This standard establishes common criteria for Synchronous Optical NETwork (SONET) interfaces at standard rates associated with the Network Interface (NI). Criteria covered in this standard include maintenance and operation functionality at the SONET Section, Line and Path layers, and other necessary criteria for compliance with the proper interfacing of the connecting customer installation equipment. Compliance with this standard is intended to ensure compatibility at the SONET NI and should not be construed as a constraint on the internal operations of the network or customer installation equipment.