A file transfer protocol based on X-Modem , Y-Modem was designed by Chuck Forsberg to add batch transmission, and variable block size. The variable function was seldom ever at the maximum size, 1024 bytes. The error detection scheme was CRC (Cyclic Redundancy Check). A subsequent version was Y-Modem-G. This was an effort to send data in a stream, evaluating the blocks on the fly and only talking back to the sender if there was an error. Both programs had to be identical and the noise predominant on POTS (Plain Old Telephone Service) at the time was a problem.