Device Classes in TAPI

Device classes simplify development by letting programmers treat devices that have similar properties in a similar manner. Real-world devices such as telephones, modems, and telephone lines belong to device classes. Applications access devices belonging to a given class using the same functions.

An application never needs to know which service provider controls which device.

Device classes help make TAPI extensible by providing a framework from which to classify and support new equipment.

Application developers should keep in mind the existence of other applications that share telephony services, as explained in Multiple-Application Programming.

There are two device classes: line device and phone device.

It also defines two sets of functions and messages, one used for line devices and one used for phone devices.

The line device class is a device-independent representation of a physical line device, such as a modem. It can contain one or more identical communications channels (used for signaling and/or information) between the application and the switch or network. Because channels belonging to a single line have identical capabilities, they are interchangeable. In many cases (as with POTS), a service provider will model a line as having only one channel. Other technologies, like ISDN, offer more channels, and the service provider should treat them accordingly.

A service provider may allow an application to request that multiple channels be combined in a single call (as, for example, when ISDN "B" channels are combined into "H" channels) to give the call wider bandwidth, using a technique often referred to as inverse multiplexing. This added bandwidth enables the call to transmit more information at the same time. For most current telephonic purposes, inverse multiplexing is not necessary.

In POTS, it is normally necessary to assign one channel per line, but with ISDN, a line's channels are dynamically allocated when an application makes or answers a call. Because these channels have identical capabilities and are interchangeable, the application need not identify which channel is to be used in a given function call. Channels are owned and assigned by the service provider for the line device in a way that is transparent to applications. This channel management is a method of abstraction that eliminates the need to introduce the naming of channels by TAPI.

Just as a line device class is an abstraction of a physical line device, the phone device class represents a device-independent abstraction of a telephone set. TAPI treats line and phone devices as devices that are independent of each other. In other words, you can use a phone (device) without using an associated line, and you can use a line (device) without using a phone.

Service providers that fully implement this independence can offer uses for these devices not defined by traditional telephony protocols. For example, a person can use the handset of the desktop's phone as a waveform audio device for voice recording or playback, perhaps without the switch's knowledge that the phone is in use. In such an implementation, lifting the local phone handset need not automatically send an offhook signal to the switch.

This independence also allows an application to ring the local telephone in a manner that is independent of inbound calls. The capabilities of service providers is limited by the capabilities of the hardware and software used to interconnect the switch, the phone, and the computer. For detailed information about specific device classes, see Device Classes.

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