Editing ZMODEM (section)

==Improvements==
===Older systems===
Generally, early file transfer protocols break down a file into a series of ''[[network packet|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 {{mono|ACK}} message and the sender then starts sending the next packet.

The telephone system introduces a small delay known as ''[[Latency (engineering)|latency]]'' that interferes with this process. Even if the receiver sends the {{mono|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 have been sent during the delay, decreasing the ''[[channel efficiency]]''.

[[XMODEM]] used 128-byte payloads with a three-byte header and one-byte checksum for a total of 132 bytes per packet. In the era of 300&nbsp;bit/s modems, a packet took about four seconds to send, and typical latencies were on the order of {{frac|10}} of a second, so the performance overhead was not significant. As speeds increase the problem becomes more problematic; at 2400&nbsp;bit/s a packet takes about 0.55&nbsp;seconds to send, so about {{frac|5}} of the available bandwidth is wasted waiting for {{mono|ACK}}s. At 9600&nbsp;bit/s a packet requires only 0.13&nbsp;seconds to send, so about {{frac|2}} of the bandwidth is wasted.

===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.

===Restart===
XMODEM, and most protocols based on it, managed packet order by prefixing the data with a packet number from 1 to 255. Windowed versions used this packet number to indicate which packets had been received properly, or specify one that had not. Since the packets were 128&nbsp;bytes long, this meant the maximum amount of data that could be transferred before the packet numbers rolled over was 32&nbsp;KB.

ZMODEM replaced the packet number with the actual location in the file, indicated by a 32-bit number. This allowed it to send {{mono|NAK}} messages that re-wound the transfer to the point of failure, regardless of how long the file might be. This same feature was also used to re-start transfers if they failed or were deliberately interrupted. In this case, the receiver would look to see how much data had been previously received and then send a {{mono|NAK}} with that location, automatically triggering the sender to start from that point.

===Auto-start===
Auto-starting simplified management by allowing the sending machine to start the transfer. Previously the user had to first request the file from the sender, placing it into a "waiting" state, then return to their local program and invoke a command to start the transfer. With auto-transfer, they simply requested the file, the sender would then automatically trigger the transfer in the user's program.