363-206-285
Transmission and Synchronization Interfaces
Issue 3 June 2001
5-35
available. The capability also allows the DDM-2000 OC-3 and OC-12 Multiplexers
to inform a local BITS clock when the DS1 timing output has been degraded and
should no longer be used as a reference. This synchronization messaging feature
is based on the scheme developed in the
ANSI
T1X1 standards committee.
Applications 5
The applications that are currently supported with the synchronization messaging
feature can be divided into three categories:
1. DS1 timing output integrity
2. Automatic synchronization reconfiguration
3. Synchronization provisioning integrity.
DS1 Timing Output Integrity 5
The DS1 timing outputs are typically used to distribute timing from the BITS clock
in one office (master) to the BITS clock in the next office (slave), using the SONET
transmission facilities between them as the synchronization vehicle. The BITS are
typically of stratum 2 or stratum 3 accuracy. The DDM-2000 Multiplexer does not
introduce its own internal timing source onto the SONET facility but merely
transfers the quality of its timing reference. This ability preserves the required
hierarchical structure of the timing network and should be maintained at all times.
If the DDM-2000 Multiplexer at the master office enters holdover due to a
disconnected reference cable or a local BITS failure, the quality of the DS1 timing
output at the slave office will now be traceable directly to the DDM-2000
Multiplexer. If the master DDM-2000 system contains a TG3 circuit pack, stratum
3 accuracy will be maintained indefinitely. This provides acceptable timing for
stratum 3 NEs at slave offices. If the master DDM-2000 contains a TGS circuit
pack, stratum 3 accuracy will be maintained for the first 24 hours of holdover. After
the first 24 hours, the DDM-2000 system could exceed stratum 3 accuracy.
If a DDM-2000 system contains a TGS pack, it may change the output of its
derived DS1 under some conditions. If the incoming synchronization message is
better than internal clock (IC) the derived DS1 will be an ‘all ones’ with framing.
This format is suitable for providing synchronization to a BITS of stratum 3
accuracy. If the co-located DDM-2000 enters holdover or the incoming
synchronization message is IC or worse, the derived DS1 format will change to ‘all
ones’ without framing (AIS). This will prevent a BITS from using this input and
force the BITS to either select a new input or enter holdover. This change to AIS
preserves the stratum timing hierarchy by allowing the BITS to synchronize from a
clock of equal or higher quality.
Available in R13.0 and later is the threshold AIS feature. This feature allows the
user to select the value of the incoming message that causes the derived DS1
output to output AIS. This feature will allow stratum 2 BITS clock to receive timing
from a TGS or TG3 without violating stratum timing rules.
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