Page 60 - M. Abrahim Thesis

Chapter (3) Networking Media and Data Link Layers Protocols

3.3.1.1 Multimode Fiber

Multimode fiber allows multiple modes (paths) of light to propagate through the
fiberoptic core, as compared to single-mode fiber, which allows only one mode. Multiple modes
of light propagating through fiber might travel different distances, depending on their entry
angles. This angle causes them to arrive at the destination (receiving end of the cable) at slightly
different times—a phenomenon called modal dispersion. Multimode uses a type of glass, called
graded index glass, which has a lower index of refraction towards the outer edge of the core.
This glass causes the light to slow down when passing through the center of the core and
accelerate when passing through the outer areas of the core, ensuring that all modes of light
reach the end at approximately the same time. This design is used because a light ray following
a mode that goes straight down the center of the core does not have to go as far as a ray
following a mode that bounces around in the fiber. All rays should arrive at the end of the fiber
together. Then, the receiver at the end of the fiber receives a strong flash of light rather than a
long, dim pulse. A standard multimode fiber-optic cable (the most common type of fiber-optic
cable used in LANs) uses an optical fiber with either a 62.5- or a 50-micron core and a 125-micron
diameter cladding. This cable is commonly designated as 62.5/125 or 50/125-micron optical
fiber. A micron is one millionth of a meter. Because the diameter of the cladding is considerably
larger than the wavelength of the light being transmitted, the light bounces around (reflects)
inside the core as it is propagated along the transmission line. Infrared light emitting diodes
(LEDs) or vertical cavity surface emitting lasers (VCSELs) are usually the light source used with
multimode fiber. LEDs are a little cheaper to build and require somewhat less safety concerns
than lasers. However, LEDs cannot transmit light over cable as far as the lasers. Multimode fiber
(62.5/125) can carry data distances of up to 2000 meters (6560 feet). Multimode fiber is mainly
used in LAN applications including backbone cabling.

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