Unit 5

Lesson 3 - Token Ring and FDDI


Because fiber optic cable supports both higher transmission speeds and greater distances than copper wire, it is easy to see how we can speed up a network by replacing twisted pair cable with optical fiber. FDDI does this; however, it uses more than just a faster medium to achieve high speed and throughput.

FDDI is a token-passing ring protocol based on the Token Ring standard. A FDDI ring can be combined with Token Rings or Ethernet buses to provide a high-throughput "backbone" for a network, as illustrated on the FDDI Diagram.



Although FDDI is similar to Token Ring, three important differences make the technology and protocols of FDDI more complicated:

Dual channels

ETR and multiple messages

Synchronous traffic

Dual Channels

The FDDI standard specifies dual channels running in opposite directions. (In contrast, a Token Ring can have only one physical link between adjacent nodes.) This double-ring design provides much greater reliability and recoverability, a highly desirable goal for a standard aimed at network backbones. The dual-channel architecture is implemented as follows:

A node can communicate on one or two of the channels. (The standard does not require communication on both channels, thus lower cost, single-channel machines can be built.) The two types of FDDI stations are Dual Attachment Station (DAS) and Single Attachment Station (SAS).

If the node communicates on both channels and one channel fails, the node will often still be able to communicate on the other channel.

When a break occurs in both channels on the ring because of a node failure or damaged link, nodes on either side of the break connect the two channels, effectively turning them into one longer ring. This allows operation to continue until the problem is resolved.

Each DAS on a double-ring network, whether it communicates on one or both channels, must still connect to both channels and provide shunt circuits to handle recovery.

ETR and Multiple Messages

It takes time for a token to make its way around a ring, because each node's repeater must copy each bit from its input to its output. In a Token Ring network, this time is wasted, because a single token serves the entire ring. The token must travel all the way around and be freed by the originating node before any other node can transmit.

However, a FDDI backbone ring may contain as many as 1,000 nodes and up to 200 kilometers of optical fiber. There can be a long delay from the time a node transmits the final bit of a message until it receives the first bit of the returning token. Therefore, the FDDI standard uses a Token Rotation Timer (TRT) to control token operations.

When a FDDI node has finished transmitting a message and is not receiving, it immediately transmits a free token. This creates a new token where there was none before. The token can then be used by the next node on the ring that needs to transmit, and the transmission will occur simultaneously with the transmission of other messages that may be making their way around the ring. In other words, both 16-Mbps Token Ring and FDDI use ETR.

Synchronous Traffic

A portion of a FDDI ring's data transmission capacity can be allocated to "synchronous" traffic, such as voice traffic. This essentially layers one or more high-speed communication channels onto the ring.

For example, imagine a FDDI ring that serves as the backbone for a large network. If network traffic is lower at night than during the day, a significant portion of the backbone capacity goes unused during those hours. Thus, the FDDI standard allows a portion of its 100-Mbps bandwidth to be dedicated to other types of traffic, such as telephone calls or multimedia applications.


See the Activities and Extended Activities section in Unit 5 Lesson 3 in your textbook Introduction to Networking to test what you have learned so far.

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