 clocks as sources of frequency for a syn- chronization network. From the scope of the recommendation: This Recommendation outlines the requirements for primary reference clocks (PRCs) suit- able for synchronization supply to digital networks. These requirements apply under the normal environmental conditions specified for digital equipment. This recommendation includes the following main topics: Frequency accuracy requirements (1 part in 10 11 over one week) Noise generation (wander and jitter at the output interface) Output interfaces defined Even though the original recommendation goes back to the 1970s, the latest version of G.811 is "only" 25 years old, although there was an amendment published in 2016. The amendment covers some changes for the enhanced primary reference time clock (ePRTC) and for 10-MHz interfaces. Thankfully, the amendment was published with full text, SO only that single document is needed to read the complete recommendation. Link to recommendation: https://handle.itu.int/11.1002/1000/12792 G.811.1: Timing Characteristics of Enhanced Primary Reference Clocks ITU-T G.811.1 defines the (timing) characteristics of enhanced PRC clocks as sources of frequency for a synchronization network. The frequency accuracy of an ePRC is ten times that of a PRC. From the scope of the recommendation: This Recommendation outlines the requirements for enhanced primary reference clocks (ePRCs) suitable for frequency synchronization. These requirements apply under normal environmental conditions specified for the equipment. This recommendation includes the following main topics: Frequency accuracy requirements (1 part in 10 12 over one week) Noise generation (wander and jitter at output) Output interfaces defined G.811.1 is a recent recommendation and covers the latest ePRC clocks. Like G.811, it is a relatively short and straightforward document. Link to recommendation: https://handle.itu.int/11.1002/1000/13301 G.812: Timing Requirements of Slave Clocks Suitable for Use as Node Clocks in Synchronization Netwo