3Com CoreBuilder 3500, Руководство по внедрению

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Key Concepts 93
All physical connections in an FDDI topology are duplex links (a pair of
insulated fiber-optic conductors). Both the FDDI ring and the ring of trees
that are created through concentrators are made up of duplex links.
Interconnect the nodes in an FDDI network to form at
most one ring.
If a topology is legal, when physical connections and nodes fail or are
removed from the network, one or more legal FDDI topologies are
formed. So subsets of legal topologies are also legal. Examples of legal
FDDI topologies include the dual ring with trees, the dual ring without
trees, and the single tree. For information about legal topologies, see
“Setting the Connection Policies” later in this chapter.
Logical Topology: The Dual Ring
A legal FDDI topology consists of at most two separate logical rings: the
primary ring and the secondary ring. These logical rings are formed from
the physical links that make up the Physical Layer connections. For
example, a set of DASs that are connected into a closed loop form an
FDDI dual ring (that is, A to B; B to A). Each ring is a logical ring, that is, a
separate data path with its own token.
Functionally, the dual ring provides a high degree of reliability to a LAN.
When an FDDI network is in normal operation, only the primary ring
transmits and receives data. The secondary ring may also carry data, but it
is typically used as a backup in case there is a connectivity problem in the
primary ring or in one of the nodes on the ring.
When a single fault takes place on an FDDI dual ring, recovery can be
made by joining the two rings between the two nodes that are adjacent
to the fault. Doing this creates a single logical ring, which results in a
wrapped configuration. A wrapped ring is a legal FDDI topology. In the
same way, when many faults take place, several disjointed logical rings
are created, producing multiple FDDI topologies.
Nodes and
Attachments
An FDDI network is made up of stations, concentrators, and switches that
contain active services or management elements that conform to the
ANSI FDDI standards. These stations and concentrators are connected to
optical fiber medium and are attached in the prescribed manner set forth
in the FDDI standards to allow reliable data transmission. Connections are
made through FDDI ports and are managed by FDDI MACs.