Editing XMODEM (section)

==Transfer details==

Files were transferred one packet at a time. When received, the packet's checksum was calculated by the receiver and compared to the one received from the sender at the end of the packet. If the two matched, the receiver sent an <kbd><[[Acknowledge character|ACK]]></kbd> message back to the sender, which then sent the next packet in sequence. If there was a problem with the checksum, the receiver instead sent a <kbd><[[Negative acknowledge character|NAK]]></kbd>. If a <kbd><NAK></kbd> was received, the sender would re-send the packet,{{r|meeks198902}} and continued to try several times, normally ten, before aborting the transfer.

A <kbd><NAK></kbd> was also sent if the receiver did not receive a valid packet within ten seconds while still expecting data due to the lack of a <kbd><EOT></kbd> character. A seven-second timeout was also used ''within'' a packet, guarding against dropped connections in mid-packet.

The block numbers were also examined in a simple way to check for errors. After receiving a packet successfully, the next packet should have a one-higher number. If it instead received the same block number this was not considered serious, it was implied that the <kbd><ACK></kbd> had not been received by the sender, which had then re-sent the packet. Any other packet number signalled that packets had been lost.

Transfers were receiver-driven; the transmitter would not send any data until an initial <kbd><NAK></kbd> was sent by the receiver. This was a logical outcome of the way the user interacted with the sending machine, which would be remotely located. The user would navigate to the requested file on the sending machine, and then ask that machine to transfer it. Once this command was issued, the user would then execute a command in their local software to start receiving. Since the delay between asking the remote system for the file and issuing a local command to receive was unknown, XMODEM allowed up to 90 seconds for the receiver to begin issuing requests for data packets.