Unit 4

Fiber Distributed Data Interface (FDDI)

In considering the use of fiber optics for the transmission medium in a LAN, one might think that the fiber optic medium might simply be substituted for copper to provide greater speed of transmission. In fact, this can and has been done. But the Fiber Distributed Data Interface (FDDI) allows for more than just a faster medium.


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What advantages are there to using fiber optic cabling in a network compared to coaxial cable?

FDDI is a ring protocol, based on the IEEE 802.5 Token Ring standard. An FDDI ring can be combined with 802.5 rings or 802.3 buses to provide a high throughput "backbone" for a network. (We'll discuss combining rings more in the next unit.)

Although FDDI is similar to 802.5, three important differences make the technology and protocols of FDDI more complicated. See the Fiber Distributed Data Interface (FDDI) diagram below.

An 802.5 ring can have only one physical link between two nodes; FDDI has two links, with data circulating in opposite directions.
An 802.5 ring can have only one token and at most one message circulating at one time; FDDI can have many on each ring.
A portion of an FDDI ring data transmission capacity can be given up for "synchronous" traffic, essentially layering one or more high-speed communications channels onto the ring.

Fig. 63

Figure 4-21. Fiber Distributed Data Interface (FDDI)

   Multiple Message

Consider the case of a single message transmitted from a node around a ring back to the node. If transmission were truly instantaneous, the node would see the first bit of the message on its input connection the moment it transmitted it on its output connection. But it takes time for a signal to make its way around the ring — time for the signal to get from one end of each segment of the ring to the other and time for the repeater to copy each bit from its input to its output. As many as 1,000 nodes and up to 200 km of fiber might be involved in an FDDI backbone ring. There can be a long delay between the time when a node transmits the final bit of a message and when the first bit of the token is received. In an 802.5 ring of that size, this time is wasted, because the token must come all the way around and be freed by the originating node before any other node can transmit.

In an FDDI ring, when a node finishes transmitting a message, if it is not receiving anything, it immediately transmits a free token, essentially creating a new token where there was none before. That token can then be used by the next node on the ring that needs to transmit, and that transmission will occur simultaneously with the transmission of data for other busy tokens on the ring.

Dual Channels

The FDDI standard specifies dual channels, running in opposite directions. The purpose of this requirement is to provide much greater reliability and recoverability than would be possible with a single link-this is a highly desirable goal for a standard aimed at network backbones.

Synchronous Traffic

If an FDDI ring serves as the backbone for a large network, for example, in a large engineering facility such as one might find in an aerospace firm, then network traffic might be far less at night than during the day. At night, a significant portion of the FDDI ring capacity goes unused. For reasons such as this, the FDDI standard allows a portion of the 100 Mbps bandwidth of the ring to be given up to emulate "T1" or "T3" communications channels. Tl and T3 (E1 and E3 in Europe) are high-speed digital communications channels that can be used for either voice or data.

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