Juniper MX10016, Руководство по оборудованию

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To determine the power budget and power margin needed for fiber-optic connections, you need to
understand how signal loss, attenuation, and dispersion affect transmission. The MX10016 router uses
various types of network cables, including multimode and single-mode fiber-optic cables.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cables
Multimode fiber is large enough in diameter to allow rays of light to reflect internally (bounce off the walls
of the fiber). Interfaces with multimode optics typically use LEDs as light sources. However, LEDs are not
coherent light sources. They spray varying wavelengths of light into the multimode fiber, which reflects
the light at different angles. Light rays travel in jagged lines through a multimode fiber, causing signal
dispersion. When light traveling in the fiber core radiates into the fiber cladding (layers of lower refractive
index material in close contact with a core material of higher refractive index), higher-order mode loss
occurs. Together, these factors reduce the transmission distance of multimode fiber compared to that of
single-mode fiber.
Single-mode fiber is so small in diameter that rays of light reflect internally through one layer only. Interfaces
with single-mode optics use lasers as light sources. Lasers generate a single wavelength of light, which
travels in a straight line through the single-mode fiber. Compared to multimode fiber, single-mode fiber
has a higher bandwidth and can carry signals for longer distances. It is consequently more expensive.
Attenuation and Dispersion in Fiber-Optic Cable
An optical data link functions correctly provided that modulated light reaching the receiver has enough
power to be demodulated correctly. Attenuation is the reduction in strength of the light signal during
transmission. Passive media components such as cables, cable splices, and connectors cause attenuation.
Although attenuation is significantly lower for optical fiber than for other media, it still occurs in both
multimode and single-mode transmission. An efficient optical data link must transmit enough light to
overcome attenuation.
Dispersion is the spreading of the signal over time. The following two types of dispersion can affect signal
transmission through an optical data link:
•
Chromatic dispersion, which is the spreading of the signal over time caused by the different speeds of
light rays.
•
Modal dispersion, which is the spreading of the signal over time caused by the different propagation
modes in the fiber.
For multimode transmission, modal dispersion, rather than chromatic dispersion or attenuation, usually
limits the maximum bit rate and link length. For single-mode transmission, modal dispersion is not a factor.
However, at higher bit rates and over longer distances, chromatic dispersion limits the maximum link length.
An efficient optical data link must have enough light to exceed the minimum power that the receiver
requires to operate within its specifications. In addition, the total dispersion must be within the limits
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