Over time, different versions of TAPI, applications, and service providers
may be produced. These new versions can make new definitions, such as for new
features, new members in data structures, and new bit fields. Version numbers
are therefore necessary to indicate how to interpret various data
structures.
To allow optimal interoperability of different versions of applications,
versions of TAPI itself, and versions of service providers by different vendors,
Microsoft Telephony provides a simple version negotiation mechanism for
applications. There are two different versions that TAPI and the telephony
service provider need to agree on for each line device. The first is the version
negotiated with TAPI and the telephony service provider (TSP) Basic and
Supplementary Telephony, referred to as the TAPI interface version. The other is
for provider-specific extensions, if any, and is referred to as the extension
version. The format of the data structures and data types used by TAPI's Basic
and Supplementary features is defined by the TAPI version, while the extension
version determines the format of data structures defined by the vendor-specific
extensions.
The lineNegotiateAPIVersion function
negotiates a TAPI version and lineNegotiateExtVersion negotiates the TSP
extension version. A single TSP could be capable of handling more than one
version, and an application must "fall back" to using an older version if using
an older TSP. In lineNegotiateAPIVersion, the dwApiVersion
parameter defaults to a value according to version, as follows.
TAPI
version |
dwApiVersion
default value |
| 1.3 |
0x00010003 |
| 1.4 |
0x00010004 |
| 2.0 |
0x00020000 |
| 2.1 |
0x00020001 |
| 2.2 |
0x00020002 |
However, TAPI makes this much easier as long as the TSP itself is using a
newer version than the application. If the TSP is indeed newer, then TAPI is
capable of translating "down" to the application's version. For example, TAPI
2.0 TSPs do not need to be specifically capable of dealing with TAPI version
1.4. If a TAPI 1.4 application is run, TAPI converts all TAPI 2.0 structures and
messages into TAPI 1.4 equivalents, or as close as possible. If there is no
close approximation in TAPI 1.4, then all TAPI 2.0-specific information will be
lost.
The precise meaning of an extension version is provider-specific. To use a
TSP that supports extensions, consult the provider's documentation.
There are a number of versions of TAPI. While most of these versions involved
changes to the TAPI and Telephony Service Provider Interface (TSPI)
documentation sets, there are other implications for each version, namely,
architectural differences, operating system variations, redistributables, and
TSP development issues.
| TAPI version |
Supported platforms |
| 1.0 – 1.2 |
Beta versions that should not be used any longer. |
| 1.4 |
Windows 95. |
| 1.5 |
Windows CE 1.0. |
| 2.0 |
Windows NT® 4.0 SP3. |
| 2.1 |
Part of Windows 98 and Windows NT 4.0 SP4. Available as a
redistributable for Windows 95. |
| 2.2 |
Windows 2000. |
