Cisco Systems 15310-MA 6.2 Network Timing , 6.3 Synchronization Status Messaging

6-2
Cisco ONS 15310-MA SDH Reference Manual, Release 9.1 and Release 9.2
78-19417-01
Chapter 6 Timing
Network Timing
Caution Mixed timing allows you to select both external and line timing sources. However, Cisco does not
recommend its use because it can create timing loops. Use mixed timing mode with caution.
6.2 Network Timing
Figure 6-1 shows an example of an ONS 15310-MA SDH network timing setup. Node 1 is set to external
timing. One reference is set to BITS, the two references are set to internal. The BITS output pins on the
CTX cards of Node 3 provide timing to outside equipment, such as a digital access line multiplexer.
Figure 6-1 ONS 15310-MA SDH Timing Example
6.3 Synchronization Status Messaging
Synchronization status messaging (SSM) is an SDH protocol that communicates information about the
quality of the timing source. SSM messages are carried on the S1 byte of the SDH line layer. They enable
SDH devices to automatically select the highest quality timing reference and to avoid timing loops.
Node 4
Timing Line
Ref 1: Slot 4
Ref 2: Slot 3
Ref 3: Internal (ST3)
Node 2
Timing Line
Ref 1: Slot 3
Ref 2: Slot 4
Ref 3: Internal (ST3)
Node 1
Timing External
Ref 1: BITS
Ref 2: Internal
Ref 3: Internal (ST3)
Node 3
Timing Line
Ref 1: Slot 3
Ref 2: Slot 4
Ref 3: Internal (ST3)
BITS
out
BITS
source
Third party
equipment
Slot 3
Slot 3
Slot 3
Slot 3
Slot 4
Slot 4
Slot 4
Slot 4
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