10-3

Cisco ONS 15454 Reference Manual, R8.5.x
78-18106-01
Chapter 10 Timing
10.3 Synchronization Status Messagi ng
Figure 10-1 ONS 15454 Timing Example
10.3 Synchronization Status Messaging

Synchronization status messaging (SSM) is a SONET and SDH protocol that c ommunicates 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 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