Lowest level of MNP (Microcom Network Protocol) performance. You will never see a modem with just
MNP class 1.
Level 1 uses an asynchronous byte-at-a-time half-duplex (ping pong back and forth,
but never send-receive at the same time) method. The protocol efficiency is about
70%. A 2400 BPS (Bits Per Second)
modem will realise a throughput of about 1700 BPS.