Juniper QFX5240 - Page 57

To determine the power budget and power margin needed for ber-opc connecons, you need to
understand how signal loss, aenuaon, and dispersion aect transmission. The QFX Series uses various
types of network cables, including mulmode and single-mode ber-opc cables.
Signal Loss in Mulmode and Single-Mode Fiber-Opc Cables
Mulmode ber is large enough in diameter to allow rays of light to reect internally (bounce o the
walls of the ber). Interfaces with mulmode opcs typically use LEDs as light sources. However, LEDs
are not coherent light sources. They spray varying wavelengths of light into the mulmode ber, which
reect the light at dierent angles. Light rays travel in jagged lines through a mulmode ber, causing
signal dispersion. When light traveling in the ber core radiates into the ber cladding (layers of lower
refracve index material in close contact with a core material of higher refracve index), higher-order
mode loss occurs. Together, these factors reduce the transmission distance of mulmode ber compared
to that of single-mode ber.
Single-mode ber is so small in diameter that rays of light reect internally through one layer only.
Interfaces with single-mode opcs use lasers as light sources. Lasers generate a single wavelength of
light, which travels in a straight line through the single-mode ber. Compared to mulmode ber, single-
mode ber has a higher bandwidth and can carry signals for longer distances. It is consequently more
expensive.
For informaon about the maximum transmission distance and supported wavelength range for the
types of single-mode and mulmode ber-opc cables that are connected to the QFX Series, see the
Hardware Compability Tool. Exceeding the maximum transmission distances can result in signicant
signal loss, which causes unreliable transmission.
Aenuaon and Dispersion in Fiber-Opc Cable
An opcal data link funcons correctly provided that modulated light reaching the receiver has enough
power to be demodulated correctly.
Aenuaon is the reducon in strength of the light signal during
transmission. Passive media components such as cables, cable splices, and connectors cause
aenuaon. Although aenuaon is signicantly lower for opcal ber than for other media, it sll
occurs in both mulmode and single-mode transmission. An ecient opcal data link must transmit
enough light to overcome aenuaon.
Dispersion is the spreading of the signal over me. The following two types of dispersion can aect
signal transmission through an opcal data link:
• Chromac dispersion, which is the spreading of the signal over me caused by the dierent speeds
of light rays.
• Modal dispersion, which is the spreading of the signal over me caused by the dierent propagaon
modes in the ber.
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