Software User's Guide Version 3.4

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. This diagram shows Single Access Stations (SAS), Dual Access Stations (DAS), Single Access Concentrators (SAC), and Dual Access Concentrators (DAC) in one Main Ring Path of an FDDI network.