Setting this write-only object to startTest (1) begins a V.54 local loopback test on the node; noTest (2) leaves it inactive. A local loop test loops the signal back at the near end without involving the remote modem, so it isolates whether a suspected line problem is on this node's own local port hardware rather than downstream. For example, an admin troubleshooting a noisy circuit would run this local loop first: if the loop passes clean, the fault is confirmed to be further out on the line or remote equipment.