Lesson 3:FDDI

Until the introduction of Fast Ethernet, FDDI (pronounced "fiddy"), was the only data-link layer protocol in common use to offer 100-Mbps transmission speeds using fiber optic cable. Standardized by the American National Standards Institute (ANSI), FDDI is primarily a protocol used on backbone networks, but there was also a desktop version of the protocol designed to use copper cables called Copper Distributed Data Interface (CDDI, or "siddy") that never achieved widespread deployment. Like Token Ring, FDDI networks are cabled using a ring topology and employ the token passing MAC mechanism, but there are several important differences between FDDI and Token Ring.

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After this lesson, you will be able to

• Describe the characteristics of the FDDI protocol
• Distinguish between the various types of FDDI network connections
• Diagram a FDDI frame

Estimated lesson time: 15 minutes

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The FDDI Physical Layer

Apart from its speed, which was unprecedented at the time of its introduction, the use of fiber optic cable was the primary reason for FDDI's commercial success. Like other fiber optic protocols, FDDI networks can span much longer distances than copper-based networks and are completely resistant to electromagnetic interference. FDDI supports several different types of fiber optic cable, including the 62.5/125 micron multimode cable that is the industry standard for fiber optic LANs, which provides for network segments up to 100 kilometers long with up to 500 workstations placed as far as 2 kilometers apart. Singlemode fiber optic cables provide even longer segments, with up to 60 kilometers between workstations.

The original FDDI standard calls for a ring topology, but unlike Token Ring networks, this ring is not strictly a logical one implemented in the hub. The computers are actually cabled together in a ring. To provide fault tolerance in the event of a cable break, the network is a double ring that consists of two independent rings, a primary and a secondary, with traffic flowing in opposite directions. A computer that is connected to both rings is called a dual attachment station (DAS), and when one of the rings is broken by a cable fault, the computer switches to the other ring, providing continued full access to the entire network. A double ring FDDI network in this condition is called a wrapped ring.

It's also possible to cable a FDDI network in a star topology using a hub called a dual attachment concentrator (DAC). The DAC creates a single logical ring, like a Token Ring MAU. A computer connected to the DAC is called a single attachment station (SAS). A FDDI network can be deployed using the double ring, the star topology, or both. The double ring is better suited to use as a backbone network, and the star to a segment network connecting desktop computers. To construct an entire enterprise network using FDDI, you create a double ring back- bone, to which you connect your servers and other vital computers as DASes. You then connect one or more DACs to the double ring, which you use to attach your workstations, as shown in Figure 5.10. This is sometimes called a dual ring of trees. The DAS servers have full advantage of the double ring's fault tolerance, as do the DACs, whereas the SAS computers attached to the DACs are connected to the primary ring only. If a cable connecting a workstation to a DAC fails, the DAC can remove it from the ring without disturbing communications to the other computers, as on a Token Ring network. To expand the network further, you can connect additional DACs to ports in existing DACs without limit, as long as you remain within the maximum number of computers permitted on the network.

Figure 5.10  An enterprise FDDI network

The FDDI Frames

Like Token Ring, FDDI uses several different types of frames in its communications. The most common of these is the data frame, shown in Figure 5.11. The functions of the fields in the FDDI data frame are as follows:

Figure 5.11  The FDDI data frame

• Preamble (PA, 8 bytes).  Contains series of alternating 0s and 1s, used for clock synchronization.
• Starting Delimiter (SD, 1 byte).  Indicates the beginning of the frame.
• Frame Control (FC, 1 byte).  Indicates the type of data found in the Data field. Some of the most common values are as follows:
  • 41, 4F—Station Management (SMT) Frame.  Indicates that the Data field contains an SMT Protocol Data Unit.
  • C2, C3—MAC Frame.  Indicates that the frame is either a MAC Claim frame (C2) or a MAC Beacon frame (C3), which are use to recover from token passing errors.
  • 50, 51—LLC Frame.  Indicates that the Data field contains application data in a standard IEEE 802.2 LLC frame.
• Destination Address (DA, 6 bytes).  Specifies the hardware address of the computers that will receive the frame.
• Source Address (SA, 6 bytes).  Specifies the hardware address of the system sending the frame.
• Data (variable).  Contains network layer protocol data, or an SMT header and data, or MAC data, depending on the function of the frame.
• Frame Check Sequence (FCS, 4 bytes).  Contains a cyclical redundancy check (CRC) value, used for error detection.
• Ending Delimiter (ED, 4 bits).  Indicates the end of the frame.
• End of Frame Sequence (FS, 12 bits).  Contains three indicators that may be modified by intermediate systems when they retransmit the packet, the functions of which are as follows:
  • E (Error).  Indicates that an error has been detected, either in the FCS or in the frame format.
  • A (Acknowledge).  Indicates that the intermediate system has determined that the frame's destination address applies to itself.
  • C (Copy).  Indicates that the intermediate system has successfully copied the contents of the frame into its buffers.

Because it is a token passing protocol, FDDI also must have a token frame, which contains only the Preamble, plus the Starting Delimiter, Frame Control, and Ending Delimiter fields, for a total of 3 bytes. The token passing mechanism used by FDDI is virtually identical to that of Token Ring, except that the early token release feature that is optional in Token Ring is standard equipment for the FDDI protocol. The third type of frame used on FDDI networks is the station management frame, which is responsible for ring maintenance and network diagnostics.

Exercise 1: FDDI Concepts

Match the acronyms in the left column with the correct definitions in the right column.

1. DAS
2. DAC
3. SAS
4. CDDI
5. SMT

1. A version of FDDI that uses copper cable
2. A computer connected to a FDDI network using the star topology
3. A FDDI frame that performs ring management functions
4. The hub used in a FDDI star network
5. A computer connected to both rings of a double ring

Lesson Review

1. A FDDI double ring network that has experienced a cable failure is called what?
   1. A wrapped ring
   2. A truncated ring
   3. A bifurcated ring
   4. A dual ring of trees
2. Which FDDI physical layer option supports the longest network segments?
   1. The double ring topology
   2. The star topology
   3. Singlemode fiber optic
   4. Multimode fiber optic
3. Which of the following fields identifies the type of data carried in a FDDI data frame?
   1. Starting Delimiter
   2. Frame Control
   3. Source Address
   4. Frame Check Sequence

Lesson Summary

• FDDI is a token passing data-link layer protocol that was at one time a popular solution for backbone networks.
• FDDI uses either a physical double ring topology or a star topology.
• A FDDI workstation attached to both rings of a double ring is called a dual attachment station (DAS), and one that is attached to a single ring is called a single attachment station (SAS).