both YMODEM and YMODEM Batch listed as two separate protocols. Further confusion was created by the similarity between XMODEM-1k and these non-compliant YMODEMs, which were similar to the point that they were often incorrectly listed as the same. Streaming support YMODEM-g is a streaming variant used for error-free connections. It does not wait for an ACK to be received before sending the next packet. The protocol is faster than YMODEM because no latency is introduced between packets, but has no capability for correcting errors. It is reliant on the underlying connection being error-free, which is the case for modems supporting MNP for instance. Normally, a YMODEM transfer would be started by the receiver sending a to indicate it wants to use the 128-byte format with CRC, or if it wishes to use the original checksum system. When g-protocol is desired, the transfer is instead triggered by sending a . If the sender does not support the g-protocol, it considers this to be an error and ignores it, but if it does support g it begins sending packets in a continual stream. It only expects a single after the final packet is received, which is indicated by the presence of an character in the data. YMODEM-g assumes 1k packets are available. However, despite this protocol potentially being faster than ZMODEM it was still rarely used. This was partially due to the lack of other functionality, but also a more serious issue. Before the emergence of the 16550 UART, there was a substantial risk of buffer overrun on the serial port. Although this would be detected by YMODEM-g it could not be corrected since no block re-transmission is possible. The receiver would have to cancel and restart the whole transmission from the beginning. ZMODEM on the other hand has transfer resume capability which made it more appealing. References XMODEM / YMODEM Protocol Reference by Chuck Forsberg, October 10, 1985 XMODEM / YMODEM Protocol Reference by Chuck Forsberg, June 18, 1988 (document reformatted October 14, 1988) (HTML version with text issues) BBS file transfer protocols