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Microcom Networking Protocol
Microcom Networking Protocol (MNP) family of error-correcting protocols were commonly used on early high-speed (2400 bit/s and higher) modems. Originally developed for use on Microcom's own family of modems, the protocol was later openly licensed and used by most of the modem industry, notably the "big three", Telebit, USRobotics and Hayes. MNP was later supplanted by v.42bis, which was used almost universally starting with the first v.32bis modems in the early 1990s.
==Error correction basics==
Modems are, by their nature, error-prone devices. Noise on the telephone line, a common occurrence, can easily mimic the sounds used by the modems to transmit data, thereby introducing errors that are difficult to notice. For some tasks, like reading or writing simple text, a small number of errors can be accepted without causing too many problems. For other tasks, like file transfers, even one error can "destroy" the entire file. As modems increase in speed by using up more of the available bandwidth, the chance that random noise would introduce errors also increases; above 2400 bit/s these errors are quite common.
To deal with this problem, a number of file transfer protocols were introduced and implemented in various programs. In general, these protocols break down a file into a series of ''packets'' containing a number of bytes from the original file. Some sort of additional data, normally a checksum or CRC, is added to each packet to indicate whether the packet encountered an error while being received . The packet is then sent to the remote system, which recomputes the checksum or CRC of the data and compares it to the received checksum or CRC to determine if it was received properly. If it was, the receiver sends back an ACK (''acknowledgement'') message, prompting the sender to send the next packet. If there was any problem, it instead sends a NAK (''not-acknowledged'') message, and the sender resends the damaged packet.
This process introduces "overhead" into the transfer. For one, the additional checksum or CRC uses up time in the channel that could otherwise be used to send additional data. This is a minor concern, however, unless the packets are very small (which they are in UUCP for instance). A more serious concern is the time needed for the receiver to examine the packet, compare it to the CRC, and then send the ACK back to the sender. This delay grows in relative terms as the speed of the modem increases; the latency of the phone line is a constant, but the amount of data that ''could'' be sent in that time grows as the speed increases. To address this problem, newer protocols use a system known as "sliding windows", allowing the sender to begin transmitting the next packet ''without'' receiving an ACK message; only if the ACK does not arrive for some time will it resend the packet.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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