2.4.2.1 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 bearer mode separate from media mode. The media mode of a call describes the type of information that is exchanged over the call of a given bearer mode. As an example, the analog telephone network (PSTN) only provides 3.1 kHz voice grade quality of service (bearer mode). However, a call with this bearer mode can support a variety of different media modes such as voice, fax, or data modem. Media modes require certain bearer modes. The Telephony SPI 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 APIs, although some limited support is provided in the Telephony SPI.
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 the SPI are:
- voice - Regular 3.1 kHz analog voice service. Bit integrity is not assured.
- speech - G.711 speech transmission on the call.
- multiuse - As defined by ISDN.
- data - Unrestricted data transfer. The data rate is specified separately.
- alternate speech and data - The alternate transfer of speech and unrestricted data on a call (ISDN).
- non call-associated signaling - This 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, the SPI provides an operation to request a change in a call's bearer mode and/or data rate. The operation is TSPI_lineSetCallParams.
TSPI_lineSetCallParams
Request a change in the call parameters of an existing call.