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 a twisted-pair copper wire.
FDDI uses a 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. The primary purpose of the dual rings is to provide superior reliability and robustness. The figure shows the counter-rotating primary and secondary FDDI 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).
The 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.
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.
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.
The figure illustrates the four FDDI specifications and their relationship to each other and to the IEEE-defined Logical Link Control (LLC) sublayer. The LLC sublayer is a component of Layer 2, the MAC layer, of the OSI reference 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), a single-attached concentrator (SAC), and dual-attached concentrator (DAC).
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 devices using DAS connections will affect the rings if they are disconnected or powered off.
An FDDI concentrator (also called a dual-attachment concentrator [DAC]) is the building block of an FDDI network. It attaches directly to both the primary and secondary rings and ensures that the failure or power-down of any SAS does not bring down the ring. This is particularly useful when PCs, or similar devices that are frequently powered on and off, connect to the ring.
FDDI Fault Tolerance
FDDI provides a number of fault-tolerant features. In particular, FDDI’s dual-ring environment, the implementation of the optical bypass switch, and dual-homing support make FDDI a resilient media technology.
FDDI’s 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. When the ring is wrapped, the dual-ring topology becomes a single-ring topology. Data continues to be transmitted on the FDDI ring without performance impact during the wrap condition.
When a single station fails, as shown in Figure devices on either side of the failed (or Powered – down) station wrap, forming a single ring. Network operation continues for the remaining stations on the ring. When a cable failure occurs, as shown in Figure b devices on either side of the cable fault wrap. Network operation continues for all stations.
It should be noted that FDDI truly provides fault tolerance against a single failure only. When two or more failures occur, the FDDI ring segments into two or more independent rings that are incapable of communicating with each other.
Optical Bypass Switch
An optical bypass switch provides continuous dual-ring operation if a device on the dual ring fails. This is used both to prevent ring segmentation and to eliminate failed stations from the ring. The optical bypass switch performs this function using optical mirrors that pass light from the ring directly to the DAS device during normal operation. If a failure of the DAS device occurs, such as a power-off, the optical bypass switch will pass the light through itself by using internal mirrors and thereby will maintain the ring’s integrity.
The benefit of this capability is that the ring will not enter a wrapped condition in case of a device failure.
The figure shows the functionality of an optical bypass switch in an FDDI network. When using the OB, you will notice a tremendous digression of your network as the packets is sent through the OB unit.
Critical devices, such as routers or mainframe hosts, can use a fault-tolerant technique called dual homing to provide additional redundancy and to help guarantee operation. In dual-homing situations, the critical device is attached to two concentrators.
The figure shows a dual-homed configuration for devices such as file servers and routers.
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.
FDDI Frame Fields
- 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.
Copper Distributed Data Interface
Copper Distributed Data Interface (CDDI) is the implementation of FDDI protocols over a twisted-pair copper wire. Like FDDI, CDDI provides data rates of 100 Mbps and uses dual-ring architecture to provide redundancy. CDDI supports distances of about 100 meters from desktop to concentrator.