Ymodem is an error-correcting protocol for a modem that uses larger data blocks for greater efficiency. Modems that agree on using the Ymodem protocol send data in 1024-byte blocks. Blocks received successfully are not acknowledged. Blocks with errors are acknowledged (with a NAK or negative acknowledgement) and the blocks are retransmitted. Ymodem is similar to Xmodem-1K except that a batch mode is provided. In batch mode, a number of files can be sent with a single command. Ymodem uses cyclic redundancy checking as the method of error detection.
See modem error-correcting protocols for a comparison with other error-correcting protocols.