Loopback can be one of the mechanisms which may be applied to fault localization and failure detection. This Recommendation provides digital loopback definitions and describes loopback applications related to the maintenance phases of Recommendation M.20.
ITU-T M.125 SPANISH-1988 Referenced Document
ITU-T X.200 Information Technology - Open Systems Interconnection - Basic Reference Model: the Basic Model - Data Networks and Open System Communications Open Systems Interconnection - Model and Notation 64 pp*， 1994-01-01 Update