Media Access

The media mode is the form in which data is transmitted on a line. The four main types of media mode are voice, speech, fax, and data. With TAPI, calls can be established independently of the call's media mode.

The media stream is the actual stream of information that travels on the line. Phone devices and calls on line devices are capable of carrying media streams. The Telephony-API line and phone device classes provide a wide range of control operations for these devices, but access to the media stream itself is not provided by TAPI. Instead, the application must use other APIs for the Win32 environments to access or manage these media streams. These APIs include the Waveform API, the Comm API, and the MCI (Media Control Interface). The Waveform API is used for multimedia programming, the MCI provides a high-level generalized interface for controlling media devices, and the Comm API is the set of communications functions provided by the Win32 SDK.

For example, for line devices, an application can use TAPI to establish a connection to another station. Once the connection is established, the application can then use the Waveform API (or the MCI Waveaudio API) on the associated device to play back (send) and record (receive) audio data over the connection. Similarly, if the connection's media stream is from a modem, an application would use the modem configuration extensions of the Comm API to control the media stream.

To provide TAPI and media-stream access to either a phone or a call on a line device, the service provider must implement both the Telephony SPI and the appropriate media stream SPI or DDI (device-driver interface). The service provider can support lines and phones simultaneously.

Because these device classes and media stream interfaces function independently of one another, coordination of their usage must occur at the application level. Multiple applications that share calls and media streams in nontrivial ways will likely need to coordinate their activities at the application level to prevent conflicting usage of TAPI and the media stream API in use. For more information on preventing conflicts, see Multiple-Application Programming.

TAPI reports changes in the type of media stream (voice, fax, data modem, and so on) to participating applications. This process is sometimes referred to as call classification. The mechanism used to determine the type of media stream is specific to the service provider. For example, a service provider may filter the media stream for energy or tones that characterize the media type, or it may use distinctive ringing, information exchanged in messages over the network, or knowledge about the caller or called ID to make this determination.

TAPI also provides limited support for control of the media stream on a call, particularly in server-based networks. The actual data does not pass through TAPI, but TAPI can be used to a limited extent to control functions that control the media stream. This control is provided to avoid latency (delay) problems that could arise in client/server configurations for which the application is forced to use the stream's media API. An application can request actions on a call's media stream if these actions are to be triggered by events normally reported by TAPI, such as the detection of a tone or DTMF digit, or the transition of a call to a specified call state.

For example, an application can request that a call's media stream be suspended (with lineSetMediaControl) when a # DTMF digit is detected on the call, and that the media stream be resumed when a * DTMF digit is detected. Note that some implementations or configurations will be unable to provide any media-control functions or media access to the phone or line. Providing media control is optional to the service provider; it should provide performance benefits primarily for client/server implementations. Because it is optional and because only limited control is provided, its usage is generally discouraged. If possible, applications should use the media stream's control functions instead.

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