There are two standards for data compression protocols, MNP-5 and CCITT V.42bis. Some modems also use proprietary data compression protocols.
A modem cannot support data compression without utilizing an error control protocol, although it is possible to have a modem that only supports an error control protocol but not any data compression protocol. A MNP-5 modem requires MNP 4 error control protocol and a V.42bis modem requires V.42 error control protocol.
Also note that although V.42 include MNP-4, V.42bis does not include MNP-5. However, virtually all high-speed modems that support CCITT V.42bis also incorporate MNP-5.
The maximum compression ratio that a MNP-5 modem can achieve is 2:1. That is to say, a 9600 bps MNP-5 modem can transfer data up to 19200 bps. The maximum compression ratio for a V.42bis modem is 4:1. That is why all those V.32 modem manufacturers claim that their modems provide throughput up to 38400 bps.
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