PURPOSE: To improve the reliability of a transmission line without incurring deterioration in the function of information transfer by using a simple protocol having no re-transmission control function and employing the re-transmission function of the layer 3 so as to ensure a fault on a user network interface and the reliability of information transfer.

CONSTITUTION: The terminal control system for an exchange in which a terminal equipment is controlled through transfer of a control message with respect to the terminal equipment based on communication protocols layered as layers 1,2,3 based on the 0SI reference model is provided with the layer 3 protocol control device having a major transfer function and with the layer 2 protocol control device implementing transfer of data unit based on the information transfer operation mode of non-confirmation type. When a transfer fault of the layer 2 data unit takes place, a timer monitor function for reply reception with respect to the incoming call message transfer provided to the layer 3 and a control message re-transmission function at the expiration of a timer are used to recover the error.