Synchronous Ethernet

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 locks to this provisioned reference selection). The operator then can ensure that all system output is locked to a stable traceable frequency source.

Fixed copper ports using Synchronous Ethernet can be used as a candidate reference or for distribution of recovered reference. If the port is a fixed copper Ethernet port and in 1000BASE-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 needed 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, 802.3 link autonegotiation is triggered to ensure that the link timing aligns correctly.

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.