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6.3 10BASE-FL Components

Network Medium

The typical fiber optic cable used for a fiber link segment is a multi-mode fiber cable (MMF) with a 62.5 micron fiber optic core and 125 micron outer cladding (62.5/125). Each link segment requires two strands of fiber, one to transmit data, and one to receive data. 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 fiber connectors used on link segments are generally known as "ST" connectors. The formal name of this connector in the ISO/IEC international standards is "BFOC/2.5." The ST connector is a spring-loaded bayonet connector, whose outer ring locks onto the connection, much like the BNC connector used on 10BASE2 segments. The ST connector has a key on an inner sleeve and also an outer bayonet ring. To make a connection you line up the key on the inner sleeve of the ST plug with a corresponding slot on the ST receptacle, then push the connector in and lock it in place by twisting the outer bayonet ring. This provides a tight connection with precise alignment between the two pieces of fiber optic cable being joined.

The wavelength of light used on a fiber link segment is 850 nanometers (850 nm), and the optical loss budget for a fiber link segment must be no greater than 12.5 dB. The loss budget refers to the amount of optical power lost through the attenuation of the fiber optic cable, and the inevitable small losses that occur at each fiber connector.

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 standard grade fiber optic cable operating at 850 nm will have something in the neighborhood of from 4 dB to 5 dB loss per 1000 meters. 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.

The older FOIRL segment typically used the same type of fiber optic cable, connectors, and had the same optical loss budget. The 10BASE-FL specifications were designed to allow backward compatibility with existing FOIRL segments. The major difference is that the 10BASE-FL segment may be up to 2,000 meters in length if only 10BASE-FL equipment is used on the segment.

Quick Reference Guide to 10BASE-FL Fiber Optic Ethernet - 04 SEP 95
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