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Q24: What is Multi-Mode Fiber Cables?
A24:Multi-mode fiber allows many "modes", or paths, of light
to propagate through the fiber optic cable. The relatively large core
of a multi-mode fiber allows good coupling from inexpensive LEDs light
sources and the use of inexpensive couplers and connectors. Multi-mode
fiber typically has a core diameter of 50 to 100 microns.
Two types of multi-mode fiber exist with a refractive index that may be
"graded" or "stepped". With graded index fiber the
index of be "graded" or "stepped". With graded index
fiber the index of refraction of the core is lower toward the outside
of the core and progressively increases toward the center of the core,
thereby reducing modal dispersion of the signal. With stepped index fiber
the core is of uniform refractive index with a sharp decrease in the index
of refraction at the core-cladding interface. Stepped index multi-mode
fibers generally have lower bandwidths than graded index multi-mode fibers.
The most popular fiber for networking is the 62.5/125 micron multi-mode
fiber. These numbers mean that the core diameter is 62.5 microns and the
cladding is 125 microns. Other common sizes are 50/125 and 100/140.
The primary advantage of multi-mode fiber over twisted pair cabling is
that it supports longer segment lengths. Multi-mode fiber can support
segment lengths as long as 2000 meters for 10 and 100 Mbps Ethernet, and
550 meters for 1 Gbps Ethernet.
The industry recommends fiber optic cable be considered for distances
greater than 295 feet, when electromagnetic interference is a problem
or when transmissions bit rates exceed 350 MHz.
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