A method of diagnosing a transmission loop in an Internet protocol television (IPTV) network includes configuring the loop as an open loop and performing a single-ended loop test (SELT), configuring the loop as a terminated-loop and performing another SELT, subtracting a trace of the first SELT from a trace of the second SELT, and determining the length of the loop from the result. A service assurance system in an IPTV network includes a test module and a DSLAM coupled to a residential gateway by a transmission loop. The test module operates to determine that a fault condition on the IPTV network is caused by a fault in the loop, direct the DSLAM to perform an open loop SELT and a terminated-loop SELT on the loop, receive the results of the SELTs, and determine a location of a tap on the loop based upon the results.