Abstract: A system includes private lines for carrying respective signals which each have at least one D channel for control information, and plural B channels for data. The system also includes a multiplexer connected to the private lines, which receives the signals and generates a multiplexed signal for transmission on a digital multiplex interface. The system further includes a demultiplexer which receives the multiplexed signal received from the digital multiplex interface, to generate signals for transmission on the private lines. The multiplexer and demultiplexer are designed so that the total number of D channels and B channels can be changed without requiring significant effort or modification of either the multiplexer or the demultiplexer. The system also includes a testing device which transmits and returns a multiplexed signal from the multiplexer, to the demultiplexer. The returned multiplexed signal can be used to test the system without requiring involvement of external devices.