With modems, you've probably been using V42bis data compression. For asynchronous V120 or proprietary calls some TA's do support V42bis for data. An active TA is one which has an on board processor and firmware which handles all of the protocols and controls the device. Performance on ISDN will still be the same 64Kbps per channel. At present, inland calls made over an ISDN line cost the same as regular analogue calls.
International calls have their own pricing structure.
CAPI is an acronym for C ommon ISDN API . CAPI is currently a system specifically for Microsoft Windows based PCs. If you connect a regular modem or TA to your PC's serial port, a software application can open the COM port and control or use that device. Because each of those programs controls the COM port directly and there is no mechanism for program co-operation, once one program has opened the port, it is unavailable for all other programs. That solution, however, is awkward and cannot be automated or used to route incoming calls to the appropriate application.