Transmission apparatus and a method for transmitting data in a data transmission system

A transmission apparatus includes a reception line selecting unit, a communication confirming frame transmitting unit, a communication confirming table, and a reception line deciding unit. The reception line selecting unit selects one of several reception lines each formed by the synchronous network. The frame transmitting unit transmits a communication confirming frame to the synchronous network at least at a predetermined period. The communication confirming table stores each reception time of the communication confirming frame received through the synchronous network. The deciding unit decides occurrence of failure on the reception line corresponding to the reception time when a difference between a reception time stored in the table and a present time at the predetermined period exceeds a predetermined value, and transmits instructions for switching the reception line to the reception line selecting unit.