Abstract: A marker controlling the establishment of connections between local subscriber lines and inter-office links, via an associated line concentrator/distributor, comprises a link allocator, a signalization unit and a service unit. The link allocator responds to requests from a local subscriber line, transmitted via a line finder, or from a remote exchange, arriving over a common signal path, to control the line concentrator/distributor for connecting that local line to an available link or a designated link to a selected line. The signalization unit coacts with a respective two-way coupler in each link, in a time-division mode, to pick up outgoing dial pulses for transmission via the common path and to transmit switching instructions from that path to the engaged subscriber line. The service unit, communicating with both the link allocator and the signalization unit, receives and sends out information relating to possible malfunctions or alarm conditions.

Abstract: A marker at a central-office terminal, controlling the establishment of connections between local subscriber lines and inter-office links via an associated line concentrator/distributor, includes a source of d-c test signals of reversible polarity connectable in a phantom circuit which extends from the central office via a transmitting and a receiving branch of a local line to a subscriber station tied to that line. The subscriber station has a master switch, establishing either an inactive position or a standby position, and a hook switch closable in the standby position to initiate or accept a call. Depending on the positions of these two switches, a current discriminator at the marker detects one of three current levels (zero, low or high) with either polarity of the test signal to determine whether a call can go through or whether an alarm condition exists.