C-2
BASIC FDDI NETWORKS
When a station has a frame waiting to transmit, the station captures
the token at the next opportunity, transmits the data frame, and then
reissues the token. (A Token Holding Timer (THT) controls the
maximum length of time that any station may retain the token.) As
the data frame circulates around the ring, it is received and repeated
by each station on the ring. When the frame arrives at the station
defined by the destination address, the frame is copied into the
receiving station’s buffer and forwarded with information reflecting
the receipt of the frame and related frame status. When the data
frame has circulated completely around the ring, the source station
strips the data frame from the ring.
FDDI networks use duplex fiber optic cable for point-to-point
connections between a number of stations to form two closed loops.
The two rings serve as redundant (primary and secondary) data paths
that operate as counter-rotating rings. Redundant rings facilitate
recovery procedures in the event of a ring segment failure. This
recovery is similar to Token Ring/IEEE 802.5 networks and is
discussed in greater detail later in this section.
The FDDI standard defines two ring access methods, single
attachment and dual attachment (see Figure C-1). Dual attached
stations (DAS) and dual attached concentrators (DAC) connect to
both primary and secondary rings and are capable of restoring ring
continuity in the event of a segment failure. Single attached
stations (SAS) and single attached concentrators (SAC) are
incapable of restoring ring continuity and therefore cannot be
installed on the main ring path. Instead, single attached devices
access the main ring through a DAC and duplex fiber optic cable
connections that form branches extending from the DAC out to each
of the attached SASs to create a Ring of Trees topology. The DAC
controls main ring access to the attached SASs, restoring the
continuity of the ring whenever an attached SAS fails or is
disconnected or turned-off.