Fiber Optic Cable

Fiber optic cable consists of a glass cylinder core, surrounded by a tube of dissimilar glass with an outer coating of protective material. The core is composed of optically pure glass that is used to transmit the light waves that carry the data. The cladding that surrounds the core is a special glass coating designed to reflect the light waves back into the core.The PMD (Physical Layer Medium Dependent) standard for FDDI, specifies the diameter of the glass core, the diameter of the glass coating, and the refractive index of the core material. The core can be either single mode or multimode type fibers.

Core/Coating Dimensions - A specification of 62.5/125 micron cable, for example, is calling out a 62.5 diameter for the core (fiber size) in micrometers. The second dimension, 125, is the outer diameter of the cladding (glass coating) also in micrometers.

Graded-index - The graded-index type of cable, used in FDDI applications, is highly refractive at the center of the core and becomes less refractive toward the core-cladding boundary. The refractive index of the core at the cladding boundary matches the refractive index of the glass used for the cladding.

Single Mode - uses only one mode of transmission (light wave). It is normally used to carry data over distances up to 25 km.

Multimode - can transmit more than one light wave at a time, but cannot sustain a quality of service beyond 2 kilometers. Note that both HP EISA FDDI and HSC FDDI use multimode.

To allow for less-than-optimal media, and still support high data rates, FDDI uses a group encoding technique that allows four bits to be encoded in five bauds. This encoding technique is called 4B/5B encoding. Using this technique, FDDI provides 100 Mbps using a 125 MHz rate.

Fiber Optic Cable Maintenance

Fiber optic cable requires proper handling. Follow these basic guidelines in addition to manufacturers specifications for good cable maintenance:

• Do not stretch, puncture, or crush the fiber cable(s) with staples, heavy equipment, doors, etc.

• Always maintain the minimum bend radii specified by the cable manufacturer. The minimum bend radii is usually 10-20 times a cable's outer diameter.

• Keep the dust caps on the cable ends, transmitter(s), and receiver(s) until you actually make the connections. Put the dust covers back on when the cable is disconnected.

• Do not polish the connectors with a cloth made of synthetic fibers, as this will charge up the fiber and attract dust.