Synchronous Ethernet

Synchronous Ethernet is a variant of line timing that derives the physical layer transmitter clock from a high-quality timing reference, traceable to a primary reference clock. Synchronous Ethernet uses the physical layer of the Ethernet link to distribute a common clock signal to all nodes in the network. Each node has a local or system clock that determines the outgoing clock rate of each interface. The system clock of each node in the network is derived from the incoming clock at an input interface or from a dedicated timing interface; for example, a BITS port.

Synchronous Ethernet works at Layer 1 and is concerned only with the precision of the timing of signal transitions to relay and recover accurate frequencies. It is not impacted by traffic load and is therefore not affected by packet loss or PDV that occurs with timing methods that use higher layers of the networking technology.

Synchronous Ethernet is automatically enabled on ports and SFPs that support synchronous Ethernet. The operator can select an Ethernet SFP port as a candidate timing reference. The recovered timing from this port is distributed to the nodes in the network over the physical layer of the Ethernet link. This allows the operator to ensure that any of the system outputs are locked to a stable, traceable frequency source. The transmit timing of all SFP ports with SFPs that support synchronous Ethernet is then derived from the node’s SSU.

Synchronous Ethernet can only be used for end-to-end network synchronization when all intermediate switching nodes in the network have hardware and software support for synchronous Ethernet.

Synchronous Ethernet is supported on the following cards and platforms:

If an SFP that does not support synchronous Ethernet is installed, the Ethernet card uses its local oscillator for transmit timing and an event is logged. If the Ethernet port is configured as a source of node synchronization and an SFP that does not support synchronous Ethernet is installed, a clock is not supplied to the SSU and an event is logged.

Each synchronous Ethernet port can be configured to recover received timing and send it to the SSU. On the 7705 SAR-M, 7705 SAR-H, 7705 SAR-Hc, 7705 SAR-A, 7705 SAR-Ax, and 7705 SAR-Wx, any synchronous Ethernet-capable port can be used as an available reference. In addition, two references are available on the 7705 SAR-X and on the 2-port 10GigE (Ethernet) module or 6-port SAR-M Ethernet module. On the 7705 SAR-8 Shelf V2 and 7705 SAR-18, two references are available on:

Synchronous Ethernet ports always use node timing from the SSU. Configuration of one port automatically configures the other port.

If timing is recovered from a synchronous Ethernet port from an upstream non-synchronous Ethernet free-running port and selected as the reference to the SSU, then this clock may not be of sufficient quality or accuracy for node operations. This reference may be disqualified because the frequency may not be within the pull-in range of the SSU Stratum 3 oscillator.

On the 7705 SAR-M, 7705 SAR-Hc, 7705 SAR-A, 7705 SAR-Ax, 7705 SAR-Wx, 7705 SAR-X, and on the Packet Microwave Adapter card, a copper-based, RJ-45 synchronous Ethernet port phy-tx-clock must be configured as slave before the port is configured to be a timing source for the node. If a copper-based, RJ-45 synchronous Ethernet port is a timing source for the node, the port phy-tx-clock cannot be changed to another mode.

Parent topic: Node Timing