This Recommendation specifies the principles of maintenance testing for public data networks using the DTE/DCE test loops. The definition of the loops, the principles for the control of the loops and the principles for the indications to be presented when the loops are activated are described herein for general application with all DTE/DCE interfaces. Specific details concerning the implementation of these principles are contained in the individual DTE/DCE interface Recommendations. The maintenance philosophy, definitions and general principles are applicable to interface Recommendations, such as X.20, X.20 bis, X.21, X.21 bis, and X.22.