A defect is any serious or persistent event that interrupts transmission service. SDH uses defect processing to report and locate failures across entire circuits or specific network nodes. An alarm indication signal (AIS) is sent downstream to notify the next network element of an event, and a remote defect indication (RDI) is sent backwards in response. Depending on the affected service, AIS can take the form of MS-AIS, AU-AIS, TU-AIS, or PDH-AIS by setting bits to "1". Enhanced RDI provides additional information to differentiate between server, connectivity, and payload defects.

1. SONET/SDH:DEFECTS
A defect is understood as any serious or persistent event that holds up the transmission service. SDH
defect processing reports and locates failures in either the complete end-to-end circuit (HP-RDI, LP-RDI)
or on a specific multiplex section between adjacent SDH nodes (MS-RDI)

2. Alarm indication signal
An alarm indication signal (AIS) is activated under standardized criteria, and sent downstream in a path
in the client layer to the next NE to inform about the event. The AIS will arrive finally at the NE at which
that path terminates, where the client layer interfaces with the SDH network .
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.

3. 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.