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An investigation of system upgradability of installed fiber-optic cable was conducted using 40-Gb/s wavelength-division-multiplexing (WDM) signals toward multiterabit optical networks. A field trial of 63-channel 40-Gb/s dispersion-managed soliton WDM signal transmission was successfully demonstrated over 320-km (4 × 80-km) installed nonzero-dispersion-shifted fibers. The average Q factor of 15.4 dB was obtained, and very stable long-term bit-error-ratio performance was confirmed without polarization-mode dispersion compensation. This system upgradability investigation in the field environment provided the confidence to introduce 40-Gb/s technologies and effectively to construct multiterabit optical networks following the demand increase in the future.