Abstract: The present invention relates to a broadband transmission path modem, a radio terminal device, a program, and a storage medium. An object of the present invention is to provide a radio terminal device with various communication services including access to the Internet etc. via a broadband transmission path at a low cost without remarkably impairing the effects of cost-cutting, downsizing, lightening, and power economization. To this end, a broadband transmission path modem according to the present invention interfaces functions, defined in advance as a specification of a radio access link, of an entity realizing a communication protocol applied to the broadband transmission path and an entity realizing the radio access link, and converts transmission information delivered to realize an added function not defined in the specification.