A method of controlling a set-top box, for determining whether a user has an authority to use contents of an image signal corresponding to a plurality of broadcasting services, the set-top box being connected to a plurality of display apparatuses within a network using the same frequency band. comprising: the set-top box asking a first display apparatus of the plurality of display apparatuses, whether the first display apparatus comprises at least one available receiver; in response to a response that the first display apparatus comprises the at least one available receiver the set-top box - requesting the first display apparatus to process the additional information of the image signal through the at least one available receiver, - receiving processed additional information of the image signal from the first display apparatus, and - storing the processed additional information of the image signal received from the first display apparatus. in response to a response that the first display apparatus does not comprise the at least one available receiver, the set-top box - suspending requesting the first display apparatus to process the additional information of the image signal and determining whether at least one second display apparatus of the plurality of display apparatuses comprises at least one available receiver to request processing the additional information of the image signal thereto, in response to a response that the at least one second display apparatus comprises the at least one available receiver - receiving processed additional information of the image signal from the at least one second display apparatus, and - storing the processed additional information of the image signal received from the at least one second display apparatus; and if a broadcasting service is changed to one of the plurality of broadcasting services of the image signal by a user's input, parsing the processed additional information being stored, to obtain entitlement management information capable of determining the authority to use contents of the image signal and comparing the obtained entitlement management information with data in a smart card of the set-top box to determine whether the user has the authority to use a content corresponding to the changed broadcasting service. the set-top box asking a first display apparatus of the plurality of display apparatuses, whether the first display apparatus comprises at least one available receiver; in response to a response that the first display apparatus comprises the at least one available receiver the set-top box - requesting the first display apparatus to process the additional information of the image signal through the at least one available receiver, - receiving processed additional information of the image signal from the first display apparatus, and - storing the processed additional information of the image signal received from the first display apparatus. - requesting the first display apparatus to process the additional information of the image signal through the at least one available receiver, - receiving processed additional information of the image signal from the first display apparatus, and - storing the processed additional information of the image signal received from the first display apparatus. in response to a response that the first display apparatus does not comprise the at least one available receiver, the set-top box - suspending requesting the first display apparatus to process the additional information of the image signal and determining whether at least one second display apparatus of the plurality of display apparatuses comprises at least one available receiver to request processing the additional information of the image signal thereto, in response to a response that the at least one second display apparatus comprises the at least one available receiver - receiving processed additional information of the image signal from the at least one second display apparatus, and - storing the processed additional information of the image signal received from the at least one second display apparatus; and - suspending requesting the first display apparatus to process the additional information of the image signal and determining whether at least one second display apparatus of the plurality of display apparatuses comprises at least one available receiver to request processing the additional information of the image signal thereto, in response to a response that the at least one second display apparatus comprises the at least one available receiver - receiving processed additional information of the image signal from the at least one second display apparatus, and - storing the processed additional information of the image signal received from the at least one second display apparatus; and if a broadcasting service is changed to one of the plurality of broadcasting services of the image signal by a user's input, parsing the processed additional information being stored, to obtain entitlement management information capable of determining the authority to use contents of the image signal and comparing the obtained entitlement management information with data in a smart card of the set-top box to determine whether the user has the authority to use a content corresponding to the changed broadcasting service.