System Management
Basic System Configuration Guide 257
DS3/E3
DS3/E3 signals are not required to be synchronous. However, it is acceptable for their
clocking to be generated from a synchronization source. The 7750 SR and the 7450 ESS
permit E3/DS3 physical ports to be specified as a central clock input reference.
DS3/E3 signals do not support an SSM channel. QL-override should be used for these ports
if ql-selection is enabled
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.
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