The transmission group (TG) trace, a joint function of VTAM® and the NCP, traces the sequence of PIUs being sent through a transmission group. The transmission group trace shows PIU traffic on a transmission group as though the transmission group were a single line. The sequence of PIUs traced is the sequence of their entry to and exit from the transmission group, not the sequence of actual transmission along the physical lines in the transmission group. The line trace shows the sequence of PIUs transmitted and received on a physical line.