Juniper EX4300 Series Hardware Manual page 157

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. EX Series switches use
various types of network cables, including multimode and single-mode fiber-optic cable.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable
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), higher-order mode loss
(HOL) occurs. (Cladding consists of layers of lower-refractive index material in close contact with a core
material of higher refractive index.) 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. Single-mode fiber is
consequently more expensive than multimode fiber.
Exceeding the maximum transmission distances can result in significant signal loss, which causes
unreliable transmission.
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.
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 transmissions. 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 usually limits the maximum bit rate and link length.
Chromatic dispersion or attenuation is not a factor.
Attenuation is the reduction in strength of the light signal during
147