Protocol spoofing was also widely used with another feature of earlier high-speed modems. Before the introduction of echo cancellation in v.32 and later protocols, high-speed modems typically had a very slow "backchannel" for sending things like these ACKs back to the sender. On the ~18,500 bit/s TrailBlazer, for instance, the modem could send as many as 35 UUCP packets a second to the receiver, but the backchannel offered only 75 bit/s, not nearly enough for the 35 bytes, 280 bits, of ACK messages generated by the remote host.