Ymodem:-

A popular file transfer protocol available in many off-the-shelf and shareware communications packages, as well as on many bulletin board systems. Ymodem, a variation of the Xmodem protocol, divides the data to be transmitted into blocks; each block consists of the start-of-header character, a block number, 1 kilobyte of data, and a checksum. YModem's larger data block means less overhead for error control when compared with Xmodem, but if the block has to be retransmitted because the protocol detects an error, there is more data to resend. Ymodem also incorporates the capabilities to send multiple files in the same session and to abort file transfer during the transmission.