Bull DPX/20 Installation And Configuration Manual - page 38
B-2
FDDI Adapters – Installation and Configuration Guide
Connection Types
There are three basic connection types that can be mixed and matched in the same
network:
• Dual attachment to the dual ring
• Single attachment to a concentrator
• Dual homing.
Dual Attachment to the Dual Ring
Class A devices can be connected directly to the FDDI dual ring. A device connected to
both rings is called ”dual attached”. Since each ring has a transmit and receive line, there
are two transmit paths out of and two receive paths into the dual attached device (DAS or
DAC in Figure 10). Because of redundant data paths, dual attachment offers fault tolerance.
Single Attachment to a Concentrator
Class B devices connect point to point to a concentrator. This connection type is called
”single attached”. For single attached devices, the concentrator acts as the central hub.
When SASs are connected to a single concentrator, the concentrator is said to be
non–attach or stand alone. In this situation, the dual ring is collapsed into the concentrator.
Both SASs and SACs can be single attached.
Dual Homing
This is a connection type for a Class A device where it connects to two different
concentrators. The connection to one concentrator is the primary connect and is active; the
connection to the other concentrator is for backup purposes and inactive. Since each
connection to the concentrator has a send and a receive path, there are two transmit paths
out of and two receive paths into the dual home device. Because of redundant data paths,
dual attachment offers fault tolerance.
Both DASs and DACs can be dual homed.
When concentrators are connected to other concentrators building a tree below the dual
ring, it is called ”cascading”. Cascading applies to both single attached and dual homed
concentrators off the dual ring.