Determining Initial Media Modes
When a service provider learns of the appearance of a call, its first task is
to determine the call's media mode to the best of its ability. (It has received
ringing voltage on a POTS line or, in the case of EPBX or ISDN, a protocol
message indicating that a call is incoming.) It may be able to tell the single
correct media mode or it may only be able to narrow down the possibilities to a
certain few. These first media mode settings are called initial media modes
, and the following are the considerations used for setting initial media mode
- Service provider setup The service provider has been configured to work with only a single media
mode or certain media modes.
- Hardware limitations Hardware limitations are usually reflected in the service provider's
configuration, but the media modes could be further restricted by a particular card in
- Call to lineOpen Possible media modes are limited by what applications have requested in their
invocation of the lineOpen function. TAPI combines all of the media modes requested by applications and
indicates the sum of them to the service provider in a call to TSPI_lineSetDefaultMediaDetection. For example, a telephony device and its service provider may be able to
handle Group 3 fax calls, but if no application is running to handle such calls,
the provider would know not to bother with probing for fax or reporting fax calls
to TAPI. (The TAPI dynamic-link library does not automatically launch an
application to handle a particular type of incoming call.)
- Caller ID and direct Inward Dialing With Direct Inward Dialing (DID) at the called address, the switch supplies
the service provider with the digits that were dialed (the called address). The
service provider can be configured to associate particular called addresses
with particular media modes. Likewise, it could associate calls from particular
numbers as being associated with particular media modes, although this is much
less commonly used.
- Distinctive ringing The ring pattern of the incoming call may match a predetermined pattern (of
several possible at the called address) that is reserved for calls of a certain
media mode. If, for example, the incoming call is using ring pattern 2, the
service provider knows it to be a fax call (based on configuration information
supplied by the user).
- ISDN On an ISDN network, the provider may analyze the call's protocol frames to
determine the media mode. If the call is indicated as a 3.1 kHz Voice call, it is
still possible that the actual media mode on the call is analog data modem,
Group 3 fax, text telephone, or any of several other voiceband modulated signals,
in addition to human voice; it is only with digital data signals that the
media mode would necessarily be clearly definedfor example as Group 4 faxat call setup time in ISDN.
- Auto answer and probe Some providers give the user an option to let the service provider autoanswer the call and do some of the probing itself. In this process, TAPI gives the
call the correct application with the correct media mode already identified.
These tools may be enough to make a final and accurate determination of the
media mode. In any case, when the service provider passes the new call to TAPI,
it sends a LINE_CALLSTATE
message and includes in the message all that it knows about the call's media
mode(s). The following topics give details on the possible cases.
- Software for developers
Software for Android Developers
- More information resources
Unix Manual Pages