Editing ZMODEM (section)

===Windows and streaming===
One solution to this problem is the use of a [[sliding window]]. These protocols address latency by allowing the sender to continue sending a number of packets without waiting for an {{mono|ACK}}. The number of packets that it allows to continue is known as the "window", which was typically between two and sixteen packets in most implementations. A number of new versions of XMODEM with sliding window support appeared in the early 1980s.

Sliding windows are useful for latencies on the order of several packet lengths, which is the case for XMODEM on conventional phone lines. However, it is not enough to address longer latencies found on overseas phone calls, satellite connections, or X.25 services such as [[PC Pursuit]], where the latencies may on the order of a second or longer. In other cases, where the reverse channel was much slower than the sending one, as was the case for [[Telebit]] or [[US Robotics]] modems, even the small number of {{mono|ACK}}s might overwhelm the return channel and cause the transfer to pause.

ZMODEM addressed these problems by removing the need for {{mono|ACK}}s at all, allowing the sender to send data continually as long as the receiver detected no errors. Only {{mono|NAK}}s had to be sent, if and only if there was a problem. Since ZMODEM was often used on links with built-in [[error correction]], like X.25, the receiver would often not send a single message back to the sender. As a result, the system would send the entire file in a continual stream, and ZMODEM referred to itself as a "streaming protocol".

ZMODEM's performance was so improved over previous common protocols that it generally replaced even special protocols such as [[YMODEM|YMODEM-g]], which included no error correction at all and instead relied on error-free links maintained by the modems. Although YMODEM-g was faster (and thus popular among "[[power user]]s"), the lack of other features such as restartable transfers made it less appealing.