Juniper MX240 Hardware Manual page 208

MX240 Ethernet Services Router Hardware Guide
Single-mode fiber is so small in diameter that rays of light can 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 with multimode fiber, single-mode fiber has higher
bandwidth and can carry signals for longer distances. It is consequently more
expensive.
For information about the maximum transmission distance and supported wavelength
range for the types of single-mode and multimode fiber-optic cable used by DPCs
on the MX240 router, see the MX-series Ethernet Services Router DPC Guide. Exceeding
the maximum transmission distances can result in significant signal loss, which
causes unreliable transmission.
Attenuation and Dispersion in Fiber-Optic Cable
Correct functioning of an optical data link depends on modulated light reaching the
receiver with enough power to be demodulated correctly. Attenuation is the reduction
in power of the light signal as it is transmitted. Attenuation is caused by passive
media components, such as cables, cable splices, and connectors. 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
have enough light available to overcome attenuation.
Dispersion is the spreading of the signal in time. The following two types of dispersion
can affect an optical data link:
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 rather than modal dispersion limits 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 less than 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. For more information about power budget,
see "Calculating Power Budget for Fiber-Optic Cable" on page 181.
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Network Cable Specifications and Guidelines
Chromatic dispersion—The spreading of the signal in time resulting from the
different speeds of light rays.
Modal dispersion—The spreading of the signal in time resulting from the different
propagation modes in the fiber.