A data terminal equipment which is coupled to an ISDN through an access line and uses a hierarchical protocol in conformity to an OSI reference standard model includes the following structural elements. A first data link protocol circuit provides a link access procedure balanced (LAPB) modulo-8 data link protocol. A second data link protocol circuit provides an LAPB modulo-128 data link protocol. A third data link protocol circuit provides a data link protocol based on a link access procedure for a data channel (LAPD). A protocol determining circuit determines a data link protocol to be used by referring to an address field and a control field of a cell set-up signal which is supplied from a second data terminal equipment. A data link protocol activating circuit selects and activates one of the first, second and third circuits on the basis of the result provided by the protocol determining circuit.