Xfps And Dispersion Considerations - Avaya 8800 Planning And Engineering

Hardware fundamentals and guidelines
SFP/XFP/GBIC
1000BASE-XD
1000BASE-ZX
1000BASE-EX
10GBASE-LRM
10GBASE-SR
10GBASE-LR/LW
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.
32 Planning and Engineering — Network Design
Maximum reach
Up to 40 km
Up to 70 km
Up to 120 km
Up to 220 m
Up to 300 m
Up to 10 km
Up to 40 km
Up to 80 km
November 2010