Fiber Distributed Data Interface (FDDI)

The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using fiber-optic cable.

FDDI is frequently used as high-speed backbone technology because of its support for high bandwidth and greater distances than copper.

It should be noted that relatively recently, a related copper specification, called Copper Distributed Data Interface (CDDI), has emerged to provide 100-Mbps service over copper. CDDI is the implementation of FDDI protocols over twisted-pair copper wire.

FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called counter-rotating). The dual rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle. As will be discussed in detail later in this article, the primary purpose of the dual rings is to provide superior reliability and robustness.

Figure: FDDI Uses Counter-Rotating Primary and Secondary Rings

FDDI Transmission Media:

FDDI uses optical fiber as the primary transmission medium, but it also can run over copper cabling.

As mentioned earlier, FDDI over copper is referred to as Copper-Distributed Data Interface (CDDI).

Optical fiber has several advantages over copper media. In particular, security, reliability, and performance all are enhanced with optical fiber media because fiber does not emit electrical signals.

A physical medium that does emit electrical signals (copper) can be tapped and therefore would permit unauthorized access to the data that is transiting the medium.
In addition, fiber is immune to electrical interference from radio frequency interference (RFI) and electromagnetic interference (EMI).

Fiber historically has supported much higher bandwidth (throughput potential) than copper, although recent technological advances have made copper capable of transmitting at 100 Mbps.
Finally, FDDI allows 2 km between stations using multimode fiber, and even longer distances using a single mode.

FDDI defines two types of optical fiber: single-mode and multimode. A mode is a ray of light that enters the fiber at a particular angle. Multimode fiber uses LED as the light-generating device, while single-mode fiber generally uses lasers.

Multimode fiber allows multiple modes of light to propagate through the fiber. Because these modes of light enter the fiber at different angles, they will arrive at the end of the fiber at different times.
This characteristic is known as modal dispersion. Modal dispersion limits the bandwidth and distances that can be accomplished using multimode fibers. For this reason, multimode fiber is generally used for connectivity within a building or a relatively geographically contained environment.

Single-mode fiber allows only one mode of light to propagate through the fiber. Because only a single mode of light is used, modal dispersion is not present with single-mode fiber. Therefore, single-mode fiber is capable of delivering considerably higher performance connectivity over much larger distances, which is why it generally is used for connectivity between buildings and within environments that are more geographically dispersed.

Figure: Light Sources Differ for Single-Mode and Multimode Fibers

FDDI Specifications: 

FDDI specifies the physical and media-access portions of the OSI reference model. FDDI is not actually a single specification, but it is a collection of four separate specifications, each with a specific function.

Combined, these specifications have the capability to provide high-speed connectivity between upper-layer protocols such as TCP/IP and IPX, and media such as fiber-optic cabling.

FDDI's four specifications are the

Media Access Control (MAC), 

Physical Layer Protocol (PHY),

Physical-Medium Dependent (PMD), and

Station Management (SMT) specifications.

The MAC specification defines how the medium is accessed, including frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error-recovery mechanisms.

The PHY specification defines data encoding/decoding procedures, clocking requirements, and framing, among other functions.

The PMD specification defines the characteristics of the transmission medium, including fiber-optic links, power levels, bit-error rates, optical components, and connectors. The SMT specification defines FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection.

FDDI is similar to IEEE 802.3 Ethernet and IEEE 802.5 Token Ring in its relationship with the OSI model. Its primary purpose is to provide connectivity between upper OSI layers of common protocols and the media used to connect network devices.


Figure: FDDI Specifications Map to the OSI Hierarchical Model

FDDI Station-Attachment Types
One of the unique characteristics of FDDI is that multiple ways actually exist by which to connect FDDI devices. FDDI defines four types of devices: single-attachment station (SAS), dual-attachment station (DAS), single-attached concentrator (SAC), and dual-attached concentrator (DAC).
An SAS attaches to only one ring (the primary) through a concentrator. One of the primary advantages of connecting devices with SAS attachments is that the devices will not have any effect on the FDDI ring if they are disconnected or powered off. Concentrators will be covered in more detail in the following discussion.

Each FDDI DAS has two ports, designated A and B. These ports connect the DAS to the dual FDDI ring. Therefore, each port provides a connection for both the primary and the secondary rings. As you will see in the next section, devices using DAS connections will affect the rings if they are disconnected or powered off.

Figure: FDDI DAS Ports Attach to the Primary and Secondary Rings

FDDI Frame Format

The FDDI frame format is similar to the format of a Token Ring frame. This is one of the areas in which FDDI borrows heavily from earlier LAN technologies, such as Token Ring. FDDI frames can be as large as 4,500 bytes.

Figure: The FDDI Frame Is Similar to That of a Token Ring 

The following descriptions summarize the FDDI data frame and token fields illustrated in the above figure.

Preamble - Gives a unique sequence that prepares each station for an upcoming frame.

Start delimiter - Indicates the beginning of a frame by employing a signaling pattern that differentiates it from the rest of the frame.

Frame control - Indicates the size of the address fields and whether the frame contains asynchronous or synchronous data, among other control information.

Destination address - Contains a unicast (singular), multicast (group), or broadcast (every station) address. As with Ethernet and Token Ring addresses, FDDI destination addresses are 6 bytes long.

Source address - Identifies the single station that sent the frame. As with Ethernet and Token Ring addresses, FDDI source addresses are 6 bytes long.

Data - Contains either information destined for an upper-layer protocol or control information.

Frame check sequence (FCS) - Is filed by the source station with a calculated cyclic redundancy check value dependent on frame contents (as with Token Ring and Ethernet).

The destination address recalculates the value to determine whether the frame was damaged in transit. If so, the frame is discarded.

End delimiter - Contains unique symbols; cannot be data symbols that indicate the end of the frame.

Frame status - Allows the source station to determine whether an error occurred; identifies whether the frame was recognized and copied by a receiving station.