a network communication system is provided for communication between network nodes , including individual nodes and a central control station node . in the network , signals are transmitted from the central control node to identify one or more frequencies that the node may transmit . when a first individual node has data to transmit , the first individual node transmits a request signal to the central control node over one or more of its assigned frequencies . in response to the request signal , the central control node transmits a grant signal assigning one or more frequencies to allow the first node to transmit its data .