page_title: Synchronous Ethernet
section_title: Messaging channel
hierarchical_section_title: Synchronous Ethernet / Overview / Messaging channel
caption_attribution_description: English: SyncE ESMC fields
context_page_description: Synchronous Ethernet, also referred as SyncE, is an ITU-T standard for computer networking that facilitates the transference of clock signals over the Ethernet physical layer. This signal can then be made traceable to an external clock.
context_section_description: In SDH, the Synchronization Status Message (SSM) provides traceability of synchronization signals and it is therefore required to extend the SSM functionality to Synchronous Ethernet to achieve full interoperability with SDH equipment.
  In SDH, the SSM message is carried in fixed locations within the SDH frame.  However, in Ethernet there is no equivalent of a fixed frame. The mechanisms needed to transport the SSM over Synchronous Ethernet are defined by the ITU-T in G.8264 in cooperation with IEEE. More specifically, the ESMC, defined by the ITU-T is based on the Organization Specific Slow Protocol (OSSP), currently specified in IEEE 802.3ay.  The ITU-T G.8264 defines a background or heart-beat message to provide a continuous indication of the clock quality level.  However, event type messages with a new SSM quality level are generated immediately.
  The ESMC protocol is composed of the standard Ethernet header for a slow protocol, an ITU-T specific header, a flag field and a type length value (TLV) structure.  The SSM encoded within the TLV is a four-bit field whose meaning is described in ITU-T G.781.
caption: Ethernet Synchronization Message Channel (ESMC) protocol data unit rec. ITU-T G.8264
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