7.2 Basics of Fiber Distributed Data Interface (FDDI)
7.2.3 FDDI MAC
Instructor Note
  The details of FDDI's MAC method are presented. Note that while FDDI relies on Token Passing, with its dual ring there are more variations possible than with typical Token Ring networks. FDDI's MAC method is one of the reasons for its reliability.

This TI relates to CCNA Certification Exam Objective #1.
FDDI uses a token passing strategy similar to Token Ring. Token-passing networks move a small frame, called a token, around the network. Possession of the token grants the right to transmit data. If a node receiving the token has no information to send, it passes the token to the next end-station. Each station can hold the token for a maximum period of time, depending on the specific technology implementation.

When a station that is in possession of the token has information to transmit, it seizes the token and alters one of its bits. The token then becomes a start-of-frame sequence. Next, the station appends the information that it transmits to the token, and sends this data to the next station on the ring.

There is no token on the network while the information frame is circling the ring, unless the ring supports early token release. Other stations on the ring must wait for the token to become available. FDDI networks have no collisions. If early token release is supported, a new token can be released when  the frame transmission has finished.

The information frame circulates around the ring until it reaches the intended destination station, which copies the information for processing. The information frame continues around the ring until it reaches the sending station, where it is removed. The sending station can check the returning frame to see whether the frame was received, and subsequently copied by the destination.

Unlike CSMA/CD networks, such as Ethernet, token-passing networks are deterministic. This means you can calculate the maximum time that will pass before any end station will be able to transmit. FDDI's dual ring assures that not only are stations guaranteed their turn to transmit, but if one part of one ring is damaged or disabled for any reason, the second ring can be used. This makes FDDI very reliable.

FDDI supports real-time allocation of network bandwidth, making it ideal for a variety of different application types. FDDI provides this support by defining two types of traffic - synchronous and asynchronous.

Synchronous

  • Synchronous traffic can consume a portion of the 100 Mbps total bandwidth of an FDDI network, while asynchronous traffic can consume the rest.
  • Synchronous bandwidth is allocated to those stations requiring continuous transmission capability. This is useful for transmitting voice and video information. The remaining bandwidth is used for asynchronous transmissions.
  • The FDDI SMT specification defines a distributed bidding scheme to allocate FDDI bandwidth.

Asynchronous

  • Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned an asynchronous priority level.
  • FDDI also permits extended dialogues, in which stations may temporarily use all asynchronous bandwidth.
  • The FDDI priority mechanism can lock out stations that cannot use synchronous bandwidth, and that have too low an asynchronous priority.