ZMODEM is an inline 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 offered restartable transfers, auto-start by the sender, an expanded 32-bit CRC, and control character quoting supporting 8-bit clean transfers, allowing it to be used on networks that would not pass control characters. In contrast to most transfer protocols developed for bulletin board systems (BBSs), ZMODEM was not directly based on, nor compatible with, the seminal XMODEM. Many variants of XMODEM had been developed in order to address one or more of its shortcomings, and most remained backward compatible and would successfully complete transfers with "classic" XMODEM implementations. This list includes Forsberg's own YMODEM. ZMODEM eschewed backward compatibility in favor of producing a radically improved protocol. It performed as well or better than any of the high-performance varieties of XMODEM, did so over links that previously didn't work at all, like X.25, or had poor performance, like Telebit modems, and included useful features found in few or no other protocols. ZMODEM became extremely popular on bulletin board systems (BBS) in the early 1990s, becoming a standard as widespread as XMODEM had been before it. Improvements Streaming Generally, file transfer protocols break down a file into a series of packets, and then send them one-at-a-time to the receiver. The main portion of the packet, the payload, is a certain number of bytes from the file being sent. After the payload comes a checksum or cyclic redundancy check (CRC) that can be used to determine if the payload was received correctly. If the packet is received correctly, the receiver sends an message and the sender then starts sending the next packet. The telephone system introduces a small delay known as latency that interferes with this process. Even if the receiver sends the 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 have been sent during the delay, decreasing the channel efficiency. XMODEM used 128-bytes payloads with a three-byte header and one-byte checksum for a total of 132 bytes per packet. In the era of 300 bps modems, a packet took about four