The generalized PIU trace collects PIU trace data collected by the NCP. The resulting trace output shows the flow of PIUs exchanged between the NCP and its attached resources. This trace is hierarchical when started for a physical unit or a line. That is, logical units associated with the physical unit are automatically traced when traffic flows to them. Likewise, when the generalized PIU trace is started for a line, physical units and logical units associated with the line are automatically traced when traffic flows to them. When the generalized PIU trace is stopped for a physical unit, the trace is reset for all logical units associated with the physical unit, regardless of how the generalized PIU trace was started.