Abstract: A plurality of control channel units in a telephone channel bank which communicate with other control channel units through a trunk line connection are serviced by a system which monitors data transmission consisting of coded commands and responses. These commands are transmitted to and response received from the channel units. A central control which provides the commands in coded characters which in the described embodiment are hexadecimal numbers. The monitoring system receives the data of the coded characters and analyzes and stores the data in memory. The data stored is thus analyzed for abnormal or erroneous conditions or status. When such condition or status or other failure occurs it is detected, the circumstances of the occurrence are reviewed and reported in a print-out on a printer. And also the report may be displayed on a screen. Thus, the report of the failure is accompanied by an evaluation of the failing channel.