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.

Maintenance philosophy

The provision of DTE and DCE test loops in public data networks is based upon the following maintenance philosophy:

a) test loops may be used by an Administration's test centre(s) to test the operation of either leased lines or circuit-switched network subscriber lines, including either all or part of the DCE, without necessarily requiring the dispatch of network maintenance personnel to the subscriber's premises;

b) where, allowed by national testing principles, DCE test loops may also be used by a DTE to test the operation of network connection or leased lines. Where provided, the intent is for the DTE to make initial tests to isolate a trouble condition either in the DTEs or in the data circuit.

Loop testing principles

The provision of loop testing capabilities should be based on the following principles:

a) the loops should be transparent, e.g. they should be bit sequence independent;

b) loop testing is a disruptive type of testing, e.g. when a loop has been established transmission of data is not possible;

c) test loops may be established from any state; however, when loop testing is commenced from the data transfer state, the same test procedures may be used for both leased lines and circuit-switched connections;

d) when allowed, the preferred DTE test sequence is: loop 1 - loop 2 - loop 3, sequentially from both ends of the circuit.