Editing YMODEM (section)

===XMODEM===
The original [[XMODEM]] was a very simple protocol and that is the reason for its success; it could be implemented on practically any machine of the era, even those with very limited processors and storage. It worked by breaking up the data to be sent into 128-byte [[network packet|packets]], adding a 3-byte header and 1-byte [[checksum]] footer, and sending the resulting 132-byte packets out in order. The receiving computer recalculated the checksum from the 128 bytes of data, and if it matched the checksum sent in the footer it sent back an {{mono|ACK}}, and if it did not match, a {{mono|NAK}}. When the sender received an {{mono|ACK}} it sent the next packet, while a {{mono|NAK}} caused it to re-send the last one.

There were a number of problems with the protocol. The use of a simple checksum meant some common errors could go unnoticed. The small packet size and requirement to wait for the {{mono|ACK}} or {{mono|NAK}} led to slow performance on higher speed links or ones with significant latency. Finally, as the transfer contained no details of the file, every file had to be manually started, which could be a chore when many small files were being transferred.

Solutions to these problems were developed during the early 1980s. XMODEM-CRC replaced the checksum with a 16-bit [[cyclic redundancy check]] (CRC), which was much more resistant to common errors. XMODEM-1k expanded the packet size from 128 bytes to 1024, improving performance on higher-speed connections, while others, like WXMODEM and SEAlink instead introduced [[sliding window]] systems to combat both performance and latency, at the cost of some complexity. Still others, like TeLink and MODEM7, added file information so that a single transfer could contain multiple files, allowing batches of files to be sent with a single command.