FDDI topology
FDDI specifies a topology consisting of a trunk area and a tree area. In the trunk area, the FDDI stations are connected to two rings operating in opposite directions, of which only one ring is usually used, namely the primary ring. The second ring, the secondary ring, serves as a backup and allows transmission to continue even in the event of a cable interruption in the trunk area.
In the event of multiple interruptions, individual sub-rings are automatically configured to allow communication to continue between station groups that are still connected.
FDDI stations connected together in the trunk area are referred to as "dual attached" because of their connection to the primary and secondary rings.
The FDDI standard defines concentrators as structuring means. These components are used to connect stations that are only to have access to the primary ring (single attached). Concentrators can be cascaded so that logical star topologies or tree topologies (tree areas) can be configured using concentrators.