Previous generation of optical long-haul communication systems operate at 1.31 µm where chromatic dispersion is zero, but attenuation is 0.5 dB/km.
Wavelength division multiplexing (WDM) system was introduced to increase the fibre bandwidth, the operating wavelength moved to 1.55 µm due to the low attenuation region of SMF and the operating wavelength window of EDFA, but the chromatic dispersion is occurred 17 ps/nm-km.