cludes the PTP mapping (IP) and message rates and which form of the best master clock algorithm (BMCA) is allowed. The profile requires support for IPv4 (IEEE 1588-2008 Annex D), with IPv6 (IEEE 1588-2008 Annex E) being an optional per- mitted transport mechanism. Annex A, "ITU-T PTP profile for frequency distribution without timing support from the net- work (unicast mode)." This annex contains the main details such as allowed values for datasets, TLVs, flag values, and SO on. Appendix I, "Use of mixed unicast/multicast mode for PTP messages." This appendix contains information on deployment of PTP, especially considering that PTP was originally designed to be used in a multicast environment. Some implementations of PTP in a telecom environment used a "mixed mode" whereby some messages were multicast and others were unicast. This recommendation outlines the implementation and deployment details for G.8265.1. This pro- file is widely used in the circuit emulation application as well as frequency synchronization for older versions of the mobile telecommunications networks. See "Chapter 2. Usage of Synchroniza- tion and Timing", for an overview of the application of frequency synchronization to the mobile space. A new full-text version of G.8265.1 with an amendment was published in 2019. This amendment included a couple of new notes in a table mapping between SSM quality levels and PTP clock class values. Link to recommendation: https://handle.itu.int/11.1002/1000/14012 G.8275.1: Precision Time Protocol Telecom Profile for Phase/Time Synchron- ization with Full Timing Support from the Network The ITU-T G.8275.1 profile is one of the most important for modern networks and for those operat- ors transitioning to 5G. It utilizes the layer 2 version of PTP for those networks that fully support PTP at every hop from the master to the slave. From the scope and summary of the March 2020 edi- tion of the recommendation: Recommendation ITU-T G.8275.1 contains the ITU-T precision time protocol (PTP) profile for phase and time