Cisco Networking Academy Del Mar College
ITNW 1392 - Beginning Router Configuration
Instructor: Michael P. Harris

Table of Contents

Fiber Distributed Data Interface (FDDI)

Fiber Distributed Data Interface (FDDI)


The Fiber Distributed Data Interface (FDDI) standard was produced by the ANSI X3T9.5 standards committee in the mid-1980s. During this period, high-speed engineering workstations were beginning to tax the capabilities of existing local-area networks (LANs) (primarily Ethernet and Token Ring). A new LAN was needed that could easily support these workstations and their new distributed applications. At the same time, network reliability was becoming an increasingly important issue as system managers began to migrate mission-critical applications from large computers to networks. FDDI was developed to fill these needs.

After completing the FDDI specification, ANSI submitted FDDI to the International Organization for Standardization (ISO). ISO has created an international version of FDDI that is completely compatible with the ANSI standard version.

Today, although FDDI implementations are not as common as Ethernet or Token Ring, FDDI has gained a substantial following that continues to increase as the cost of FDDI interfaces diminishes. FDDI is frequently used as a backbone technology as well as a means to connect high-speed computers in a local area.

Technology Basics

FDDI specifies a 100-Mbps, token-passing, dual-ring LAN using a fiber-optic transmission medium. It defines the physical layer and media-access portion of the link layer, and so is roughly analogous to IEEE 802.3 and IEEE 802.5 in its relationship to the Open System Interconnection (OSI) reference model.

Although it operates at faster speeds, FDDI is similar in many ways to Token Ring. The two networks share many features, including topology (ring), media-access technique (token passing), reliability features (redundant rings, for example), and others. For more information on Token Ring and related technologies, refer to Chapter 6, "Token Ring/IEEE 802.5."

One of the most important characteristics of FDDI is its use of optical fiber as a transmission medium. Optical fiber offers several advantages over traditional copper wiring, including security (fiber does not emit electrical signals that can be tapped), reliability (fiber is immune to electrical interference), and speed (optical fiber has much higher throughput potential than copper cable).

FDDI defines use of two types of fiber: single mode (sometimes called monomode) and multimode. Modes can be thought of as bundles of light rays entering the fiber at a particular angle. Single-mode fiber allows only one mode of light to propagate through the fiber, while multimode fiber allows multiple modes of light to propagate through the fiber. Because multiple modes of light propagating through the fiber may travel different distances (depending on the entry angles), causing them to arrive at the destination at different times (a phenomenon called modal dispersion), single-mode fiber is capable of higher bandwidth and greater cable run distances than multimode fiber. Due to these characteristics, single-mode fiber is often used for interbuilding connectivity, while multimode fiber is often used for intrabuilding connectivity. Multimode fiber uses light-emitting diodes (LEDs) as the light-generating devices, while single-mode fiber generally uses lasers.

FDDI Specifications

FDDI is defined by four separate specifications (see Figure 9-1):

Figure 9-1:
FDDI Standards

Physical Connections

FDDI specifies the use of dual rings. Traffic on these rings travels in opposite directions. Physically, the rings consist of two or more point-to-point connections between adjacent stations. One of the two FDDI rings is called the primary ring; the other is called the secondary ring. The primary ring is used for data transmission, while the secondary ring is generally used as a backup.

Class B or single-attachment stations (SAS) attach to one ring; Class A or dual-attachment stations (DAS) attach to both rings. SASs are attached to the primary ring through a concentrator, which provides connections for multiple SASs. The concentrator ensures that failure or power down of any given SAS does not interrupt the ring. This is particularly useful when PCs, or similar devices that frequently power on and off, connect to the ring.

A typical FDDI configuration with both DASs and SASs is shown in Figure 9-2.

Figure 9-2: FDDI Nodes: DAS, SAS, and Concentrator

Each FDDI DAS has two ports, designated A and B. These ports connect the station to the dual FDDI ring. Therefore, each port provides a connection for both the primary and the secondary ring, as shown in Figure 9-3.

