A data communication apparatus includes a data communication section for preforming data communication according to full duplex communication, a testing section for performing a communication test prior to the data communication, and a transmission controller for determining whether full duplex communication can be performed during a period of the communication test performed by the testing section. The testing section includes a backward signal generator, a backward signal detector, and a control signal analyzer. The backward signal generator and the backward signal detector may be replaced with an echo canceller.