363-206-295
Maintenance Description
9-18 Issue 1 December 1997
In Figure 9-5(a), transmitted optical signals in each direction are permanently
bridged to the service and protection fibers. The same signal is transmitted on
both fibers. Each service and protection fiber and its associated OLIU are
monitored as a set. The fiber/OLIU set selected by the receiving location is the
active set and the ACTIVE LED, which in Figure 9-5(a) is on the service OLIU
circuit packs, is lit. The fiber/OLIU set not selected is the standby set, and the
ACTIVE LED on its corresponding OLIU is not lighted.
Contrary to other transmission circuit packs, ACTIVE LEDs on both OLIUs may be
lit at the same time on the same shelf. For example, in Figure 9-5(b) the receive
fiber going to location C has been cut. Location C selects the protection fiber/
OLIU set as active, resulting in the lighting of ACTIVE LEDs on both the OLIUs at
location C. Location A did not switch to its protection set but continues to remain
on the service set. This example illustrates the SONET standard of unidirectional
APS. Since each location sends APS information to the other end, both ACTIVE
LEDs on the OLIUs at location A are also lit.
After the cut service fiber is repaired, location C will not switch back (revert) and
select the service set as its active fiber/OLIU set. This example, Figure 9-5(c),
illustrates the SONET standard of nonrevertive APS.
Figure 9-5(d) and Figure 9-5(e) illustrate a second fiber failure and the resulting
changes to the active OLIUs, thus showing that both protection sets could be
active and carrying service.
APS Initiation Criteria 9
The APS is initiated by signal fail and signal degrade conditions on the received
optical signal. The BER of the received optical signal is calculated from violations
in the SONET line overhead B2 parity bytes. Signal fail is declared for:
— Incoming OC-12 LOS
— OC-12 LOF
— OC-12 Line AIS
— OC-12 BER exceeding 10
-3
.
An OC-12 BER exceeding a provisionable 10
-5
to 10
-9
threshold causes the signal
degrade condition. An APS is completed within 60 milliseconds of the beginning of
a hard failure such as a fiber cut.
The DDM-2000 OC-12 Multiplexer APS is "span independent." In multispan
applications, such as hubbing, an automatic or manual protection switch on one
span does not cause a protection switch on any other span.
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