Bearer Mode and Rate

The notion of bearer mode corresponds to the quality of service requested from the network for establishing a call. It is important to keep the concept of bearer mode separate from that of media mode. The media mode of a call describes the type of information that is exchanged over a specific call of a given bearer mode. As an example, the analog telephone network (PSTN) provides only 3.1 kHz voice-grade quality of service—this is its bearer mode. However, a call with this bearer mode can support a variety of different media modes such as voice, fax, or data modem. In other words, media modes require certain bearer modes. TAPI manages the bearer modes only by passing the bearer mode parameters on to the network. Media modes are fully managed through the appropriate media stream APIs, although some limited support is provided in TAPI.

The bearer mode of a call is specified when the call is set up, or is provided when the call is offered. With line devices able to represent channel pools, it is possible for a service provider to allow calls to be established with wider bandwidth. The rate (or bandwidth) of a call is specified separately from the bearer mode, allowing an application to request arbitrary data rates.

The bearer modes defined in TAPI are:

• Voice, which is regular 3.1 kHz analog voice service; bit integrity is not assured.
• Speech, which is G.711 speech transmission on the call.
• Multiuse, as defined by ISDN.
• Data, which is unrestricted data transfer; the data rate is specified separately.
• Alternate speech and data, which is the alternate transfer of speech and unrestricted data on a call (ISDN).
• Non-call-associated signaling, which provides a clear signaling path from the application to the service provider.

Although support for changing a call's bearer mode or bandwidth is limited in networks today, TAPI provides an operation to request a change in the bearer mode or the data-rate parameters of an existing call. This function is lineSetCallParams.