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11.2 100BASE-FX Components

Network Medium

The 100BASE-FX media system is designed to allow segments of up to 412 meters in length. While it's possible to send signals over fiber for much longer distances, the 412 meter limit for fiber segments in Fast Ethernet is there to ensure that the round trip timing specifications are met.

The 100BASE-FX specification requires two strands of multimode fiber optic (MMF) cable per link, one for transmit data, and one for receive data, with the signal crossover (TX to RX) performed in the link as shown in the figure. There are many kinds of fiber optic cables available, ranging from simple two-strand jumper cables with a PVC outer jacket material on up to large inter-building cables carrying many fibers in a bundle.

The typical fiber optic cable used for a fiber link segment is a graded index MMF cable, with a 62.5 micron fiber optic core and 125 micron outer cladding (62.5/125). The wavelength of light used on a fiber link segment is 1350 nanometers (1350 nm). There is an 11 dB loss budget allowed per link, which means that the total power loss through the fiber and associated connectors must not be higher than 11 dB as measured by a fiber optic power meter.

The more connectors you have and the longer your fiber link cable is, the higher the optical loss will be. Optical loss is measured with fiber optic test instruments that can tell you exactly how much optical loss there may be on a given segment at a given wavelength of light. A typical performance rating for standard grade fiber operating at 1350 nm will provide something in the neighborhood of from 1 dB to 2 dB loss per 1000 meters of cable. You can also expect something in the neighborhood of from 0.5 to around 2.0 dB loss per connection point, depending on how well the connection has been made. If your connectors or fiber splices are poorly made, or if there is finger oil or dust on the connector ends, then you can have higher optical loss on the segment.

Quick Reference Guide to 100BASE-FX - 09 SEP 95
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