A method and mechanism for testing communication links. A transmitter contact, or transmission point, is assigned a unique identifier. During a given test, the transmitter conveys a test pattern to a receiver via a link. Following this test pattern, the transmitter transmits a bit of its unique identifier to the receiver. The receiver receives both the test pattern and the identifier bit, and determines whether the received test pattern matches an expected value. If the test pattern was correctly received by the receiver, the receiver transmits the received identifier bit back to the transmitter. However, if the received test pattern is not correct, the receiver complements the received identifier bit and transmits the complemented bit back to the transmitter. The transmitter receives the identifier bit from the receiver and determines whether it matches the identifier bit which was originally transmitted by the transmitter. If the returned identifier bit does not match what was originally transmitted, the transmitter records an error.