Understanding the network.pdf - Back to Home

combination of light refraction and reflection, using materials with differing
refraction indices. The refraction index determines how the wavelength of light is
bent when it passes through a substance and also the rate of photon passage
through the substance. Though copper and fiber cabling both transport data at the
same rate (0.69 percent of the speed of light), the difference between the two is the
amount of useable bandwidth, due to fiber optic's low attenuation rating. Fiber optic
cable suffers minimal signal loss in the form of actual resistance, because photons
instead of electrons are used. This results in a more effective use of the transmission
medium. The actual amount of bandwidth available for transmission is determined
by the width of the light source and the rise and fall rate of the light. The narrower
the light wavelength and the faster the rise and fall rate, the greater the bandwidth
of the fiber transport.
Three types of light wavelengths are used to transmit signals: short, long, and very
long. Short wavelength light has a wavelength size between 770 and 910
nanometers. Short wavelength light is usually generated by a
gallium-aluminum-arsenide infrared laser or LED with nominal yield of 850
nanometers. Long wavelength light ranges between 1,270 and 1,355 nanometers.
Very long wavelength light ranges between 1,355 and 1,500 nanometers. Both long
and very long wavelength light sources are generated with gallium-indium
arsenide-phosphate infrared lasers. Short and long wavelength light is used for LAN
transmission applications. Very long wavelength light, 1,500 nanometers, is used
exclusively for telecommunications signaling applications such as Synchronous
Optical Network (SONET).
As it passes through the fiber strand, the light slowly spreads out, and this
phenomenon is known as dispersion. Three forms of dispersion are present in fiber
transport: modal, chromatic, and material. The dispersion factor determines how
far the data can travel across the fiber path before the signal will be unusable (the
signal can be repeated or amplified if the cable length exceeds the dispersion factor).
Fiber optic cable comes in multimode and single mode. Two differences between
multimode and single-mode fiber are core size and how the light travels across the
core.
The TIA/EIA describes cable using two measurements: the core size and the
cladding size. The TIA/EIA defines two types of multimode fiber: 62.5/125 micron
and 50/125 micron. 62.5/125 micron is the standard used by FDDI, 10Base-FL,
100Base-FX, and 1000Base-SX/LX. 50/125 micron is used (and preferred over
62.5/125 micron) only with 1000Base-SX and 1000Base-LX. Single-mode fiber has
a much smaller core size. The TIA/EIA defines the single-mode fiber standard as
10/125 microns. With multimode fiber, the light bounces off the cladding in a rise
and fall pattern across the fiber core. With single-mode fiber, the light travels
directly through the core, as a single beam.