Editing YMODEM (section)

==Streaming support==
'''YMODEM-G''' is a streaming variant used for error-free connections. It does not wait for an [[Acknowledge_character|ACK]] to be received before sending the next packet; [[XON/XOFF]] is used for [[flow control (data)|flow control]]. The protocol is faster than YMODEM because no [[Latency (engineering)|latency]] is introduced between packets, but has no capability for correcting errors. It is reliant on the underlying connection being error-free,{{r|meeks198902}} which is the case for modems supporting [[Microcom Networking Protocol|MNP]] for instance.

Normally, a YMODEM transfer would be started by the receiver sending a {{mono|C}} to indicate it wants to use the 128-byte format with CRC, or {{mono|NAK}} if it wishes to use the original checksum system. When g-protocol is desired, the transfer is instead triggered by sending a {{mono|G}}. 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 {{mono|ACK}} after the final packet is received, which is indicated by the presence of an {{mono|EOT}} 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 overflow|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.