Telephone Network Services

TAPI provides access to a variety of telephone network services. Although these services may use different technologies to establish calls and transmit voice and data, TAPI makes these service-specific details transparent to applications. This means you can create applications that can take advantage of any available service without including service-specific code in your application.

Historically, most telephone connections in the world have been of the type POTS, or Plain Old Telephone Service. Most POTS calls are transmitted digitally except while in the local looppics/TAPI00090000.gifthe part of the telephone network between the telephone and the telephone company's central switching office. Within this loop, human speech from a household telephone is usually transmitted in analog format and the digital data from a computer must first be converted to analog by a modem. Digital networks are gradually replacing analog in the local loop.

Using TAPI for POTS is straightforward because POTS is comparatively simple. It normally uses only one type of information (such as data or voice) per call, supports one channel per line, and so on. The vast majority of uses for TAPI are still POTS, and most telephony programmers will use TAPI only for POTS applications.

But TAPI is not restricted to POTS. TAPI also lets you make connections over other types of networks. More advanced kinds of data transmission methods are being developed, refined, and installed. For example, one important digital service is Integrated Services Digital Network (ISDN), which is expected to grow significantly in availability. ISDN networks have these advantages over POTS:

  • All digital

  • Less prone to error

  • Faster data transmission, with speeds up to 128 kilobytes per second (Kbps) on basic service

  • From 3 to 32 channels for simultaneous transmission of voice and data

  • An international standard

On ISDN networks, error rates are lower than with analog transmission because data travels from one end of the ISDN network to the other in digital format. Speeds of up to 128 Kbps are possible on Basic Rate Interface (BRI-ISDN) standard lines and much higher on Primary Rate Interface (PRI-ISDN) standard lines. By contrast, today's maximum dial-up modem data rates of 28.8 Kbps. When ISDN connections become more widespread, users will be able to send data to the recipient simultaneously with a voice call to that or another person. Each ISDN line, depending on its transmission rate, provides at least three channels (two for voice or data and one strictly for data or signaling information) and as many as 32 channels, for simultaneous, independently operated transmission of voice and data. BRI-ISDN lines provide two 64-Kbps "B" channels (B channels carry voice or data) and one 16-Kbps "D" channel (D channels carry signaling information or packet data). The PRI-ISDN lines for the U.S., Canada, and Japan have twenty-three 64-Kbps B channels and one 64-Kbps D channel. The European PRI standard offers thirty B channels and two D channels.

TAPI can also be used with other digital networks such as T1/E1 and Switched 56 service. With Switched 56, some local and long-distance telephone companies provide signaling at 56 Kbps over dial-up telephone lines. Switched 56 is quickly becoming available throughout the U.S. and in many other countries. It requires special equipment, and though its connection capabilities are limited to calls to other specially-equipped facilities, its high speed and pricing make it a reasonable choice for many data communications needs. Switched 56 is used for data calls only.

TAPI can also be used with other services such as CENTREX, which provides a set of centralized network services (such as conferencing) without the need to install special equipment. With CENTREX, you pay for the use of telephone-company equipment over regular telephone lines. In addition, TAPI can be used with digital Private Branch Exchanges (PBXs) and key systems. Because TAPI is independent of the underlying telephone network, programming a PBX application using TAPI is the same as programming a POTS application using TAPI. An application that was originally programmed for a POTS environment can be used within a PBX environment with no changes to the application's source code.

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