10-3
Cisco ONS 15454 Reference Manual, R7.0.1
OL-9217-01
Chapter 10 Timing
10.3 10.3 Synchronization Status Messaging
10.3 Synchronization Status Messaging
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 15454,
consult your timing reference documentation to determine which message set to use. Table 10-1 and
Table 10-2 on page 10-3 show the Generation 1 and Generation 2 message sets.
Table 10-1 SSM Generation 1 Message Set
Message Quality Description
PRS 1 Primary reference source—Stratum 1
STU 2 Synchronization 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
RES — Reserved; quality level set by user
Table 10-2 SSM Generation 2 Message Set
Message Quality Description
PRS 1 Primary reference source—Stratum 1
STU 2 Synchronization traceability unknown
ST2 3 Stratum 2
TNC 4 Transit node clock
ST3E 5 Stratum 3E
ST3 6 Stratum 3
SMC 7 SONET minimum clock
ST4 8 Stratum 4
DUS 9 Do not use for timing synchronization
RES — Reserved; quality level set by user
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