Cabletron Systems MMAC-5FNB, Сетевое руководство

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Technology Basics
3-16 Fiber Distributed Data Interface
Operation
Rings
FDDI operation is based on the movement of data around a series of rings. Like
the organization of the Token Ring technology discussed earlier, FDDI data passes
from one station to another in a predetermined order. In FDDI LANs, there are
two main types of rings: dual counterrotating rings and single rings.
The dual counterrotating ring, hereafter referred to as the dual ring, is the main
ring of an FDDI network. It incorporates the most powerful fault-tolerance
features of the FDDI technology. The dual ring consists of two rings, one designed
to pass data in one direction, one designed to pass it in the opposite direction. If
there is a fault in the dual ring, the network will cause transmissions to wrap from
the primary ring to the secondary ring and travel back along the network,
avoiding the fault and keeping the ring whole.
The dual ring can be connected to a series of single rings, which extend the FDDI
network to many stations. A connection to a single FDDI ring does not provide
the same level of fault-tolerance as a connection to the dual ring, but typically
costs less. Since a series of single rings can be connected to each other through the
dual ring, single ring connections are often used to make connections to
workstations and end users, while the dual ring handles connections between
hubs.
Devices
As there are two types of rings in FDDI networks, there are also two methods that
FDDI devices may use to connect to the network. FDDI device may be
dual-attached, connecting directly to the dual ring, or single-attached, connecting
to a single ring. Dual-attached devices require two FDDI ports, one attaching to
the primary ring and one attaching to the secondary ring. Dual attached devices
are more expensive than single attached devices, but incorporate greater fault
tolerance.
FDDI devices, whether single attached or dual attached, fall into one of two
categories. Any FDDI device which is an endpoint on the network, such as a
workstation or a fileserver, is referred to as a station. Stations may be
dual-attached or single-attached, depending on the connection method they use.
If an FDDI workstation is designed to be connected to the dual counterrotating
rings, it is called a “Dual Attached Station,” abbreviated DAS.
FDDI devices which provide an access point for other FDDI devices are called
concentrators. The typical concentrator connects a series of Single Attached
Stations (SASs) to the FDDI ring. If the concentrator is attached directly to the
dual ring, it is referred to as a Dual Attached Concentrator (DAC).