Choosing a Protocol

You have four choices for protocols. The one you select will depend upon the protocols supported by your ASCII host and by your particular requirements. The following table shows the capabilities of the protocols:
  Downloading Uploading
  Single File Multiple Files Single File Multiple Files
XMODEM Yes No Yes No
XMODEM1K Yes No Yes No
YMODEM Yes Yes Yes Yes
YMODEMG Yes Yes Yes Yes
The XMODEM protocol is a single-file half-duplex protocol that performs error checking. Data is transmitted in 128-byte packets. Error checking, either by CRC or by checksum, occurs automatically. The Z and I Emulator for Windows implementation of XMODEM first tries CRC. If the sender fails to acknowledge the first three requests for CRC, XMODEM shifts to the checksum mode.
The XMODEM1K protocol is the same as XMODEM, except that it always uses CRC and has a larger packet size of 1024 bytes. Because some hosts are not able to handle the 1024-byte packets, there is a need for both XMODEM and XMODEM1K
The YMODEM protocol is similar to XMODEM, but it allows you to send multiple files in a single transfer. You may use a set of unique file names, or you may specify groups of files.
The YMODEMG protocol is the same as YMODEM, supporting multiple files, but it does not supply error checking. It assumes that the data always transfers correctly, and is only for use with error-correcting modems. For large amounts of data it can achieve much greater throughput than YMODEM because it does not wait for packet acknowledgment.