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Operation, administration, and maintenance (OAM) features 2-157
Planning and Ordering Guide—Part 1 of 2 NTRN10AN Rel 12.1 Standard Iss 1 Apr 2004
Synchronization-status messages are carried in bits 5 through 8 of the S1 byte
in the SONET line overhead and in the extended superframe (ESF) datalink of
the external DS1 signal from a BITS or a tributary. As the timing is passed
from one network element to the next, each network element sends SSMs. If
the quality of the timing changes, the SSMs inform the next network element
of the change. See Table 2-33.
Table 2-33
Synchronization status messages and quality levels
Description Quality
level
SONET S1 byte
(bit 5 to bit 8)
Designation ESF datalink
code byte
Stratum 1—Traceable 1 0001 ST1 (PRS) 000010
Synchronized—
Traceability unknown
2 0000 STU 000100
Stratum 2—Traceable 3 0111 ST2 000110
Stratum 3—Traceable 4 1010 ST3 001000
SONET minimum
clock—Traceable
5 1100 SMC 010001
Stratum 4—Traceable
(see Note 1)
6 n/a ST4 010100
Do not use for
synchronization
8 1111 DUS 011000
Reserved for network
synchronization use
(holdover)
7 1110 RES 100000
Note 1:
The ST4 message is carried in the ESF datalink of an external DS1 timing
signal, not in the SONET overhead.
Note 2:
Any unsupported synchronization status message S1 or DS1 ESF datalink
is byte-mapped to a DUS designation.