A method and system for asynchronously transporting narrowband and broadband transmissions over a link includes providing at least one host terminal or at least one network unit for receiving and transmitting communications over a transmission line. Narrowband transmissions are converted to and from composite asynchronous transfer mode (ATM) cells by separating data and signaling of each channel into separate byte positions in the composite ATM cells. The composite asynchronous transfer mode (ATM) cells are transferred over the transmission link. The composite asynchronous transfer mode (ATM) cells include both narrowband and broadband composite cells such that an entire bandwith of the transmission line is available for both narrowband and broadband transmissions.