Current ISDN BRI integrated U interface Loop status and NT maintenance mode. The definition of maintenance modes are as follows: none - The U interface is not in either Loopback or maintenance mode. type2Loopback - Type 2 (CO to U interface) Loopback mode; this mode is set to perform the loopback between CO and the ISDN BRI integrated U interface. type3Loopback - Type 3 (processor to U interface) Loopback mode; this mode is set by router to perform the loopback between the router processor and the ISDN BRI integrated U interface. ntQuietMode - NT quiet mode; this mode is set by CO for metallic loop tests. In the NT quiet mode, the ISDN U interface will not attempt a start-up or will not initiate transmission during metallic loop tests conducted by the network. ilmtMode - Insertion loss measurement test (ILMT); this mode is set by CO for the Insertion loss measurement test. The insertion loss measurement test will cause a known test signal to be generated by the NT. This test will be used in network measurements of DSL (Digital Subscriber Line) transmission characteristics.