|  | 7.2 | Basics of Fiber Distributed Data Interface (FDDI)
| 7.2.5 | FDDI media

FDDI specifies a 100 Mbps, token-passing, dual-ring LAN that uses a fiber-optic transmission medium. It defines the physical layer and media access portion of the link layer, which is similar to IEEE 802.3 and IEEE 802.5 in its relationship to the OSI Model. Although it operates at faster speeds, FDDI is similar to Token Ring. The two networks share a few features, such as topology (ring) and media access technique (token-passing). A characteristic of FDDI is its use of optical fiber as a transmission medium. Optical fiber offers several advantages over traditional copper wiring, including such advantages as:

- security - Fiber does not emit electrical signals that can be tapped.
- reliability - Fiber is immune to electrical interference.
- speed - Optical fiber has much higher throughput potential than copper cable.

FDDI defines the two specified types of fiber: single-mode (also mono-mode); and multi-mode. Modes can be thought of as bundles of light rays entering the fiber at a particular angle. Single-mode fiber allows only one mode of light to propagate through the fiber, while multi-mode fiber allows multiple modes of light to propagate through the fiber. Multiple modes of light propagating through fiber may travel different distances, depending on their entry angles. This causes them to arrive at the destination at different times, a phenomenon called modal dispersion. Single-mode fiber is capable of higher bandwidth, and greater cable run distances, than multi-mode fiber. Because of these characteristics, single-mode fiber is often used for inter-building connectivity while multi-mode fiber is often used for intra-building connectivity. Multi-mode fiber uses LEDs as the light-generating devices, while single-mode fiber generally uses lasers.

FDDI specifies the use of dual rings for physical connections. Traffic on each ring travels in opposite directions. Physically, the rings consist of two or more point-to-point connections between adjacent stations. One of the two FDDI rings is called the primary ring; the other is called the secondary ring. The primary ring is used for data transmission; the secondary ring is generally used as a back up.

Class B, or single-attachment stations (SAS), attach to one ring; Class A, or dual attachment stations (DAS), attach to both rings. SASs are attached to the primary ring through a concentrator, which provides connections for multiple SASs. The concentrator ensures that a failure, or power down, of any given SAS, does not interrupt the ring. This is particularly useful when PCs, or similar devices that frequently power on and off, connect to the ring. A typical FDDI configuration with both DASs and SASs is shown in Figure . Each FDDI DAS has two ports - designated A and B. These ports connect the station to the dual FDDI ring, therefore, each port provides a connection for both the primary and the secondary ring.