Multimedia devices generate two streams of data on two distinct virtual circuits. One is the actual data stream which was cursorily described above. The other is a control stream; this is a bi-directional low-bandwidth stream that is used to control the device and for purposes of synchronization.
Both data and control virtual circuits are established through the normal mechanism of ATM signalling, although in the case of many of the ATM devices, this signalling is handled by a management process on the attached workstation, rather than by the device itself.
Typically, the device manager will connect the data stream directly to the sink or source; however, the control stream would normally be connected to a local synchronization process. For example, a host that wishes to send synchronized audio and video, will do so by having the audio node and camera send the audio and video data streams separately (they have to end up in different devices too, at the other end), while a local process will merge the two control streams into a combined control stream for the playback control process at the rendering end. The playback control process is then responsible for the synchronization of the play-out of the various streams arriving at it, based on the source synchronization information from the remote manager(s) and data arrival events.
The Pegasus File Server, which can also be viewed as a multimedia device in this context, uses the control stream associated with an incoming data stream to generate index information that can later be used to go to specific time offsets into a media file or a set of synchronized files.