Xmodem 
A protocol for transferring files during direct dial-up communications. Developed by Ward Christensen in 1977, Xmodem does basic error-checking to ensure that information isn't lost or corrupted during transfer; it sends data in 128-byte blocks. Xmodem has undergone a couple of enhancements: Xmodem CRC uses a more reliable error-correction scheme, and Xmodem-1K transfers data faster by sending it in 1,024-byte blocks.