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 conversion.

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.