A Dual Attachment Concentrator (DAC) has connections to both FDDI rings, Single Attachment Concentrators( SAC) have only one connection to the primary ring. DACs have one or more master ports. They can be used to create star topologies or tree topologies within the FDDI ring by connecting multiple single- attachment components. As a rule, a concentrator does not have a MAC component, although this is provided as an option. In contrast to Single Attachment Stations, the Dual Attachment Concentrator( DAS), like the Dual Attachment Station (DAS), has an optional bypass.