TAPI (short for Telephony Application Programming Interface) was the result of cooperation of Microsoft and Intel and functions under Windows 95, 98 and Windows NT. It is included in a DLL and configurable via control panel. The aim of TAPI is to make an application independent of the hardware being used. This means a TAPI-enabled program does not care whether the computer is connected to the telephone net via modem, an ISDN card or any other proprietary solution.
The TAPI specification contains, at the hardware side, the TSPI (Telephony Service Provider Interface) to offer a standard interface to the providers, not only to the users. The hardware manufacturers use this to set up their drivers.
With TAPI, Microsoft has created a standard for telephony applications. Microsoft offers this interface since Windows 95.
The currently active ISDN application transfers its data to the TAPI centrally integrated in Windows. This transfers the data via SPI (Service Provider Interface) and its drivers to the end device.
Generally there are two different structures: First party- solutions (TAPI 1.3 to 2.1) and third party solutions (TAPI 2.1). First party means that the telephone end device is directly connected via serial interface with the local computer. This solution is recommended if the telephone system does not have an own interface or only few work places have to be equipped with a CTI interface.
If you have a third party solution, there is no connection between the local computer and the telephone. Instead the telephone system is directly connected to the NT-server. If the user wants to make a call from the computer, the command is directed to the server. The server logs in to the telephone which makes the connection. The work place phone rings once and the connection is established.