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XMODEM File Transfer Protocol
XMODEM is a standard file transfer process developed by Ward Christensen in 1977. It sends data-related data blocks and awaits receipt of block receipt. XModem is used in both hardware and software.
XMODEM is a half-dup communication protocol with an effective error detection device.
It violates the original data in a series of packets sent to the recipient and other information that allows the recipient to determine whether the packets have been received correctly.
Marked files are completed with the end-of-file character sent behind the last block. This character is not in the packet but is sent as a single byte.
Since the lengths of the files are not transferred as part of the protocol, the final packages are filled with known characters, which can be released.
Files are transferred one datagram at a time. The package checksum is calculated and compared with that found at the end of the package on the acquisition side. When the recipient sends a confirmation message to the sender, the subsequent set of packages is sent.
If there is a problem with the checksum, the recipient sends a message requesting to be resubmitted.
Upon receiving a negative feedback, the sender re-sends the package and attempts to transfer continuously approximately ten times before issuing the transfer.
The recipients conduct the transfer, i.e., the sender will not send any data until the recipient sends the first NAK. The transmitter waits until the recipient sends a NAK byte.
The NAK byte is a signal that the recipient is ready to start. This is the first byte that indicates that the file transfer is ready to start.
If the recipient takes too long or an error occurs, the sender will stop waiting or "Time Out". File transfers will restart when they expired.
NAK is also sent if the recipient does not receive a valid package within ten seconds while waiting for data due to a lack of EOT character.
The recipient performs the following functions and transfers one packet at a time:
Assume that the packet number sent is the same as the actual packet number, i.e., the second byte is checked in the package.
In the event of a discrepancy, send a CAN byte to cancel the transfer. The packet's checksum is calculated after retrieval and compares with that received from the sender at the end of the package.
If the condition SUM = CHECKSUM is true, then the receiver sends an ACK byte to the sender, which indicates to send the next package in sequence.
If the situation is not satisfactory, the NAK byte is sent, signifying that the transmitter will re-send the byte.
When the sender sends EOT byte instead of SOH byte, the recipient sends a NAK byte.
If the sender sends another EOT immediately after that, the recipient sends the ACK byte, and the transfer is complete.
The recipient has the power to cancel the transfer at any time by sending the CAN byte, and the sender can only cancel between blocks by sending the CAN byte.
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