracy is obviously dependent on the quality of the implementation. In fact, it is scalable to higher accuracies, as some implementations can deliver synchronization accuracies of less than 1 ns-one example being White Rabbit-but even standard PTP can deliver accuracy in the range of tens of nanoseconds. For information on White Rabbit and other profiles, see the sec- tion "Profiles" later in this chapter. As mentioned in the preceding overview, the IEEE has published a newer version of PTP, known as IEEE 1588-2019 (PTPv2.1). So, over the next several years, we will see SDOs start to adopt fea- tures from the new version and develop updates to their own profiles incorporating new features from this updated version of PTP. Later in this chapter, we cover the definitions and use of specific profiles of PTP defined by the ITU-T, called telecom profiles. But first, we address the concepts and common features from the IEEE 1588-2008 standard before addressing the more specialized versions. Note that some features of IEEE 1588-2008 are not used in the telecom profiles, SO we will not cover those specific features in depth. 7.3.1 General Overview PTP is a two-way time protocol (like NTP) and functions by sending timestamps between the source of time (a master) and a receiver of time (a slave). The method involves sending timestamps from the master to the slave, SO that the slave can recover the clock, meaning (1) frequency synton- ize its oscillator, (2) align its clock phase, and (3) recognize and track the correct time of day. As the slave receives the time information, it has an exact measurement of what the time was on the master, representin