ZMODEM

/ˈziːˌmoʊdɛm/

n. “The speedy, robust file transfer protocol that learned from its predecessors.”

ZMODEM is a file transfer protocol developed in the mid-1980s as an advancement over XMODEM and YMODEM. It was designed for faster, more reliable transfers over serial connections and early modems, incorporating features like error recovery, batch file transfers, and streaming to maximize efficiency.

Key characteristics of ZMODEM include:

• Streaming Mode: Unlike the stop-and-wait approach of XMODEM/YMODEM, ZMODEM can continuously send data without waiting for an acknowledgment for each block, improving speed.
• Automatic Resume: If a transfer is interrupted, ZMODEM can resume from the point of failure rather than restarting the entire file.
• Batch Transfers: Supports sending multiple files in a single session with full metadata (file name, size, timestamp).
• Error Detection: Uses robust CRC for high reliability on noisy connections.
• Backward Compatibility: Can emulate XMODEM or YMODEM when needed for compatibility.

Conceptual example of a ZMODEM transfer:

1. Sender initiates ZMODEM session
2. Receiver signals ready
3. Sender streams file data continuously
4. Receiver detects errors → requests only the corrupted blocks to be resent
5. Transfer completes; multiple files can be sent in sequence
6. Interrupted transfers can resume from last confirmed byte

Conceptually, ZMODEM is like sending an entire conveyor belt of packages with a tracking system: if one package is lost or damaged, only that package is resent, while the rest keep moving smoothly.

In essence, ZMODEM is the pinnacle of the classic modem-era file transfer protocols, offering speed, reliability, and resume capability, making it the preferred choice for transferring large files over unreliable serial connections.