 The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels each running on their own set of two wires.The "Bell 103A dataset" standard was also introduced by AT&T in 1962.It provided full-duplex service at 300 bit/s over normal phone lines.