Link Check - Siemens SCALANCE XM-400 Configuration Manual

Technical basics
4.6 Redundancy mechanism
For a redundant link as shown in the figure, two devices must be configured as standby
redundancy switches within a network segment. In this case, network segments are rings with
a redundancy manager. Instead of rings, network segments might also be linear.
The two standby redundancy switches connected in the configuration exchange data frames
with each other to synchronize their operating statuses (one device is master and the other
slave). If there are no problems, only the link from the master to the other network segment is
active. If this link fails (for example due to a link-down or a device failure), the slave activates
its link as long as the problem persists.
4.6.5

Link Check

Monitoring optical connections in the ring
On optical connections disturbances are possible in which the optical connection is not
completely interrupted, but frames are lost sporadically. Such problems can, for example, be
caused by defective optical cables, dirty connectors or device defects.
The redundancy manager of an HRP or MRP ring with optical connections detects a "non-
recoverable ring error" with such a disturbance. The redundancy manager cannot eliminate
the disturbance by closing the ring. Closing the ring in this case, would lead to circulating
frames.
With the Link Check function, you can monitor the transmission quality of optical sections within
an HRP or MRP ring, identify disturbed connections and under certain conditions turn them
off. When the disturbed section is turned off, the redundancy manager can close the ring and
restore communication.
How Link Check works
Behavior with an undisturbed connection
If you enable Link Check on two connected ring ports, the two connection partners exchange
Link Check frames cyclically on these ports. The frames received by one connection partner
are sent back to the other.
When the devices receive back the frames they sent from the connection partner, the
connection is prepared for Link Check. The connection partners then increase the send
frequency of the Link Check test frames and the actual connection monitoring is active.
Behavior with a disturbance
When connection monitoring is enabled, you can see the number of sent and received Link
Check test frames on the "Information > Redundancy >Link Check" page. Based on these
statistics you can recognize smaller disturbances for which the disturbance does not yet cause
the transmission line to be closed down by Link Check.
Link Check recognizes a connection as being disturbed and closes it down when too many
test frames are lost within a given period. Link Check uses several intervals to be able to
recognize sudden occurrences of errors as well as a continuous low error rate.
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SCALANCE XM-400/XR-500 Web Based Management (WBM)
Configuration Manual, 05/2017, C79000-G8976-C248-12