It's a file transfer protocol that came out decades ago when modems rocked along at a whomping 300 baud.

It was developed in the CP/M days, and is a standardized version of "Modem 7" protocol created by Ward Christiansen(sp?) and Randy Seuss in Chicago ("Ward & Randy's Bulletin Board").

It was developed because the standard terminal equipment only used seven bit ASCII ... and data transfer (for files) required all eight bits.

Xmodem provides a way to send the data as block, each block is CRC'd and if it is found to be bad, it gets "nak"d and is resent in a future slot.

Kermit was developed later, and came up witha different way of passing binary data across a seven bit ASCII async line.

From Xmodem, we derived Xmodem1K (1024 byte blocks), Ymodem (batch) and Zmodem (remote features, better flow).

Xmodem is still a good way to get data across an ASYNC line ... Cisco uses it as a mechanism to get an image into a device with a trashed firmware.

Good Luck

Scott