A TSP (TAPI Service Provider) is what TAPI uses to talk to the underlying hardware. In
essence, it is a driver that takes a physical device (modem) and turns it into a logical
device that TAPI can use (line). It is an abstraction layer that makes it so TAPI itself
doesn't need to know how the hardware works, TAPI just tells the TSP to do the actual
implementation. In theory, every piece of hardware that TAPI wants to use should have a
TSP specially written for it. Windows 95 and NT come with a TSP called Unimodem which a
modem's manufacturer can configure to support their modem. Modem manufacturer find it
easier to configure Unimodem rather than write their own TSP. However, Unimodem does
have some problems. Unimodem/V is another TSP that supports Voice modems (/V for
Voice). Voice modems can handle Caller ID, voice connections, and DTMF. Unimodem/V is
only available on Windows 95, 98, 2000, and ME, and NOT on NT4. Most PBXs also come with a
TSP to allow generic or special phones to do computer telephony. ISDN also needs a TSP to
make it available to be used in TAPI. Most ISDN manufactures write their own TSP of
various qualities although some use Unimodem.

Some of the people in the TAPI newsgroup are TSP developers, and some are TAPI application
developers.