Synchronous Ethernet; Figure 10: Hypothetical Synchronous Ethernet Reference Network Architecture - Alcatel-Lucent 7710 SR Configuration Manual

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 as 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 one could
elect to 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, as compared to methods relying on sending timing
information in packets over an unlocked 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.
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Figure 10: Hypothetical Synchronous Ethernet Reference Network Architecture

7710 SR OS Basic System Configuration Guide
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