Figure 9-3: FDDI DAS Ports

Traffic Types

FDDI supports real-time allocation of network bandwidth, making it ideal for a variety of different application types. FDDI provides this support by defining two types of traffic: synchronous and asynchronous. Synchronous traffic can consume a portion of the 100-Mbps total bandwidth of an FDDI network, while asynchronous traffic can consume the rest. Synchronous bandwidth is allocated to those stations requiring continuous transmission capability. Such capability is useful for transmitting voice and video information, for example. Other stations use the remaining bandwidth asynchronously. The FDDI SMT specification defines a distributed bidding scheme to allocate FDDI bandwidth.

Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned an asynchronous priority level. FDDI also permits extended dialogues, where stations may temporarily use all asynchronous bandwidth. The FDDI priority mechanism can essentially lock out stations that cannot use synchronous bandwidth and have too low an asynchronous priority.

Fault-Tolerant Features

FDDI provides a number of fault-tolerant features. The primary fault-tolerant feature is the dual ring. If a station on the dual ring fails or is powered down or if the cable is damaged, the dual ring is automatically "wrapped" (doubled back onto itself) into a single ring, as shown in Figure 9-4. In this figure, when Station 3 fails, the dual ring is automatically wrapped in Stations 2 and 4, forming a single ring. Although Station 3 is no longer on the ring, network operation continues for the remaining stations.

Figure 9-4: Station Failure, Ring Recovery Configuration

Figure 9-5 shows how FDDI compensates for a wiring failure. Stations 3 and 4 wrap the ring within themselves when wiring between them fails.

Figure 9-5: Failed Wiring, Ring Recovery Configuration

As FDDI networks grow, the possibility of multiple ring failures grows. When two ring failures occur, the ring will be wrapped in both cases, effectively segmenting the ring into two separate rings that cannot communicate with each other. Subsequent failures cause additional ring segmentation.

Optical bypass switches can be used to prevent ring segmentation by eliminating failed stations from the ring. This is shown in Figure 9-6.

Figure 9-6: Use of Optical Bypass Switch

Critical devices such as routers or mainframe hosts can use another fault-tolerant technique called dual homing to provide additional redundancy and help guarantee operation. In dual-homing situations, the critical device is attached to two concentrators. One pair of concentrator links is declared the active link; the other pair is declared passive. The passive link stays in backup mode until the primary link (or the concentrator to which it is attached) is determined to have failed. When this occurs, the passive link is automatically activated.

Frame Format

FDDI frame formats (shown in Figure 9-7) are similar to those of Token Ring.

Figure 9-7: FDDI Frame Format

The fields of an FDDI frame are as follows:


The high cost of fiber-optic cable has been a major impediment to the widespread deployment of FDDI to desktop computers. At the same time, shielded twisted-pair (STP) and unshielded twisted-pair (UTP) copper wire is relatively inexpensive and has been widely deployed. The implementation of FDDI over copper wire is known as Copper Distributed Data Interface (CDDI).

Before FDDI could be implemented over copper wire, a problem had to be solved. When signals strong enough to be reliably interpreted as data are transmitted over twisted-pair wire, the wire radiates electromagnetic interference (EMI). Any attempt to implement FDDI over twisted-pair wire had to ensure that the resulting energy radiation did not exceed the specifications set in the United States by the Federal Communications Commission (FCC) and in Europe by the European Economic Council (EEC). Three technologies reduce energy radiation:

In June 1990, ANSI established a subgroup called the Twisted Pair-Physical Medium Dependent (TP-PMD) working group to develop a specification for implementing FDDI protocols over twisted-pair wire. ANSI approved the TP-PMD standard in February 1994. Approval of the standard is pending in Europe.

Note Of the many categories and types of twisted-pair wire, the ANSI standard only recognizes Category  5 UTP and Type 1 STP.

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