Wikipedia
Loopback (also written loop-back) is the routing of electronic signals, digital data streams, or flows of items back to their source without intentional processing or modification It is primarily a means of testing the communications infrastructure There are many example applications It may be a communication channel with only one communication endpoint Any message transmitted by such a channel is immediately and only received by that same channel In telecommunications, loopback devices perform transmission tests of access lines from the serving switching center, which usually does not require the assistance of personnel at the served terminal Loop around is a method of testing between stations that are not necessarily adjacent, wherein two lines are used, with the test being done at one station and the two lines are interconnected at the distant station
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