Fiber Data Distributed Interface (FDDI) is a fiber-optic networking technology utilizing a ring topology to provide local area network connectivity at up to 200 Mbps.


  • Fiber Optic Connections
  • Single or Dual Ring Topology
  • Token passing for media access control
  • 100 Mbps or 200 Mbps
  • 4,352 byte frame size
  • Ring sizes in miles
  • Dual Ring
  • Counter rotating


  • A ports
  • B ports
  • M ports
  • S ports
  • Ring Wraps
  • Beaconing


  • Media Access Control (MAC)
  • Physical Layer (PHY)
  • Physical Layer, Medium Dependant (PMD)
  • Station Management (SMT)


  • ANSI Standards (ANSI Standards Website)
    • Media Access Control - ANSI X3.139-1987, Media Access Control (MAC)
    • PHY - ANSI X3.148-1988, Media Access Control (PHY)
    • PMD - ANSI X3.166-1989, Media Access Control (PMD)
    • SMF-PMD - ANSI X3.184-1993, Media Access Control (SMF-PMD)
    • SMT - ANSI X3.229-1994, Media Access Control (SMT)
  • ISO Standards
    • PHY - ISO 9314-1
    • MAC - ISO 9314-2
    • PMD - ISO 9314-3
    • SMF-PMD - ISO 9314-4
    • SMT - ISO 9314-6

Concepts of FDDI Operation


FDDI utilizes fiber optic technology as the communications media. FDDI transmits light through glass or clear plastic strands that are thinner than a human hair. These strands carry signals from place to place and are connected to laser-light emitters. The strands are lit and unlit to provide data communication. This makes FDDI very expensive however, and so you rarely see it in a computer LAN. You will more often see it in a cross campus network, but even this too is fading with the advent of gigabit fiber-optic ethernet (gig-e).

100 Mbps (or 200 Mbps with FFDT)

The FDDI system is 100 megabits per second, or 100,000,000 bits per second. This is REALLY fast, but not quite as fast as some of the high speed Optical Cable links used in many telecommunications companies (like Sprint, AT&T, Qwest, WorldCom etc).


Using the FDDI Full Duplex Technology, two FDDI rings can be built, one primary and one backup. Unlike an Ethernet network card, a FDDI network card has TWO network ports, an 'A' port, and a 'B' port. The 'B' port is the primary connection, and the 'A' port is used only in case the B port fails. Traffic runs in opposite directions.


Data travels around the rings in opposite directions, but both rings are rarely used. There is an 'A' ring, and a 'B' ring. The 'B' ring is the primary ring used for communication and the A ring is the backup. When the 'B' ring breaks, it 'wraps' the connection back around on the 'A' ring, hoping to run the connection back around the opposite direction. Ring Wraps are a major troubleshooting headache in FDDI systems, as when they wrap, the traffic heads back in the opposite direction.


FDDI passes a token around the ring. This restricts transmit use of the circuit to solely the station that controls the token. This is three octet frame



FDDI has AB ports for connecting to the A and B rings, but a FDDI switch will have M ports, also called 'concentrator' ports. An M port can be either an A or B port. Switches allow FDDI to be used more efficently and to concentrate FDDI connections to a single device.


A ring wrap is when the FDDI ring has a failure on the primary B path. The FDDI controllers on either side of the broken link then send their A links into 'wrap' mode and forward the data over the A chain of links, restoring connectivity to the severed station.


This layer controls the physical communications properties and allows the Data Distributed Interface functions to run over copper (CDDI) or fiber optics (FDDI)

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