Since the XMODEM protocol required the sender to stop and wait for an <ACK> or <NAK> message from the receiver, it tended to be quite slow. In the era of 300 bit/s modems, the entire 132-byte packet required just over 3.5 seconds to send (132 bytes * (8 bits per byte + 1 start bit + 1 stop bit) / 300 bits per second). Assuming it takes 0.2 seconds for the receiver's <ACK> to make it back to the sender and the next packet to start hitting the receiver (0.1 seconds in both directions), the overall time for one packet would be 3.7 seconds, just over 92% throughput.