Glossary definition of Y Modem

Also known as: Ymodem

Y-Modem is an error-correction protocol used in the transfer of serial data files between modems.

Y-Modem is based on the earlier X-Modem protocol, but has the additional feature of being able to cope with a batch transmission mode, in which a number of files can be sent with one command. For greater efficiency Y-Modem can also handle data with a variable block size of up to a maximum 1024 bytes (1 kB). During transmission, the size of text and binary files is included in the Y-Modem header, so that the exact file length will be retained after transfer. The error detection method in Y-Modem is cyclic redundancy check (CRC).

A later version of Y-Modem, Y-Modem-G, transmits data in a continuous stream and does not acknowledge the blocks successfully received; it is only the blocks with detected errors that cause a negative acknowledgement (NAK), thus prompting the resending of data.