ZMODEM
ZMODEM is the high-speed file transfer protocol Chuck Forsberg released in 1986, with sliding windows, crash recovery, and 32-bit CRC; it was the most efficient serial file protocol of the BBS era.
Purpose
It transfers files reliably over unreliable serial links, retransmitting damaged frames automatically and supporting resume after interruption.
Mechanism
The sender frames the file with a CRC32 attached, and the receiver verifies and replies ACK/NAK; a sliding-window pipeline, selective retransmission, and file attribute (size, time) negotiation are supported.
Use cases
- BBS file upload and download
- Embedded serial firmware transfer
- File exchange on legacy Unix terminals