YMODEM

YMODEM is a file transfer protocol used between microcomputers connected together using modems. YMODEM was developed by Chuck Forsberg as the successor to XMODEM and MODEM7, and was first implemented in his CP/M YAM program. It was formally given the name "YMODEM" in 1985 by Ward Christensen.

The original YMODEM was essentially the same as XMODEM except that it sent the file's name, size, and timestamp in a regular XMODEM block, "block 0", before actually transferring the file. Sending the file size solved XMODEM's problem of superfluous padding at the end of the file.

Forsberg built the standard with a number of optional features, believing that programmers would want to implement as many as possible on any given platform. He was dismayed to find that the majority of implementations were actually providing nothing more than 1 kilobyte block size with CRC-16, while continuing to use the "YMODEM" name. The result was a large number of mutually incompatible YMODEMs.

Common variants

YMODEM-1K uses a block size of one kilobyte instead of the standard 128 bytes. 1K blocks were an option in the original YMODEM standard, but this variant is missing the rest of the features, and is best described as a 1k variant of XMODEM.