Each terminating device (router or otherwise) monitors the incoming signal for CP-bit path errors.
If an error is detected on the incoming DS3, the terminating elements transmit a FEBE bit on the outgoing direction of the DS3.
Network monitoring equipment located anywhere along the path then measures these FEBEs in each direction to gauge the quality of the circuit while in service.
Technical jargon: An error detected by extracting the 4-bit FEBE field from the path status byte (G1).
A maintenance cell indicates that an error occurred with a data block at the far end of the link.