Down indicates that the SONET/SDH line in at least one direction on a bidirectional link is degraded to the point of no longer passing traffic.
The East/West directionality for each topological link is determined during discovery.
The symptoms for TopologicalLink Down are as follows:
LOS from downstream PTP connected to the TopologicalLink
RFI for the upstream PTP connected to the TopologicalLink
Signal Failure (SF) on the receiving PTP connected to the TopologicalLink