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
- Non-call-associated signaling, which provides a clear signaling path from the application to the service
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
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