Here the modems improve on class 4 by compressing the data. The compression achieved will run from about 1.3:1
to 2:1. PKZIPped files will not compress at all. The protocol efficiency will average about 200%. A 2400 BPS (Bits Per Second) modem
will realise a throughput of about 4800 BPS. MNP-5 is not part of V.42bis.