The CCITT,considering(a) the savings to be made by reducing interruption time on TDM links;(b) the importance of being able to determine responsibilities between the several parties who, of necessity are involved in maintenance questions for the networks;(c) the advantages of standardization regarding maintenance;(d) maintenance loops are standardized in Recommendation R.115,unanimously declares the following:that when the quality of the TDM-Iink has deteriorated beyond the alarm limit or if the local muldex gives an alarm, supplementary measurement should be performed.The following test and supervision methods can be used.
ITU-T R.116-1988 Referenced Document
ITU-T R.115 Maintenance Loops for TDM-Systems - Telegraphy Telegraph Transmission Study Group IX; 11 pp*， 1993-03-01 Update
ITU-T R.116-1988 history
1988ITU-T R.116-1988 MAINTENANCE TESTS TO BE CARRIED OUT ON INTERNATIONAL TDM SYSTEMS