Frequency and time synchronization over a network are key requirements for precise timekeeping. Synchronous Ethernet (SyncE) is a physical layer technology that supports frequency transfer from hop to hop that can be made traceable to an external source such as a network clock. It distributes synchronization over the physical layer.

Although SyncE can deliver stable frequency synchronization, it cannot deliver time or phase synchronization. A separate protocol is required for delivery of time or phase synchronization.

SyncE uses the physical layer (Ethernet interfaces) to distribute frequency from the primary reference clock (PRC) to downstream devices. It supports frequency transfer from hop to hop and is used to provide frequency synchronization in networks. Because SyncE operates at the physical level, all Ethernet devices along the synchronization path must be SyncE-capable.

The advantage of SyncE for network synchronization over PTPv2 is that it provides high-quality frequency synchronization irrespective of the network load. When SyncE is enabled on the physical Ethernet interface and switch, frequency recovery from the received signal on the physical line occurs, and frequency synchronization is performed in the hardware.

SyncE uses the Synchronization Status Message (SSM) and the Ethernet Synchronization Message Channel (ESMC) for clock selection, traceability, and failover.

The Synchronization Status Message (SSM) indicates the quality level (QL) of the transmitting clock to neighboring nodes.

Clock quality helps a synchronous Ethernet node derive timing from the most reliable source and prevent timing loops by selecting the clock source with the best QL.

Other key benefits of SSM include:

• Selects the reference clock when there are multiple clock sources
• Provides fast recovery when a part of the network fails by switching to the next best available clock source

In SyncE, SSM is provided through ESMC, as described in the following section.

Ethernet Synchronization Message Channel (ESMC) is the logical channel that uses an Ethernet PDU (protocol data unit) to exchange SSM information over the SyncE link.

ESMC packets are received and processed on the SyncE-enabled ports that are configured as clock sources. The clock selection algorithm uses the QL values to select the best clock source.

For the SyncE functionality to work correctly, ensure that each network element along the network supports SyncE.