Juniper EX2300 Series Hardware Manual page 89

mode loss (HOL) 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.
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 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
specified for the type of link in Telcordia Technologies document GR-253-CORE (Section 4.3) and
International Telecommunications Union (ITU) document G.957.
When chromatic dispersion is at the maximum allowed, its effect can be considered as a power penalty
in the power budget. The optical power budget must allow for the sum of component attenuation,
power penalties (including those from dispersion), and a safety margin for unexpected losses.
Attenuation is the reduction in strength of the light signal during
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