What is a Line Device?

A line device is a physical device such as a fax board, a modem, or an ISDN card that is connected to an actual telephone line (although the device may not be physically connected to the computer on which the telephony application is running). Line devices support telephonic capabilities by allowing applications to send or receive information to or from a telephone network. A line device contains a set of one or more homogeneous channels that can be used to establish calls.

Within TAPI applications, a line device is the logical representation of a physical line device. Although "line" often connotes something with two endpoints, it is possible to abstract a line device to a single point because TAPI views it only as a point of entry to the line that leads to the switch.

Although the three lines in the preceding illustration are composed of different hardware and used for different functions, they are abstracted to the same device type and governed by the same rules. The telephone represents not a phone device but a line device used for voice calls. When using this line device for incoming or outgoing calls, the application would also need to open and control an instance of the phone-device class, which is described in detail in later sections.

TAPI requires that every TAPI-capable line device support all of Basic Telephony. If an application needs to use capabilities beyond those of Basic Telephony (namely Supplementary or Extended Telephony), it must first determine the line device's capabilities, which can vary according to network configuration (client versus client/server), hardware, service-provider software, and the telephone network. The lineNegotiateAPIVersion function allows the application to identify the set of Extended capabilities supported on a line device, and the lineNegotiateExtVersion function allows for different versions of that set to be used. The function lineGetDevCaps returns the telephonic capabilities implemented through the use of the Supplementary and Extended (if any) TAPI functions of a given line device in a data structure of the type LINEDEVCAPS.

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