If you need to make an Internet connection using the serial port of your PC,
but your ISP doesn't support V.110 or V.120, you will need to use protocol
Many Terminal Adapters support protocol conversion between synchronous PPP
and asynchronous PPP.
These protocols are not identical and conversion
is required. The serial port of the PC is asynchronous so this determines the
type of PPP used between PC and Terminal Adapter. The ISP, however, in natively
supporting ISDN (rather than using Rate Adaptation), requires synchronous PPP.