Windows Telephony Overview

Line Sharing


The Windows 3.x implementation of the API for accessing COM ports didn't allow multiple applications to share a single phone line. This makes it impossible, for example, for one application to wait for an incoming fax call while another application makes an outgoing voice call. TAPI provides extensive support for this kind of sharing. Any number of applications can share a single line device by opening it with one of several privileges:

None Application can make outbound calls only.
Monitor Application can watch inbound and outbound calls only.
Owner Application can make outbound calls and will be given inbound calls that match requested media modes.
Owner + Monitor Application wants to watch inbound calls of media modes other than it has requested ownership of.



A line owner application needs to specify one or more media modes that it is interested in for inbound calls. The media mode describes the type of media available on the call. The media modes and who might be interested in them are:

Interactive Voice Person or call control application
Automated Voice Automated application like an answering machine
Data Modem Communications application
Group 3 Fax
Group 4 Fax 		Fax application
Digital Data Communcations application or network driver
Telephony Devices for the Deaf (TDD):
  • Teletex
  • Videotext
  • Telex
    		• Display application
    Mixed ISDN application
    Analog Display Services
    Interface (ADSI)
    VoiceView     		Mixed voice and data application
    Unknown Any application looking for incoming calls whose media mode has yet to be determined



    An application can ask to handle one or more types of media mode. When an incoming call is available, TAPI has the unfavorable job of working with the applications to determine the media mode of the incoming call. For the vast majority of calls, which are analog, the call type can't be determined until it has been answered. However, a call can't be answered until it has been given to an application, who will then answer the call. Therefore, all analog calls come in as media mode unknown.

    Once the call type has been determined, the answering application has a couple of options. It can handle the call itself or it can hand-off the call to TAPI, specifying the new media mode. TAPI will then attempt to give the call to an application who can handle that media type. When an application performs a hand-off, it can either give up the call altogether or maintain access to it for later use.

    For more information about line ownership and media modes, see Chapter 2, Assisted Telephony and Chapter 6, Answering a Call.



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