Another important idea is introduced here; this is the Terminal Adapter.

A Terminal Adapter (TA) is always necessary to connect non-ISDN devices (such as a serial port of a PC) to the ISDN. However, a TA can also contain a CODEC if it is intended to support analogue phones, fax machines and modems.

In diagram below, the modem at the top left can plug in to the POTS ports on the TA. The TA will then convert sounds generated by the modem on its POTS port into a bit stream (and vice-versa). This bit stream is identical to that created by an ISDN telephone; that's to say it represents sounds.