A communication system is disclosed, including: a network in which the transmission speeds of a upstream channel and a downstream channel for transmitting data can be changed independently; a data storage server connected to the network, the data storage server configured to store data uploaded thereto via the network; and a communication terminal connected to the network, the communication terminal configured to upload old data stored therein to the data storage server via the network thereby to move the old data to the data storage server, and download new data via the network thereby to store the new data therein. The communication terminal is further configured to request the network to change at least one of the transmission speeds of the upstream channel and the downstream channel based on remaining memory capacity thereof and the transmission speeds such that there remains memory capacity available for the new data.