Abstract: A digital data transmission system comprising a data transmission network formed by several node stations (A1-A4) and interconnecting high-speed trunk lines (1-5). Each node station comprises several low-speed parallel subscriber interfaces controlled by a network management system (6), subscriber stations (B1-B5) being connectable to the subscriber interfaces either directly or through a serial modem link (10-13), depending on the distance. The network management can be extended up to the subscriber stations so that said modem link (10-13) transfers all signals of the parallel subscriber interface in serial form in both directions, forming, at the end close to the subscriber station (B2-B5), a parallel slave interface corresponding to the subscriber interface.

Abstract: The invention relates to an interface unit intended to be connected to a parallel connection of a data communication equipment (DCE) or a terminal equipment (DTE). The interface unit comprises multiplexers means (52, 54) for multiplexing connection signals to serial form and vice versa. In the invention, the multiplexers means (52, 54) are connected to parallel connection signals through I/O device (51) comprising an I/O line (L1 . . . L16) for each connection signal. At least some of the I/O lines are automatically selectable as an input or an output and/or connectable to at least two alternative inputs or outputs in the multiplexer (52, 54) for achieving cross-connections, depending on whether the interface is connected to the terminal equipment (DTE) or to the data communication equipment (DCE).