cmLoopInterface is a read-write integer specifying which interface -- identified by its shelf, slot, port, and user channel -- a loopback test should be run against. An administrator sets this before starting a test to target the exact DSL line or channel suspected of a problem. For example, to test a subscriber's line in shelf 1, slot 4, port 12, the operator would set cmLoopInterface to encode that specific location before starting the loopback.