Recommendation ITU-T G.781.1 specifies a functional architecture model and corresponding atomic functions for the transport of time and frequency synchronization via packet-based methods using the precision time protocol (PTP). This revision of G.781.1 provides the following updates: • Revised terminology based on inclusive language • Updated Figure 6-1 "The high-level functional architecture for transport of packet-based synchronization via PTP ober transport layers" • Updated Figure 8-11 "Clock mode state transitions", the definitions of the modes and Table 8-10 "Transition information" • Updated clause 9.2 "OTUk to SD-p adaptation functions" • Corrected OTSn to OSC in clause 9.3 • Added clause 9.4 "FlexE to SD-p adaptation functions" • Added clause 9.6 "MTNSy to SD time sync adaptation functions" • Added details in Appendix III "Analysis of message delay times for NEs in packet-based synchronization networks"