I am trying to troubleshoot a problem with a voice circuit on my PBX. I have been able to isolate the problem to the public phone network. In working with my vendor I have demonstrated the problem using a T-bird that displays the 8 bits on each channel. When the problem occurs the indicators for the first 6 bits go blank with the 7th indicator lit solid. Can anyone tell me what each of the 8 bits is used for on an AMI D4 circuit?