In the mid 1980's, high-speed engineering workstations had pushed the capabilities of existing Ethernet and Token Ring to their limits. Engineers needed a LAN that could support their workstations, and their new applications. At the same time, system managers became concerned with network reliability issues as mission-critical applications were implemented on the high-speed networks.

The ANSI X3T9.5 standards committee, to resolve these issues, produced the Fiber Distributed Data Interface (FDDI) standard. After completing the specifications, ANSI submitted FDDI to the International Organization for Standardization (ISO), who, then created an international version of the FDDI that is completely compatible with the ANSI standard version.

Although FDDI implementations are not as common today as Ethernet or Token Ring, FDDI has a substantial following, and continues to grow as its costs decrease. FDDI is frequently used as a backbone technology, and to connect high-speed computers in a LAN.

FDDI has four specifications:

1. Media Access Control (MAC) - defines how the medium is accessed, including:
• frame format
• token handling
• addressing
• algorithm for calculating a cyclic redundancy check and error recovery mechanisms
2. Physical Layer Protocol (PHY) - defines data encoding/decoding procedures, including:
• clocking requirements
• framing
• other functions
3. Physical Layer Medium (PMD) - defines the characteristics of the transmission medium, including:
• fiber optic link
• power levels
• bit error rates
• optical components
• connectors
4. Station Management (SMT) - defines the FDDI station configuration, including:
• ring configuration
• ring control features
• station insertion and removal
• initialization
• fault isolation and recovery
• scheduling
• collection of statistics

The fields of an FDDI frame are as follows:

• preamble - prepares each station for the upcoming frame

• start delimiter - indicates the beginning of the frame, and consists of signaling patterns that differentiate it from the rest of the frame

• frame control - indicates the size of the address fields, whether the frame contains asynchronous or synchronous data, and other control information

• destination address - contains a unicast (singular), multicast (group), or broadcast (every station) address; destination addresses are 6 bytes (like Ethernet and Token Ring)

• source address - identifies the single station that sent the frame; source addresses are 6 bytes (like Ethernet and Token Ring)

• data - control information, or information destined for an upper-layer protocol

• frame check sequence (FCS) - filled by the source station with a calculated cyclic redundancy check (CRC), value dependent on the frame contents (as with Token Ring and Ethernet). The destination station recalculates the value to determine whether the frame may have been damaged in transit. If it has been, the frame is discarded.

• end delimiter - contains non-data symbols that indicate the end of the frame

• frame status - allows the source station to determine if an error occurred and if the frame was recognized and copied by a receiving station

FDDI uses a token passing strategy similar to Token Ring. Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit data. If a node receiving the token has no information to send, it passes the token to the next end-station. Each station can hold the token for a maximum period of time, depending on the specific technology implementation.

When a station that is in possession of the token has information to transmit, it seizes the token and alters one of its bits. The token then becomes a start-of-frame sequence. Next, the station appends the information that it transmits to the token, and sends this data to the next station on the ring.

There is no token on the network while the information frame is circling the ring, unless the ring supports early token release. Other stations on the ring must wait for the token to become available. FDDI networks have no collisions. If early token release is supported, a new token can be released when  the frame transmission has finished.

The information frame circulates around the ring until it reaches the intended destination station, which copies the information for processing. The information frame continues around the ring until it reaches the sending station, where it is removed. The sending station can check the returning frame to see whether the frame was received, and subsequently copied by the destination.

Unlike CSMA/CD networks, such as Ethernet, token-passing networks are deterministic. This means you can calculate the maximum time that will pass before any end station will be able to transmit. FDDI's dual ring assures that not only are stations guaranteed their turn to transmit, but if one part of one ring is damaged or disabled for any reason, the second ring can be used. This makes FDDI very reliable.

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

• 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. This is useful for transmitting voice and video information. The remaining bandwidth is used for asynchronous transmissions.
• The FDDI SMT specification defines a distributed bidding scheme to allocate FDDI bandwidth.

Asynchronous

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

FDDI uses an encoding scheme called 4B/5B. Every 4 bits of data are sent as a 5 bit code. The signal sources in FDDI transceivers are LEDs or lasers.

FDDI specifies a 100 Mbps, token-passing, dual-ring LAN that uses a fiber-optic transmission medium. It defines the physical layer and media access portion of the link layer, which is similar to IEEE 802.3 and IEEE 802.5 in its relationship to the OSI Model. Although it operates at faster speeds, FDDI is similar to Token Ring. The two networks share a few features, such as topology (ring) and media access technique (token-passing).  A characteristic of FDDI is its use of optical fiber as a transmission medium. Optical fiber offers several advantages over traditional copper wiring, including such advantages as:

• security - Fiber does not emit electrical signals that can be tapped.
• reliability - Fiber is immune to electrical interference.
• speed - Optical fiber has much higher throughput potential than copper cable.

FDDI defines the two specified types of fiber: single-mode (also mono-mode); and multi-mode. 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 multi-mode fiber allows multiple modes of light to propagate through the fiber. Multiple modes of light propagating through fiber may travel different distances, depending on their entry angles. This causes 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 multi-mode fiber. Because of these characteristics, single-mode fiber is often used for inter-building connectivity while multi-mode fiber is often used for intra-building connectivity. Multi-mode fiber uses LEDs as the light-generating devices, while single-mode fiber generally uses lasers.

FDDI specifies the use of dual rings for physical connections. Traffic on each ring 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; the secondary ring is generally used as a back up. 

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 a 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 . 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.