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.
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. the primary purpose of the dual rings is to provide superior reliability and robustness. Figure shows the counter-rotating primary and secondary FDDI rings.
FDDI defines two types of optical fiber:
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 depicts single-mode fiber using a laser light source and multimode fiber using a light emitting diode (LED) light source.
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 shows the frame format of an FDDI data frame and token.
FDDI Frame Fields
The following descriptions summarize the FDDI data frame and token fields illustrated in 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.
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. the primary purpose of the dual rings is to provide superior reliability and robustness. Figure shows the counter-rotating primary and secondary FDDI rings.
FDDI defines two types of optical fiber:
- single-mode
- multimode.
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 depicts single-mode fiber using a laser light source and multimode fiber using a light emitting diode (LED) light source.
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 shows the frame format of an FDDI data frame and token.
FDDI Frame Fields
The following descriptions summarize the FDDI data frame and token fields illustrated in 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.