ZMODEM

ZMODEM is a file transfer protocol used over serial connections, such as RS-232, and over dial-up networks. Developed by Chuck Forsberg in 1986 as an improvement to XMODEM and YMODEM, it became widely adopted in bulletin board systems and early Internet gateways. Its design emphasized reliability, speed, and convenience for transferring files between hosts.

ZMODEM employs a block-based transfer with large block sizes and a sliding-window style flow control to keep the link busy. It supports streaming data, allowing the sender to push data continuously rather than waiting for individual acknowledgments. Error detection is performed with cyclic redundancy checks, and the protocol can automatically retransmit corrupted blocks. A key feature is the ability to resume interrupted transfers from the point of failure, without redoing already received data.

In addition to single-file transfers, ZMODEM supports directory and batch transfers, enabling multiple files and basic directory metadata (such as sizes and timestamps) to be transferred in a single session. It negotiates transfer parameters and can adapt block size and timeouts to the connection quality, making it efficient on a range of serial links from slow to relatively fast.

Implementation and usage: The protocol has been implemented in many terminal emulators and BBS software. In Unix, Linux, and Windows environments, the lrzsz package provides rz (receive) and sz (send) utilities that implement ZMODEM. ZMODEM tools are commonly used for remote file copying in shells that lack full FTP/SFTP support.

Legacy: While largely supplanted by modern protocols such as FTP, SFTP, and HTTP in mainstream use, ZMODEM remains popular in retro-computing and in some embedded or legacy serial systems due to its robustness and resume capability. See also XMODEM and YMODEM.