HP 200 Series Services And Applications page 239

How FDDI Works
FDDI is based on two counter-rotating 100-Mbps fiber-optic token-
passing rings. If one ring should fail, FDDI automatically becomes a single,
not dual, FDDI ring. The rings consist of point-to-point connections between
adjacent stations. Stations negotiate for ring access at initialization and
when new stations are added to the ring. The two rings act as a primary
carrier and a backup (secondary) carrier. Data flows in opposite directions
on the two rings.
Primary
Secondary
Figure 3. Dual Attachment Stations on an FDDI Network
FDDI Stations
There are three types of FDDI stations: Dual Attachment Stations (DAS),
Single Attachment Stations (SAS), and concentrators. See figure 3.
Dual Attachment Stations are devices that connect to both the primary ring
and the secondary ring simultaneously. Single Attachment Stations are
devices that connect to the ring through the primary ring only, and are
usually attached to the ring through concentrators.
Dual Attachment Stations have two ports, one labeled port A, the other port
B. On each port, there is a primary and a secondary ring connection; port A
is primary IN secondary OUT, port B is primary OUT secondary IN. If either
port fails, the other port then becomes primary IN and OUT on the same port.
Shining a Light on FDDI
DAS
Port Port
A B
Port Port
B A
DAS
How FDDI Works
3-45