Time divison multiplex ring

A Time Divison Multiplex Ring (TDM Ring) comprising a plurality of nodes (central offices) connected together by interchange trunks (main and standby paths) in a ring type configuration. Each node contains a multiplexer, a demultiplexer, data, alarm, bridging, switching circuits, transmitters and receivers on the main and standby paths, all arranged so that (a) data and alarm signals are transmitted in a first direction on the main path and in an opposite direction on the standby path, (b) the bridging circuit forms a transmission path between the transmitter and receiver on the standby path of each node only in response to a predetermined alarm signal originating from an alarm circuit in another node received at a given node on the main transmission path and (c) the switching circuit is responsive to the presence of a predetermined alarm status to switch the data and alarm circuits at each node from a connection where all transmission is on the main path to a connnection whereby such signals are transmitted at least in part over the standby path.