A method of controlling bidirectional data transmissions from a destination terminal in an in-band communication system, the method comprising: repeatedly transmitting a start signal from the destination terminal; discontinuing transmission of the start signal upon detection of a first received signal, wherein the first received signal indicates a successful reception of the start signal by a source terminal; repeatedly transmitting a NACK signal from the destination terminal after reception of the first received signal, whereby the source terminal is constrained to respond by repeatedly transmitting a source terminal data message; discontinuing transmission of the NACK signal upon detection of the successfully received source terminal data message; repeatedly transmitting a send signal from the destination terminal; discontinuing transmission of the send signal upon detection of a second received signal, wherein the second received signal indicates a successful reception of the send signal by the source terminal; transmitting a synchronization signal from the destination terminal; repeatedly transmitting a user data segment from the destination terminal using a first modulation scheme; discontinuing transmission of the user data segment upon detection of a third received signal, wherein the third received signal indicates a successful reception of the user data segment by the source terminal; and switching to a second modulation scheme if the destination terminal receives a predetermined number of a fourth received signal, wherein the fourth received signal indicates an unsuccessful reception of the user data segment by the source terminal, and further wherein the predetermined number is greater than one. repeatedly transmitting a start signal from the destination terminal; discontinuing transmission of the start signal upon detection of a first received signal, wherein the first received signal indicates a successful reception of the start signal by a source terminal; repeatedly transmitting a NACK signal from the destination terminal after reception of the first received signal, whereby the source terminal is constrained to respond by repeatedly transmitting a source terminal data message; discontinuing transmission of the NACK signal upon detection of the successfully received source terminal data message; repeatedly transmitting a send signal from the destination terminal; discontinuing transmission of the send signal upon detection of a second received signal, wherein the second received signal indicates a successful reception of the send signal by the source terminal; transmitting a synchronization signal from the destination terminal; repeatedly transmitting a user data segment from the destination terminal using a first modulation scheme; discontinuing transmission of the user data segment upon detection of a third received signal, wherein the third received signal indicates a successful reception of the user data segment by the source terminal; and switching to a second modulation scheme if the destination terminal receives a predetermined number of a fourth received signal, wherein the fourth received signal indicates an unsuccessful reception of the user data segment by the source terminal, and further wherein the predetermined number is greater than one.