over OTN Ethernet equipment clock (EEC): A clock that uses synchronous methods such as SyncE As mentioned earlier in the section "G.781," in the original recommendations, such as G.813, "SDH" was used to refer to the SDH equipment clock alone. But since about 2018, the Q13/15 transitioning to using "SEC" as a more generic term for any clock that supports synchronous meth- ods of frequency distribution. (See Appendix VI of G.781 for more details.) Therefore, many re- commendations are now adopting this nomenclature to avoid having a different name for every clock using a different form of transport technology. That is the reason "SEC" is used in several dif- ferent ways in G.8261 (and elsewhere). The G.8261 recommendation covers two main use cases: When carrying a reference timing signal for frequency synchronization, what is referred to in the recommendation as packet network timing (PNT) domain. When using circuit emulation, and where the circuit has its own service clock that is being transported using packet techniques such as adaptive clock recovery (ACR). This is referred to in the recommendation as the circuit emulation service (CES) domain. Some of the information for this second case is for further study. This recommendation includes the following main topics, annexes, and appendixes: Packet network synchronization and TDM timing requirements (SDH and PDH). Included are aspects of synchronization network engineering in packet networks including the PNT domain and CES domain. Reference timing signal distribution over packet networks (PNT domain). Timing recovery for constant bit rate services transported over packet networks (CES domain), including synchronous, differential, and adaptive techniques. Network limits for CES and PNT using several deployment cases. Lots of information on the impact of impairments to the transport of frequency. Annex A, "Proposed network architecture for synchronous Ethernet." Annex B, "IWF functional partitioning into CES and PNT IWF and network examples" (de- scribes interwor