In contrast to most transfer protocols developed for bulletin board systems (BBSs), ZMODEM was not directly based on, nor compatible with, the seminal XMODEM.
ZMODEM became extremely popular on bulletin board systems (BBS) in the early 1990s, becoming a standard as widespread as XMODEM had been before it.
As speeds increase the problem becomes more problematic; at 2400 bit/s a packet takes about 0.55 seconds to send, so about 1⁄5 of the available bandwidth is wasted waiting for ACKs.
Sliding windows are useful for latencies on the order of several packet lengths, which is the case for XMODEM on conventional phone lines.
ZMODEM addressed these problems by removing the need for ACKs at all, allowing the sender to send data continually as long as the receiver detected no errors.
Although YMODEM-g was faster (and thus popular among "power users"), the lack of other features such as restartable transfers made it less appealing.
Previously the user had to first request the file from the sender, placing it into a "waiting" state, then return to their local program and invoke a command to start the transfer.
LeechZmodem was a mischievous ZMODEM variant (among similar XMODEM and YMODEM derivatives) that cheated BBS download quotas.
In more current times, the developers of Synchronet have created a modern X/Y/ZMODEM implementation named SEXYZ, loosely based on the zmtx/zmrx package, which runs natively on Windows and Unix variants, supports long filenames and faster, more reliable data transfers.
A similar improvement allows ZMODEM-90 to work on 7-bit networks, whereas earlier protocols (with the notable exception of Kermit) had all demanded 8-bits to one degree or another.
Finally, ZMODEM-90 includes a basic run-length encoding compression system to further improve performance on uncompressed files.