rks ITU-T G.812 outlines minimum requirements of timing devices used as nodes in synchronization networks. This covers aspects such as frequency deviation; pull-in, hold-in, and pullout range; noise generation, tolerance, and transfer; and transient response and holdover performance. From the scope of the recommendation: The function of a node clock is to select one of the external synchronization links coming into a telecommunication station as the active synchronization reference, to attenuate its jit- ter and wander and subsequently to distribute the reference to the telecommunication equip- ment in the station. These node clocks are basically timing devices that are better than network element clocks and can be used in a frequency distribution network to provide better performance (especially during hold- over when the connection to the PRC fails). So, they are better than network elements but not as good as PRC clocks. G.812 defines six separate clock types: Type I: For use in networks optimized for the 2048-kbps hierarchy Type II: For use in networks optimized for the 1544-kbps hierarchy with more stringent hold- over requirements Type III: For use in networks optimized for the 1544-kbps hierarchy with less stringent hold- over requirements-for use in end offices Type IV: For use in networks optimized for the 1544-kbps hierarchy Type V: Transit node clock from the 1988 version of this recommendation (historical) Type VI: Local node clock from the 1988 version of this recommendation (historical) This recommendation includes the following main topics, annex, and appendix: Frequency accuracy Pull-in, hold-in, and pull-out ranges Noise generation (locked wander, non-locked wander, jitter) Noise tolerance (wander and jitter) Noise transfer Short-term and long-term (holdover) phase transient response Interface definitions Annex A, "Specifications for Types IV, V and VI clocks" Appendix II, "Measurement method for noise transfer" Link to recommendation: https://www.itu.int/rec/T-REC-G.812/en G.813: Timing Characte