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 or public switched telephone network (PSTN) only provides 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. TSPI only manages the bearer modes by passing the bearer mode parameters on to the network. Media modes are fully managed through the appropriate media mode functions, although some limited support is provided in TSPI.
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 Win32 Telephony are:
Although support for changing a call's bearer mode or bandwidth is limited in networks today, the SPI provides an operation to request a change in specific call parameters, namely the call's bearer mode and/or data rate. The operation is TSPI_lineSetCallParams.