Editing ZMODEM (section)

===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.