A communication system includes a network-side apparatus and a user-side apparatus. The network-side apparatus includes a transmission function unit configured to perform processing of a transmission function on data received from the user-side apparatus, a quality calculation unit configured to calculate a quality of media data which is transmitted from the user-side apparatus and on which the processing is performed by the transmission function unit; and a transmission function changing unit configured to change the transmission function performed by the transmission function unit in a range in which the calculated quality satisfies a quality required by an application.