Desktop VT systems become really useful if, in addition to head-to-head talking, they support application sharing and whiteboard collaboration. The ITU also has provided a family of standards to cover these activities:

Not covered in the T.120 series is Application Sharing, the mechanism that, for example, permits collaborative views of a CAD system to be shared by geographically separated engineers. Currently, this activity can be conducted only among VT systems from the same manufacturer.

Lately, a synergistic relationship has developed among VT system manufacturers and Microsoft, the developer of NetMeeting. Most manufacturers have replaced their T.120 susbsystems with hooks into NetMeeting, which they distribute with their VT product packages.

NetMeeting also can run independently in a Windows PC, using the PC's built-in sound board plus an optional (usually low-cost) camera, such as a Logitech QuickCam. In this standalone configuration, NetMeeting does not totally follow the ITU standards. However, it complies with enough of them to be able to participate on a LAN as an unsophisticated H.323 Visual Terminal.

In this spirit, NetMeeting only can support collaboration with other NetMeeting terminals. While it contains internal T.120 "plumbing" developed for Microsoft by DataBeam Corp., NetMeeting only simulates the T.126 Workspace Model. You have to overinstall DataBeam's Meeting Tools package to extend NetMeeting's capabilities and make it fully T.120 compliant.

Calling, Answering and Managing a Conference
A previous section described the codec algorithms available under the H.323 architecture, which, you'll remember, is a superset that evolved from H.320. In this section, we'll explain how calls are set up among conferencing systems and how audio, video and data are transmitted. We'll start by looking at call setup under H.320. In our first example, we have a single BRI line and IMUX (inverse multiplexing) equipment that can combine the BRI's two channels into a 128-Kbps clear transmission path in each direction. In an H.320 system, dialing and call setup is handled in the IMUX or other circuitry separate from the codec.