9000

90 Appendix B

Fiber Cabling

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.

Table B-1 FDDI Fiber Optic Cabling

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.

Cable Type

Core/Coating

(microns)

Refractive

Index

Maximum

Length

Single Mode 8.5/125 Graded 15 km

Multimode 62.5/125 Graded 2 km