Extended Line Functions
Extended Line Services
(or device-specific line services) include all service-provider defined
extensions to the API. The API defines a mechanism that enables service-provider
vendors to extend TAPI using device-specific extensions. The API only defines the
extension mechanism, and by doing so provides access to device-specific
extensions, but the API does not define their behavior. Behavior is completely defined
by the service provider.
TAPI consists of scalar and bit-flag constant definitions, data structures,
functions, and messages. Procedures are defined that enable a vendor to extend
most of these as follows.
For extensible scalar data constants, a service-provider vendor can define new
values in a specified range. As most data constants are DWORDs, typically the
range 0x00000000 through 0x7FFFFFFF is reserved for common future extensions,
while 0x80000000 through 0xFFFFFFFF are available for vendor-specific
extensions. The assumption is that a vendor would define values that are natural
extensions of the data types defined by the API.
For extensible bit-flag data constants, a service-provider vendor can define
new values for specified bits. As most bit-flag constants are DWORDs, typically
a specific number of the lower bits are reserved for common extensions while
the remaining upper bits are available for vendor-specific extensions. Common bit
flags are assigned from bit zero up; vendor-specific extensions should be
assigned from bit 31 down. This provides maximum flexibility in assigning bit
positions to common extensions versus vendor-specific extensions. A vendor is
expected to define new values that are natural extensions of the bit flags defined by
Extensible data structures have a variably sized field that is reserved for
device-specific use. Being variably sized, the service provider decides the
amount of information and the interpretation. A vendor that defines a
device-specific field is expected to make these natural extensions of the original data
structure defined by the API.
Two functions, lineDevSpecific
, and two related messages, LINE_DEVSPECIFIC
, provide a vendor-specific extension mechanism. The lineDevSpecific
function and associated LINE_DEVSPECIFIC message enable an application to
access device-specific line, address, or call features that are unavailable with
the Basic or Supplementary Telephony Services. The parameter profile of the lineDevSpecific
function is generic in that little interpretation of the parameters is made
by the API. The interpretation of the parameters is defined by the service
provider and must be understood by an application that uses them. The parameters are
simply passed through TAPI from the application to the service provider. An
application that relies on device-specific extensions will not generally work
with other service providers; however, applications written to the Basic and
Supplementary Telephony Services will work with the extended service provider.
For convenience, a more specialized escape function is also provided. It is
similar to lineDevSpecific
, but places interpretation on some of the parameters. This more specialized
function is lineDevSpecificFeature
, a device-specific escape function to allow sending switch features to the
switch. The message LINE_DEVSPECIFICFEATURE
is the device-specific message sent to the application as an indication of
features sent to the switch. This function and its associated message allow an
application to emulate button presses at the line's feature phone. As feature
phones and the meanings of their buttons are vendor-specific, feature invocation
is also vendor specific.
As mentioned earlier, there is no central registry for manufacturer IDs.
Instead, a unique ID generator (EXTIDGEN) is made available.
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