The X-Modem is a character-oriented transmission protocol and was developed in 1977.
It is supported by many terminal programs, but only has a block size of 128 bytes. Because of this, the transmission speed is also very low and only one data packet can be sent on a working day.
Each packet consists of a start of header, a consecutive number between 0 and 255, 128 data bytes and a checksum.
Further developments followed and so later the Y modem and the Z modem were created.