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Network Synchronization
Synchronous Ethernet
Traditionally, Ethernet-based networks employ the physical layer transmitter clock to be derived
from an inexpensive +/-100ppm crystal oscillator and the receiver locks onto it. There is no need
for long term frequency stability because the data is packetized and can be buffered. For the same
reason there is no need for consistency between the frequencies of different links. However, you
can derive the physical layer transmitter clock from a high quality frequency reference by
replacing the crystal with a frequency source traceable to a primary reference clock. This would
not effect the operation of any of the Ethernet layers, for which this change would be transparent.
The receiver at the far end of the link would lock onto the physical layer clock of the received
signal, and thus itself gain access to a highly accurate and stable frequency reference. Then, in a
manner analogous to conventional hierarchical master-slave network synchronization, this
receiver could lock the transmission clock of its other ports to this frequency reference and a fully
time synchronous network could be established.
The advantage of using Synchronous Ethernet, compared with methods that rely on sending timing
information in packets over an unclocked physical layer, is that it is not influenced by impairments
introduced by the higher levels of the networking technology (packet loss, packet delay variation).
Hence, the frequency accuracy and stability may be expected to exceed those of networks with
unsynchronized physical layers.
Synchronous Ethernet allows operators to gracefully integrate existing systems and future
deployments into conventional industry-standard synchronization hierarchy. The concept behind
synchronous Ethernet is analogous to SONET/SDH system timing capabilities. It allows the
operator to select any (optical) Ethernet port as a candidate timing reference. The recovered timing
from this port will then be used to time the system (for example, the CPM will lock to this
provisioned reference selection). The operator then could ensure that any of system output would
be locked to a stable traceable frequency source.
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 (refer to ITU-T
G.8262). The 802.3 link Master-Slave 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
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7950 SR OS Basic System Configuration Guide
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