Preparing for Installation 2-99
Preparing Network Connections
FDDI Station Descriptions
Fiber Distributed Data Interface (FDDI), which specifies a 100-Mbps, token-passing dual-ring
network using fiber-optic transmission media, is defined by the ANSI X3.1 standard and by
ISO 9314, the international version of the ANSI standard. An FDDI network comprises two
token-passing fiber-optic rings: a primary ring and a secondary ring.
An FDDI ring consists of two or more point-to-point connections between adjacent stations. On
most networks, the primary ring is used for data communication, and the secondary ring is used as
a backup. Class B, or single attachment stations (SAS), attach to one ring and are typically attached
through a concentrator; Class A, or dual attachment stations (DAS), attach to both rings. Figure 2-23
shows a typical FDDI configuration with both DAS and SASs.
Figure 2-23 Typical FDDI Configuration with DAS, Concentrator, and SASs
SASs (Class B) typically attach to the primary ring through a concentrator, which provides
connections for multiple SASs. The concentrator ensures that a failure or power-down of any SAS
does not interrupt the ring. SASs use one transmit port and one receive port to attach to the single
ring. DASs (Class A) have two physical ports, designated PHYA and PHY B, each of which
connects the station to both the primary and secondary rings. Each port is a receiver for one ring and
a transmitter for the other. For example, PHY A receives traffic from the primary ring, and PHY B
transmits to the primary ring.
The dual rings in an FDDI network provide fault tolerance. If a station on a dual ring shuts down or
fails, such as Station 3 in Figure 2-24, the ring automatically wraps (doubles back on itself) to form
a single contiguous ring. This removes the failed station from the ring, but allows the other stations
to continue operation. In Figure 2-24, the ring wraps to eliminate Station 3 and forms a smaller ring
that includes only Stations 1, 2, and 4.
SAS SAS SAS
DAS
FDDI
H1553a
Concentrator
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