System Management
7210 SAS M, T, X, R6 Basic System Configuration Guide Page 221
Synchronous Ethernet, for interaction with office clocks, such as BITS or SSUs and embedded
network element clocks.
SSM allows equipment to autonomously provision and reconfigure (by reference switching) their
synchronization references, while helping to avoid the creation of timing loops. These messages
are particularly useful to allow synchronization re-configurations when timing is distributed in
both directions around a ring.
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 affect 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 un-clocked physical layer, is that it is not influenced by
impairments introduced by the higher levels of the networking technology (packet loss, packet
delay variation). Therefore, 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.
The use of Synchronous Ethernet as a candidate reference and/or use of Synchronous Ethernet for
distribution of recovered reference, is supported on 7210 SAS-M, 7210 SAS-D ETR, 7210 SAS-
X, and 7210 SAS-T platforms. It is not supported on 7210 SAS-E. Synchronous Ethernet using
fiber Ethernet ports, including 10G XPF ports, is supported on 7210 SAS-M, 7210 SAS-D ETR,
7210 SAS-X, and 7210 SAS-T platforms
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