This Recommendation outlines the requirements on Ethernet networks with respect to frequency transfer. It specifies the synchronization status message (SSM) transport channel namely the Ethernet synchronization messaging channel (ESMC)@ protocol behaviour and message format. A number of methods may be used to transfer frequency which may be physical-layer based or protocol-layer based. The method used will be dependent on the actual architecture and what may be supported. This Recommendation focuses on physical layer synchronization. The physical layer that is relevant to this Recommendation is the Ethernet media types defined in [IEEE 802.3]. Other physical layers may also be relevant and may be addressed@ along with other packet technologies@ in future versions of this Recommendation. This Recommendation also details the required architecture in formal modelling language. Timing flows are used to describe where and how time and timing will flow through the architecture. Such flows describe what is functionally allowed as an acceptable timing source. Such a source may only be available for use within the equipment or may be available outside as a client service.