return to zero differential quadrature phase shift keying (RZ-DQPSK)
In optical networks, data is transported in optical transport units( OTU). The International Telecommunication Union( ITU) has defined the data frames and transmission rates in the ITU standard G.709.
In general, optical networks use optical fibers that are wavelength-division multiplexed to increase transmission capacity. Polarization mode dispersion( PMD) is used as the modulation technique. However, this modulation technique is sensitive to asymmetries of the optical fiber, which affect the transmission performance. For this reason, at the highest data rates of over 40 Gbit/s, RZ- DQPSK is used as the modulation, which is insensitive to these unbalances in the optical fiber. Since Dense Wavelength Division Multiplexing( DWDM) has 160 different wavelengths available for transmission, if each transmits 43 Gbit/s, data rates of up to 6.8 Tbit/s can be transmitted over an optical fiber.