System Management
7750 SR OS Basic System Configuration Guide Page 227
The Table 27 shows the selection followed for two reference in both revertive and non-revertive
modes:
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
Table 27: Revertive, non-Revertive Timing Reference Switching Operation
Status of
Reference A
Status of
Reference B
Active Reference
Non-revertive Case
Active Reference
Revertive Case
OK OK A A
Failed OK B B
OK OK B A
OK Failed A A
OK OK A A
Failed Failed holdover holdover
OK Failed A A
Failed Failed holdover holdover
Failed OK B B
Failed Failed holdover holdover
OK OK A or B A
To Next Page
Loading...
Need help?
General
