The synchronous digital communications system according to the invention serves to transmit electric signals optically. The electric signals to be transmitted are converted from electrical to optical form (E/O1, E/O2, E/On) and are transmitted using wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM). At least one optical connection is configured as a nonswitched connection using at least one wavelength per transmission section between optical network elements or optical and electrical network elements, and serves to transmit synchronization and information signals. This has the advantage that independently of the switched communication links, synchronization is constantly ensured throughout the network. Each network element (NE1, NE2, NE3) has at least one interface unit that is reserved for synchronization and that continuously receives signals at the wavelength (&lgr;1) reserved for synchronization.