TAPI is an abstraction layer, and by itself does not provide any useful functionality. As an abstraction layer, its function is to provide a consistent, device-independent programming model to applications that use it. TAPI is a generalized API that covers a wide range of telephony equipment with diverse capabilities. But TAPI is not a "Modem API," and can't perform device-specific tasks such as modifying initialization strings or setting up auto-answer. TAPI uses components provided by the operating system or third parties to provide call control functionality.

TAPI DLLs

The TAPI DLLs, along with the TAPI Server (Tapisvr.exe), are crucial abstractions separating end-user or server applications from service providers. A TAPI DLL in conjunction with the TAPI Server provides a consistent interface between these two layers.

A TAPI application loads the appropriate DLL into its process space. During initialization, TAPI establishes an RPC link with Tapisvr.exe. In Windows NT, the TAPI Server runs as a separate service. In Windows 2000/XP, the TAPI Server runs in the context of SVCHOST.

There are three DLLs associated with TAPI:

Tapi.dll – interface for 16 bit applications
Tapi32.dll – “C” interface for 32 bit applications that use the TAPI 2.X interface
Tapi3.dll. – “COM” interface for applications that use the TAPI 3.X interface.

Existing 16-bit applications link to Tapi.dll. In Windows 3.1 and Windows 95, this DLL is the core of Windows Telephony. Under Windows 98/Me and Windows NT/2000/XP, Tapi.dll is simply a thunk layer that maps 16-bit addresses to 32-bit addresses and passes requests to Tapi32.dll.

Existing 32-bit TAPI 2.x applications link to Tapi32.dll. In Windows 95, Tapi32.dll is a thunk layer to Tapi.dll. In Windows 98/Me and Windows NT/2000/XP, Tapi32.dll is a thin marshaling layer that transfers function requests to the TAPI Server (TAPISRV) and, when needed, loads and invokes media service provider DLLs in the application's process.

TAPI 3.x applications link to Tapi3.dll. The Microsoft Remote TSP enables non-server resident applications (remote clients) to use the server based TSP capabilities (on the Telephony Server).

TAPI / TSPI Versions

Applications can access the capabilities of the TSP through the various versions of TAPI. Version-ing negotiations between the application and the TSP are handled by TAPI. The Mitel TSP was written to the TSPI interface version 2.2. This does not limit the application from accessing the TSP with an API version higher that the TSP’s, as long as the required functions are supported by the TSP and properly mapped by TAPI to the TSPI version 2.2 interface.

Here is some general information as to what was released in the different versions of the TAPI and TSPI specifications.

TAPI Version 1.3

Included in Windows 3.1

TAPI 1.3 (sometimes referred to as TAPI 1.03) was the first released version of the TAPI specification.

TAPI Version 1.4

Included in Windows 95

TAPI 1.4 added a number of APIs, messages, constants, and structure elements to the 1.3 specification. TAPI 1.4 is a component of Windows 95—it is always present and cannot be separated out for separate distribution. TAPI 1.4 supports only 16- it TSPs. However, it does allow 32-bit applications to be developed without having to worry about the limitations of 16-bit Windows.

TAPI Version 1.5

Included in Windows CE 1.0

TAPI 1.5 was designed specifically for Windows® CE version 1.0. It is a subset of TAPI 1.4, is only available on Windows CE, and is documented in the Windows CE developer documentation.

TAPI Version 2.0

Included in Windows NT® 4.0 SP3

TAPI 2.0 provides a small number of enhancements to the basic TAPI 1.4 functionality. However, there were some major architectural changes that greatly improved its stability. Most of the changes were fundamental changes necessary to bring TAPI to Windows NT
4.0 and take advantage of its features (full 32-bit support, services, and Unicode). However, these changes were internal to TAPI and had little effect on TAPI applications.

TAPI Version 2.1

Included in Windows 98 and Windows NT 4.0 SP4. Available as a redistributable for Windows 95.

TAPI 2.1 provides one addition to the previous TAPI 2.0 specification: client/server functionality. With TAPI 2.1 and later, telephony hardware can be installed on a telephony server that can be accessed from any computer on a network. A new set of functions allows administration DLLs to be written to control who has access to remote resources.

TAPI Version 2.2

Included in Windows 2000 and Windows XP

TAPI 2.2 provides specialized call center management functionality. Under TAPI 2.1 and earlier, call center software required careful mapping of lines and addresses to call center Microsoft’s TAPI function. The TAPI 2.1 functions coordinate with TAPI 3.x call center capabilities and makes the creation of ACD systems much easier.

TAPI Version 3.0

3.0 requires Windows 2000 or later

The COM-based telephony API, TAPI 3.x, is available starting with Microsoft® Windows®
XP. TAPI 3.x provides greatly enhanced development tools for the modern world of communication programming, where a "call" may be a video stream on an IP-based network, and there is no phone set involved in the session. Generally speaking, application writers will find TAPI 3.x provides better support to implement flexible communications controls. However, TAPI 2.x may be the right choice in some situations. If minor modifications are needed for a legacy application, or there are legacy platforms to support, conversion to the COM API may not be cost effective.

TAPI Version 3.1

TAPI 3.1 adds detailed phone device controls and a number of specialized terminal interfaces.

TSPI 3.0 adds functions and messages to support communication with a Media Service Provider (MSP). Functions have also been added which enhance the ease of tracking conferences using the Call Hub concept.