interfaces defined in G.703 are not especially interesting for timing, but there are several defined that are highly relevant for carrying physical timing signals. G.703: Physical/Electrical Characteristics of Hierarchical Digital Interfaces ITU-T G.703 defines the physical and electrical characteristics of a whole range of interfaces ap- plicable to PDH and SDH. From the scope of the recommendation: Recommendation ITU-T G.703 specifies the recommended physical and electrical charac- teristics of the interfaces at hierarchical bit rates as described in Recommendations ITU-T G.702 (PDH) and ITU-T G.707 (SDH). The interfaces are defined in terms of general char- acteristics, specifications at the output ports and input ports and/or cross-connect points, earthing of outer conductor or screen and coding rules. This recommendation includes the following main topics: Many interfaces from 64 kbps up to 139,264 kbps Timing interfaces, including 2048 kHz, 1 pulse per second (1PPS), 10 MHz, and time of day (ToD) Definitions of modified alternate mark inversion codes such as High Density Bipolar of Order 3 (HDB3) The main reason for including G.703 in this chapter is that it defines the electrical characteristics of three interfaces used expressly to carry timing signals, for example, between a GNSS receiver and a packet grandmaster. These three interfaces are defined in the following G.703 clauses: Clause 15, "2048 kHz synchronization interface (T12)": Defines the 2048-kHz synchronization signal (for frequency) Clause 19, "Time synchronization interfaces defined in ITU-T G.8271/Y.1366": Defines the V.11-based (using RJ-45) time/phase distribution interface to carry 1PPS and ToD signals Clause 20, "10 MHz synchronization interface": Defines the 10-MHz interface (for frequency) This recommendation is very old, but the latest version from 2016 was released with full text, SO only that version of the document is required to get the complete recommendation. Link to recommendation: https://handle.itu.int/11.1002/1000/12788 8.3 I