Ip multicast service over a broadcast channel
A method of and apparatus for providing an Internet Protocol multicast service wherein multicast data is transmitted through multicast routers ( 8 ) to user stations ( 9 ) over a digital broadcast channel (especially DVB-T) together with digital broadcast signals, at least if a request has been received from a user station ( 9 ) for the multicast data,. The transmission of the multicast data by a multicast router is conditional upon the reception of a request for said multicast data from that multicast router by a plurality of the user stations ( 9 ). The number of requests from the user stations ( 9 ) has to be greater than a threshold number, which is a function of the available bandwidth for multicast transmission over the broadcast channel. Information concerning the requests for the multicast data by the user stations ( 9 ) is unavailable to the other user stations ( 9 ).
1 . A method of providing an Internet Protocol multicast service wherein multicast data is transmitted through multicast routers ( 8 ) to user stations ( 9 ) over a digital broadcast channel together with digital broadcast signals, at least if a request ( 18 ) has been received from a user station ( 9 ) for the multicast data,
characterised in that the transmission of said multicast data ( 22 ) by said multicast routers is conditional upon the reception of said request ( 18 ) for said multicast data at said multicast routers from a plurality of said user stations ( 9 ).
2 . A method as claimed in claim 1 , wherein the transmission of said multicast data ( 22 ) by said multicast router is conditional upon the reception of a number of said requests ( 18 ) from said user stations ( 9 ) greater than a threshold number, said threshold number being a function of the available bandwidth for multicast transmission over said broadcast channel.
3 . A method as claimed in claim 1 or 2 , wherein said user stations ( 9 ) are repeatedly interrogated and the continued transmission of said multicast data ( 22 ) by said multicast router is conditional upon the reception of responses ( 18 ) from a plurality of said user stations ( 9 ).
4 . A method as claimed in any preceding claim, wherein information concerning said requests ( 18 ) for said multicast data from said user stations ( 9 ) is unavailable to the others of said user stations ( 9 ).
5 . A method as claimed in any preceding claim, wherein said signals and data are transmitted through gateways ( 6 ) between Internet and broadcast infrastructures ( 4 , 5 and 10 ) to respective broadcast areas and the transmission of said multicast data ( 22 ) within a broadcast area by the corresponding gateway ( 6 ) is conditional upon the reception at that gateway of said request ( 18 ) for said multicast data from a plurality of said user stations ( 9 ) served by said broadcast area.
6 . A method as claimed in any preceding claim, wherein announcements ( 17 ) of future multicast data transmission sessions are transmitted to said user