9-8
Cisco ONS 15454 SDH Reference Manual, R5.0
April 2008
Chapter9 Security and Timing
9.2.2 Synchronization Status Me ssaging
Figure9-1 ONS15454 SDH Timing Example
9.2.2 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 section overhead. They
enable SDH 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 enab le SSM for the
ONS 15454 SDH, consult your timing reference documentation to determine which message set to use.
Table 9 -5 shows the SDH message set.
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
Table9-5 SDH SSM Message Set
Message Quality Description
G811 1 Primary reference clock
STU 2 Sync traceability unknown
G812T 3 Transit node clock traceable