e synchronization of SDH, Ether- net, and OTN network elements and how these elements are involved in network synchronization. From the scope of the recommendation: This Recommendation specifies a library of basic synchronization distribution building blocks, referred to as "atomic functions" and a set of rules by which they are combined in order to describe a digital transmission equipment's synchronization functionality. Although G.781 is quite a large document and somewhat dense for the new reader, it does contain quite a deal of useful information around the basic concepts. This recommendation breaks the avail- able TDM network technologies into the following options: Option I: Based on the 2048-kbps hierarchy (for example, E1). It includes dedicated timing cir- cuits (without traffic) of 2048 kHz and 2048 kbps. Option II: Based on the 1544-kbps hierarchy that includes the rates 1544 kbps, 6312 kbps, and 44,736 kbps (for example, T1). It includes dedicated timing circuits (without traffic) of 64 kHz and 1544 kbps. Option III: Based on the 1544-kbps hierarchy that includes the rates 1544 kbps, 6312 kbps, 33,064 kbps, 44,736 kbps, and 97,728 kbps. It includes dedicated timing circuits (without traffic) of 64 kHz and 6312 kHz. G.781 includes the following main topics, annex, and appendixes: Synchronization basics, including an overview of synchronization interfaces, definitions of quality levels (QL), synchronization status message (SSM), and type, length, value (TLV) for the various quality levels. Also covers concepts of the hold-off and wait-to-restore timers, source priorities, automatic source selection, and timing loop prevention. Atomic function of the various layers of synchronization. Annex A, "Synchronization selection process." This is a very important annex because it details the process flow for a clock to select the best frequency source under numerous conditions. Appendix IV, "Interworking of option II equipment supporting second-generation SSM and first-generation SSM using a translation function