Toshiba V.90 - Manual (Page 71)

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MNP and V.42
The internal modem has built-in protocols MNP ® (Microcom
Networking Protocol) Class 4 and V.42 for error correction and
MNP class 5 and V.42bis for data compression.
Error-correction overview
Telephone line noise, or electrical interference, can cause errors in
data communication. Noise is especially a problem at high speeds
of 14,400 bps or greater.
The MNP and V.42 protocols were developed to ensure reliable
data communications despite the high probability of error
generation from line noise.
MNP error correction
MNP was developed by Microcom. MNP class 4 enables data
correction of full-duplex communications and also provides
asynchronous/synchronous conversion and data packet-size
negotiation to increase throughput.