Traditionally, Ethernet-based networks employ a physical layer transmitter clock derived from an inexpensive +/-100ppm crystal oscillator and the receiver locks onto it. Because data is packetized and can be buffered, there is no need for long-term frequency stability or for consistency between frequencies of different links.
Synchronous Ethernet is a variant of the line timing that derives the physical layer transmitter clock from a high-quality frequency reference, replacing the crystal oscillator with a frequency source traceable to a primary reference clock. This change is transparent to the other Ethernet layers and does not affect their operation. The receiver at the far end of the link is locked to the physical layer clock of the received signal, and ensures access to a highly accurate and stable frequency reference. In a manner analogous to conventional hierarchical network synchronization, this receiver can lock the transmission clock of other ports to this frequency reference, and establish a fully time-synchronous network.
Unlike methods that rely on sending timing information in packets over an unclocked physical layer, Synchronous Ethernet is not affected by impairments introduced by higher levels of networking technology (packet loss, packet delay variation). The frequency accuracy and stability in Synchronous Ethernet typically exceeds networks with unsynchronized physical layers.
Synchronous Ethernet allows operators to gracefully integrate existing systems and future deployments into a conventional industry-standard synchronization hierarchy. The concept is analogous to SONET/SDH system timing capabilities. The operator can select any (optical) Ethernet port as a candidate timing reference. The recovered timing from this port is used to time the system (for example, the CPM will lock to this provisioned reference selection). The operator then can ensure that all system output is locked to a stable traceable frequency source.
The use of Synchronous Ethernet as a candidate reference and for distribution of recovered reference is supported on all 7210 SAS platforms as described in this document, except those operating in standalone-VC mode.
Synchronous Ethernet using fiber Ethernet ports, including 10G and 100G (if available), is supported on all 7210 SAS platforms as described in this document, except those operating in standalone-VC mode.
Please ensure that the SFP or XFP or SFP+ parts used with the SFP, XFP, and SFP+ ports support Synchronous Ethernet.
Synchronous Ethernet is not supported on virtual chassis (VCs).
Synchronous Ethernet using fixed copper ports is supported only on the 7210 SAS-T, 7210 SAS-R6, 7210 SAS-R12, 7210 SAS-Sx 1/10GE, 7210 SAS-S 1/10GE and 7210 SAS-Mxp. The fixed copper ports can be used as a candidate reference or for distribution of recovered reference . If the port is a fixed copper Ethernet port and in 1000 BASE-T mode of operation, there is a dependency on the 802.3 link timing for the Synchronous Ethernet functionality (see ITU-T G.8262). The 802.3 standard link timing states must align with the desired direction of Synchronous Ethernet timing flow. When a fixed copper Ethernet port is specified as an input reference for the node or when it is removed as an input reference for the node, an 802.3 link auto-negotiation is triggered to ensure the link timing aligns properly.
The SSM of Synchronous Ethernet uses an Ethernet OAM PDU that uses the slow protocol subtype. For a complete description of the format and processing, see ITU-T G.8264.