Devices connect to FDDI networks in four different ways, depending on whether they are end stations or concentrators, and are described according to their attachments. A dual attached device will have 2 ports, one marked A and the other marked B. Equipment is cabled such that the B port from one station connects into the A port of another on the primary ring, while A attaches to B on the standby or secondary ring. The four FDDI device types are listed below.
Single-attachment station (SAS). An SAS attaches only to the primary ring through a concentrator (hub-type device). This allows the station to be powered down or disconnected without affecting the network.
Dual-attachment station (DAS). A DAS attaches to both the primary and backup rings. Because they connect directly to both rings, powering down and/or removing these devices will impact the network.
Single-attachment concentrator (SAC). A SAC connects to only the primary ring.
Dual-attachment concentrator (DAC). A DAC connects to both rings, acting as a device into which single-attachment stations can be connected.