Abstract: A concentrator which couples stations to a communications network includes internal devices connected to form a primary loop which merges with the communications network if the concentrator is connected to said communications network. The internal devices include a Station MAC which communicates with stations outside of the concentrator. A local ring including a Roving MAC and at least one Concentrator Configuration Element (CCE) is also provided. A microprocessor, with an output bus interconnecting the Roving MAC, the CCE, and the Station MAC, is also provided. The concentrator insert/de-insert in the communications network without disrupting data already on the network.

Abstract: Described is an improved method and apparatus which allows graceful insertion/de-insertion of a station into an FDDI network. The circuit arrangement known as "Roving MAC" captures a local token which is circulating on a local loop established between the Roving MAC and the station. The "Roving MAC" issues void frames while holding the local token. A circuit arrangement monitors the FDDI network. If the token on the FDDI network is sensed before the local token is released, a data frame is allowed to pass through, but the token concatenated to the data frame is switched into the station thus joining it to the FDDI network. A scrubber circuit simultaneously removes the void frames and issues idle signals on the FDDI network.