The most important thing to remember when troubleshooting an HDSL2/4 facility is that it is an analog transport facility for the digital DS1 payload. Hence the TC-PAM line encoding. BERT testing with traditional digital stress patterns will help isolate trouble to the circuit packs(HLU,HRU,HRE)themselves or to equipment or wiring on either side of the HDSL span, but it will not just cause errors on the HDSL facility because you send different patterns of 1's & 0's. If you are infact working with Adtran HDSL4 technology on this trouble their should be a wealth of PM Data info on the facility to help you troubleshoot. SNR margins & Attenuation between the network elements on the span, splice detector/histogram, facility and alarm history, DSX-1 and DS1 performance data, and HDSL span performance data(ES,SES,UAS)can all be used to help isolate the trouble without actually taking the circuit down