Understanding Synchronization Protection Basics in Transmission network

Let us consider a ring network. Normal synchronization works around a ring. In this case, Nodes B-F are line timed. Node A is timed to an external reference. When a sync source is failed,  new time source should be selected in a reasonable amount of time

If synchronization is not restored , BER  will be  increasing through time.

SPS Timing Loops (SPS = Synchronization Protection Switching):

During a ring failure, simple reference switching would result in timing loops as shown below.

Operations – Normal Flow :

Let us read about the operation. Following diagram shows the normal flow of operation. Synchronization messaging in normal operation.

S1 = Stratum 1 Traceable

DU = Don’t Use

HO = Holdover

Operations – Fiber Cut:

In the ring, if fiber cuts between B & C,  Node C goes into short term holdover as shown below.

Then,   Node F switches to timing from Node A as shown below.

Finally ring is reconfigured and all nodes are again synchronized to BITS as shown below.

Please let me know if any clarification required :)