FDDI Media
Class B or SAS (single-attachment stations) attach to one ring Class A or DAS (dual attachment stations) attach to both rings.
SASs is attached to the primary ring by a concentrator which provides connections for multiple SASs. The concentrator makes sure that a failure or power down of any given SAS doesn't interrupt the ring. This is particularly useful when PCs or similar devices that frequently power on and off connect to the ring.
Every FDDI DAS has two ports designated A and B. These ports connect the station to the dual FDDI ring so each port provides a connection for both the primary and the secondary ring.