ZMODEM

ZMODEM is a file transfer protocol developed by Chuck Forsberg in 1986, in a project funded by Telenet in order to improve file transfers on their X.25 network. In addition to dramatically improved performance compared to older protocols, ZMODEM also offered restartable transfers, auto-start by the sender, an expanded 32-bit CRC, and control character quoting, allowing it to be used on networks that might "eat" control characters. ZMODEM became extremely popular on bulletin board systems (BBS) in the early 1990s, displacing earlier protocols such as XMODEM and YMODEM.

The key improvement in ZMODEM was the introduction of sliding window support for improved performance. Generally file transfer protocols break down a file into a series of packets, and then send them one-at-a-time to the receiver. If the packet is received correctly an ACK message is sent and the sender then starts sending the next packet.

However, the telephone system introduces a small delay known as latency that interferes with this process. Even if the receiver sends the ACK immediately, the delay in the phone lines means there will always be some time before the sender receives it and sends the next packet. As modem speeds increase, this delay represents a larger and larger number of packets that could be sent during the delay, decreasing the overall performance.