Method and apparatus for transmitting data

摘要

Data relating to the status of users 12 in a mobile communications system is broadcast via satellite 8 from a network register 4 to local registers 10 in an HDLC format. Each of the local registers 10 requests retransmission of any HDLC frames which are incorrectly received. The network register 4 records the earliest transmitted frame not yet acknowledged by all of the local registers and inhibits transmission of new frames if they fall outside a transmission window relative to the earliest transmitted frame. The network register 4 polls the local registers 10 for retransmission requests and the local registers 10 also send unsolicited requests to the network register 4. The network register 4 only retransmits a frame once if multiple requests for that frame are transmitted within a predetermined period. When a new local register 10 enters the broadcast reception group, the network register 4 informs the new local register 10 which new frame will next be transmitted. The number of bits used for frame sequence numbers is greater than that defined in the HDLC protocols.