1. A method of performing transmission tests of access lines from the serving switching center, which method usually does not require the assistance of personnel at the served terminal.
2. A method of testing between stations (not necessarily adjacent) wherein two lines are used, with the testing being done at one station and the two lines interconnected at the distant station.
3. A patch, applied manually or automatically, remotely or locally, that facilitates a loop-back test.
Source: from Federal Standard 1037C and from MIL-STD-188