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78-11719-02 July 2001 Using Cisco Transport Controller 3
Cisco ONS 15327 User Documentation 3-37
Figure 3-21 An ONS 15327 timing example with external, BITS, and internal timing
Synchronization Status Messaging (SSM) is a SONET protocol that communicates information
about the quality of the timing source. SSM messages are carried on the S1 byte of the SONET Line
layer. They enable SONET devices to automatically select the highest-quality timing reference and
to avoid timing loops.
SSM messages are either Generation 1 or Generation 2. Generation 1 is the first and most widely
deployed SSM message set. Generation 2 is a newer version. If you enable SSM for the ONS 15327,
consult your timing-reference documentation to determine which message set to use. Table 3-13 and
Table 3-14 show the Generation 1 and Generation 2 message sets.
Table 3-13 SSM Generation 1 Message Set
Message Quality Description
PRS 1 Primary reference source - Stratum 1
STU 2 Sync traceability unknown
ST2 3 Stratum 2
ST3 4 Stratum 3
SMC 5 SONET minimum clock
ST4 6 Stratum 4
DUS 7 Do not use for timing synchronization
Node 4
Timing Line
Ref 1: Slot 6
Ref 2: Slot 5
Ref 3: Internal (ST3)
Node 2
Timing Line
Ref 1: Slot 5
Ref 2: Slot 6
Ref 3: Internal (ST3)
Node 1
Timing External
Ref 1: BITS1
Ref 2: BITS2
Ref 3: Internal (ST3)
Node 3
Timing Line
Ref 1: Slot 5
Ref 2: Slot 6
Ref 3: Internal (ST3)
BITS1
out
BITS2
out
BITS1
source
BITS2
source
Third party
equipment
34726
Slot 5
Slot 5
Slot 5
Slot 5
Slot 6
Slot 6
Slot 6
Slot 6