Xfps And Dispersion Considerations - Avaya 8800 Planning And Engineering, Network Design

Hardware fundamentals and guidelines
Optical device (SFP/SFP+/
XFP)
10GBASE-LRM
10GBASE-ER/EW
10GBASE-ZR/ZW

XFPs and dispersion considerations

The optical power budget (that is, attenuation) is not the only factor to consider when you are
designing optical fiber links. As the bit rate increases, the system dispersion tolerance is reduced.
As you approach the 10 Gbit/s limit, dispersion becomes an important consideration in link design.
Too much dispersion at high data rates can cause the link bit error rate (BER) to increase to
unacceptable limits.
Two important dispersion types that limit the achievable link distance are chromatic dispersion and
polarization mode dispersion (PMD). For fibers that run at 10 Gbit/s or higher data rates over long
distances, the dispersion must be determined to avoid possible BER increases or protection
switches. Traditionally, dispersion is not an issue for bit rates of up to 2.5 Gb/s over fiber lengths of
less than 500 km. With the availability of 10 Gbit/s and 40 Gbit/s devices, you must consider
dispersion.
Chromatic dispersion
After you have determined the value of the chromatic dispersion of the fiber, ensure that it is within
the limits recommended by the International Telecommunications Union (ITU). ITU-T
recommendations G.652, G.653, and G.655 specify the maximum chromatic dispersion coefficient.
Assuming a zero-dispersion fiber at 1550 nanometers (nm) and an operating wavelength within
1525 to 1575 nm, the maximum allowed chromatic dispersion coefficient of the fiber is 3.5 ps/(nm-
km). The total tolerable dispersion over a fiber span at 2.5 Gb/s is 16 000 ps, at 10 Gb/s it is 1000
ps, and at 40 Gb/s it is 60 ps.
Using these parameters, one can estimate the achievable link length. Using a 50 nm-wide optical
source at 10 Gbit/s, and assuming that the optical fiber is at the 3.5 ps/(nm-km) limit, the maximum
link length is 57 km. To show how link length, dispersion, and spectral width are related, see the
following tables.
Table 7: Spectral width and link lengths assuming the maximum of 3.5 ps/(nm-km)
Spectral width (nm)
1
10
50
June 2016
Maximum reach
Up to 220 m
Up to 40 km
Up to 80 km
Maximum link length (km)
285
28.5
5.7
Planning and Engineering — Network Design
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