Nways Multiprotocol Access Services Software User's Guide Version 3.3

Using FDDI

This chapter describes how to set software-configurable information for the Fiber Distributed Data Interface (FDDI) in the router.

This chapter contains the following sections:

• "Fiber Distributed Data Interface (FDDI) Overview"

Fiber Distributed Data Interface (FDDI) Overview

Fiber Distributed Data Interface (FDDI) is described by the ANSI X3T9.5 and ISO 9314 committees as a dual counter-rotating ring that operates at a defined speed of 100 Mbps.

In many ways, FDDI is similar to the IEEE 802.5 token-ring, although there are differences, some of which are described in "Differences Between FDDI and Token-Ring".

Token-Passing Ring Network

FDDI is defined as a token-passing protocol. Each station has the chance to transmit data when a token passes. A station can decide how many frames it will transmit using an algorithm that permits "bandwidth" allocating.

FDDI also allows a station to transmit many frames without releasing the token in a way that is similar to the IEEE 802.5 token-ring standard.

An FDDI ring network consists of a set of stations/devices connected as a serial string of stations/devices and transmission media to form a physically closed loop. Information is transmitted sequentially as a stream of suitably encoded signals from one active station/device to the next active one.

Each station/device generally regenerates and repeats each token and can serve as the means of attaching one or more stations/devices to the network.

Primary and Secondary Rings

FDDI defines two rings:

• The primary ring, which is similar to the main ring path in a token-ring network.
• The secondary ring, which is similar to the backup ring path in a token-ring network.

Each ring path consists of two fibers, each fiber transmitting one signal; one is pushed and one is pulled in a device. Each fiber is equivalent to a pair of copper conductors. The physical approach in terms of fiber optics is similar to physical fiber optic token-ring paths.

Attachment of Devices

FDDI permits many attachment units:

• Stations or devices
• Concentrators
• Bridges

These units can be attached to FDDI networks in various ways, similar to those for token-ring networks.

Differences Between FDDI and Token-Ring

The main differences between FDDI and token-ring techniques are:

• A device can be attached directly to rings without a concentrator, such as a multi-station access unit (MSAU) on a token ring.

• A device can be attached to either or both of the primary and secondary rings.

FDDI defines two device classes, A and B, to differentiate between devices that attach to one ring or both rings, as described in the next section.

Device Classes A and B

FDDI defines two device classes:

• A Class A device attaches to both rings directly.

  It can be a station, called a Class A station or Dual Access Station (DAS), or it can be a Concentrator, called a Dual Access Concentrator (DAC)

• A Class B device attaches to only one of the rings directly or through a concentrator.

  It can be a station, called a Class B station or Single Access Station (SAS), or it can be a Concentrator, called a Single Access Concentrator (SAC)

FDDI Network Diagram

Figure 14. FDDI Network Diagram


View figure.