Conventional modems coupled in parallel to a coupling need a sharing apparatus, which can be avoided by providing each modem with a detector for detecting upstream/downstream signals to be transceived by other modems coupled in parallel. The detector has a distinguisher for distinguishing upstream signals and downstream signals. An additional filter between the coupling and detector prevents the modem from interfering with the other modems coupled in parallel. The modem may have an active/passive state corresponding with a lower/higher impedance.