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The following concepts are important for understanding the material presented in this section:

• Initial media modes. On a network other than ISDN, service providers usually do not know the media mode of an arriving call. For such calls, the service provider indicates a number of initial media modes, from which the correct one is eventually selected during the next stepthe probing process. The initial media mode(s) identified by the service provider as being possible on the call will be reflected in the dwMediaMode field of the LINECALLINFO data structure, which the application can obtain by calling lineGetCallInfo after the initial LINE_CALLSTATE message announcing a new call is received.

• Call control. Having control of a call means that the application has received a LINE_CALLSTATE message stating that it has become an owner of the call. With this event, the application acquires a handle to the call with owner privileges. A way for a monitoring application to obtain call control (ownership) is to call lineSetCallPrivilege to set its call privilege to owner.

• Call monitor. An application that has a handle to a call with monitor privileges is a monitor of that call. Such an application cannot control the existence or other aspects of the call, but it can record (log) facts about the call. The application can also reset its call privilege to owner, thus becoming an owner of the call, in the event that the application determines (by monitoring media modes or other events) that it should take control.

• Call owner. An application that has control of a call is an owner of that call. An application can become an owner of a call in several ways, all discussed in this section. A call can have several owners simultaneously, although the usual situation is for only one application to be the owner of a call.

• Probing. Probing is the sending of signals on the phone line as an attempt to determine an incoming call's media mode. This search for the media mode is conducted only by applications. Some service providers may also be configured to do some amount of probing automatically to narrow down the initial media modes reported on a new call.

• The Unknown application. An application that has opened a line requesting ownership privilege for calls of as-yet undetermined, or UNKNOWN media mode, is referred to as the unknown application. (The LINEMEDIAMODE_UNKNOWN bit is set in the dwMediaMode field of the LINECALLINFO structure, along with other bits indicating other modes that are potentially present on the call.) An application that does this may actually be capable of handling calls of a number of different media modes. Alternatively, it may simply act as a traffic director, passing calls on to other applications that can use calls of the specific media modes.

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