Abstract-We showed that long-distance transmission is affected by the source pulse tails. Fast tail dropoff dramatically improves the transmission. We found that the tails must be at least 20 dB down 50 ps from the peak in order to successfully transmit signals over 20 000 km at 20 Gbit/s with anomalous path average dispersion. While tails that are 15 dB down 50 ps from the peak are sufficient to transmit data at 10 Gbit/s over 20 000 km with anomalous path average dispersion, the tails must be down at least 25 dB 50 ps from the peak to transmit data with slightly normal path average dispersion.Index Terms-Optical fiber communication, optical solitons, optical fiber dispersion, optical propagation in nonlinear media.