Telephony Server Application Programming Interface

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TSAPI, short for Telephony Server Application Programming Interface, was a computer telephony integration standard developed and promoted by Novell and AT&T. It consisted of a number of call control commands for switching calls, voice mail and call logging using NetWare servers. Unlike the competing TAPI from Microsoft/Intel, TSAPI was a server-based system that did not expect client-side equipment to handle call switching. This was important to AT&T, who sold large telephone switches that TSAPI was intended to work with.


TSAPI consisted of two primary parts, the TSAPI application programming interface itself, and a "telephony service provider" that ran on a server and talked to TSAPI clients. Novell produced one such provider, "TServer" that ran, unsurprisingly, on NetWare servers. TServer, in turn, talked to a driver specific to the brand of telephony switch being used. NetWare acted primarily as the operating system for TServer to run within, although TServer did make use of NetWare Directory Services for security and provisioning. The whole system from client-side drivers to server to PBX driver was known as "NetWare Telephony Services" (NTS), at least when using Novell software from top to bottom.

The TSAPI message formats were based on a standard promoted by the European Computer Manufacturers Association, which was directly supported by a number of European-built switches. When used with one of these switches, the driver between the TSAPI server and the switch was "thin". "Thicker" drivers were needed for switches that did not directly support these standards. The client-side TSAPI API was available for Windows 3.1, Windows 95, Windows NT, OS/2, Mac OS and UnixWare.

TSAPI is a control protocol only, it does not send actual voice data across the network for use with software-based phones. It includes commands for dialing, hanging up and so forth. TSAPI is a stateful protocol that required a channel, referred to as a stream, to be set up for all communications.

TSAPI and Versit[edit]

TSAPI was being created in an era when major telephony vendors were promoting a vision of wide-area networking based on dedicated circuit-switched links. Unlike modern networking systems where each piece of data is separately routed to its destination, these networking systems were essentially a version of the existing phone system carrying data instead of voice, setting up dedicated channels between endpoints. Looking for applications that might make use of such a network, vendors promoted numerous new standards for videotelephony, high-speed fax, etc.

TSAPI standardized only the control system; getting data into and out of a computer remained an issue. There were a number of efforts by various vendors to support this functionality. Apple Computer was promoting GeoPort as the computer-end of such a system, and selected TSAPI as the messaging protocol. The various vendors formed the Versit Consortium and published the "The Versit CTI Encyclopedia." The Encyclopedia expanded the basic TSAPI system to include a data-type identifier to allow it to switch any sort of "call". Additionally, Versit allowed that data to be switched to the computer using GeoPort or a number of other connection options.