How is voice and modem traffic carried over an ISDN line?
Analog traffic, such as voice, Group 3 fax, and modem data must be converted to a digital format before it can be carried over an ISDN line. This conversion is done by an adapter know as an ISDN Terminal Adapter or TA. By using a TA, you can connect your present analog telephone, fax machine or modem to an ISDN line. When using ISDN equipment which interfaces directly to your computer (e.g., from your PC’s COM port to the data port of an ISDN Terminal Adapter), you cannot connect end-to-end with an analog modem or fax machine without special software or firmware which “emulates” the analog modulated waveforms of modems and fax machines. This emulation capability is available with equipment by Adtran (ISU Express), UK Robotics (Courier I-Modem) and 3Com. This add-on capability would allow an ISDN TA to connect to another ISDN TA or to a standard V.32bis/V.42bis analog modem.
