SONET/SDH:DEFECTS

As an answer to a received AIS, a remote defect indication is sent backwards. An RDI is indicated in a specific byte, while an AIS is a sequence of “1s” in the payload space. The permanent sequence of “1s” tells the receiver that a defect affects the service, and no information can be provided.

Depending on which service is affected, the AIS signal adopts several forms:-

• MS-AIS: All bits except for the RSOH are set to the binary value “1.”
• AU-AIS: All bits of the administrative unit are set to “1” but the RSOH and MSOH maintain their codification.
• TU-AIS: All bits in the tributary unit are set to “1,” but the unaffected tributar- ies and the RSOH and MSOH maintain their codification.
• PDH-AIS: All the bits in the tributary are “1.”

Enhanced remote defect indication 

Enhanced remote defect indication (E-RDI) provides the SDH network with addi- tional information about the defect cause by means of differentiating:

• Server defects: like AIS and LOP;
• Connectivity defects: like TIM and UNEQ;
• Payload defects: like PLM.

Enhanced RDI information is codified in G1 (bits 5-7) or in k4 (bits 5-7), depending on the path.