6-3
Cisco ONS 15600 SDH Reference Manual, Release 9.0
78-18400-01
Chapter 6 Timing
6.3 Synchronization Status Messaging
Figure 6-1
ONS 15600 SDH Timing Example
6.3 Synchronization Status Messaging
Synchronization status messaging (SSM) is a SONET and SDH protocol that communicates information
about the quality of the timing source. SSM messages are transported as follows:
•
If SSM is carried over an optical line, for both SONET and SDH the SSM is transported in the S1
byte.
•
If SSM is carried over an electrical line:
–
for SDH, the SSM is transported in the Sa bit of E1.
–
for SONET, the SSM is transported in the outband loop code.
The SSM messages enable SONET and SDH devices to select the highest quality timing reference
automatically and to avoid timing loops.
Node 4
Timing: Line
Ref 1: Slot 12, Port 1
Ref 2: Slot 11, Port 1
Ref 3: Internal (STE)
Node 2
Timing: Line
Ref 1: Slot 11, Port 1
Ref 2: Slot 12, Port 1
Ref 3: Internal (STE)
Node 1
Timing: External
Ref 1: BITS1
Ref 2: BITS2
Ref 3: Internal (STE)
Node 3
Timing: Line
Ref 1: Slot 11, Port 1
Ref 2: Slot 12, Port 1
Ref 3: Internal (STE)
BITS1
source
BITS2
source
120368
Slot 11
Port 1
Slot 11
Port 1
Slot 11
Port 1
Slot 11
Port 1
Slot 12
Port 1
Slot 12
Port 1
Slot 12
Port 1
Slot 12
Port 1
BITS1
Out
BITS2
Out
Third party
equipment
