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FDDI and ANSI X3T9.5

Fiber Distributed Data Interface (FDDI) is a type of media access defined by the American National Standards Institute (ANSI) X3T9.5 specification. Although FDDI also uses a MAC addressing scheme, it is different from Ethernet and Token-Ring. The difference lies in the fact that, instead of referring to the MAC address in terms of a 6-byte address as with the other topologies, FDDI uses 4-bit symbols to refer to the MAC addresses.

FDDI incorporates token passing in a dual-ring physical topology, which provides a self-healing redundancy. In the event of a problem with the primary ring, the secondary ring serves as a backup. If a break occurs, the data is rerouted from the primary ring to the secondary ring at two or more locations in what is called a ring wrap.

General FDDI Operation

FDDI has become a favorite standard for network backbones because it enables transmission speeds of up to 100Mbps and, unlike copper, is immune to EMI and RFI. In fact, fiber-optic cabling has many advantages over conventional copper cable, including the following:

  • Speed at which data can travel

  • Signal distance achieved before attenuation

  • Immunity to EMI and RFI

  • Redundancy in having counter-rotating rings

With regard to the last advantage, you should know two ways are available to connect devices to an FDDI network:

  1. Connect all stations to the dual counter-rotating rings for fault tolerance.

  2. Connect only critical devices such as switches and routers to both rings and connect end hosts only to the primary ring.

FDDI also has some drawbacks:

  • Both the fiber cable and the hardware needed to implement it are quite expensive.

  • Because it allows for larger packet sizes than in other LAN specifications, data frames must go through fragmentation and reassembly.

  • It suffers slowdowns due to latency when translating to and from Ethernet.

  • It cannot use full-duplex capability.

Key Concept

Be able to distinguish between single-mode fiber and multi-mode fiber, including their light sources and distance limitations.

Two types of fiber-optic cabling exist: multi-mode and single-mode (see Figure 3.19). A mode is simply a bundle of light rays that enters the cable at varying angles.

Figure 3.19 Single-mode has a single light source, and multi-mode has multiple light sources.

Multi-mode uses Light Emitting Diodes (LEDs) to generate its light. It enables several modes to enter the cable simultaneously. The angles and their points of entry ultimately affect the speed at which the various modes arrive at their destination, a consequence known as modal dispersion. This limits bandwidth and, consequently, the distance the transmission can travel. Thus, the maximum distance for multi-mode fiber is two kilometers.

Single-mode, on the other hand, uses a laser for its light source and allows only one mode at a time to enter the cable. Because it is not subject to modal dispersion, single-mode fiber enables higher bandwidth than multi-mode and can thereby extend the transmission distance up to 10 kilometers.

Channel Access Method

FDDI uses a token access method for transmission of packets. The following three layers define the specification:

  • Physical Layer Medium Dependent (PMD)

  • Physical Layer Protocol (PHY)

  • Media Access Control (MAC)

In the case of FDDI, communications access the physical medium (the ring) through the token being passed around the ring. When a node wants to transmit, it simply grabs a token, sends its transmission, and then releases a new token on the ring. Note that unlike with Token-Ring, which allows only one token to circulate at a time, FDDI allows multiple tokens to circulate at any given time.

FDDI Frames

FDDI frames are similar to Token-Ring frames in that they include token and data/command frames. In fact, FDDI data/command frames and their fields are identical to Token-Ring with one exception: FDDI frames do not include the AC (Access Control) field. All other fields and their general functions are the same. The Token frame in FDDI is a 3-byte frame beginning with a start delimiter, followed by a frame control field specifying the frame contents as either synchronous or asynchronous data and finishing with the end delimiter.

Figure 3.20 summarizes the characteristics of the frame.

Figure 3.20 This example of an FDDI frame identifies the fields and their functions.

SAS Versus DAS

An FDDI network has two types of nodes: Single Attached Stations (SASs) and Dual Attached Stations (DASs). As their names suggest, the SASs are attached to only one of the two physical rings and tend to be less critical equipment, such as workstations. Logically, DASs are more often mission-critical machines such as servers or routers.

Following the logic of the scheme, units that are SASs are attached to only one of the rings using a concentrator. The benefit of this configuration is that, if one of the machines were to fail or simply power down, it would have no effect on the integrity of the ring.

Figure 3.21 shows an example of both node types.

Figure 3.21 End devices attach to an FDDI ring's primary or primary and backup ring paths.

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