Juniper SRX 5800, Hardware Manual

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sources, however. 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, higher-order mode loss (HOL) results.
Together these factors limit the transmission distance of multimode fiber compared to single-mode fiber.
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.
Attenuation and Dispersion in Fiber-Optic Cable for the SRX5800 Services
Gateway
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. While 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:
•
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.
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.
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