Isochronous Distributed Multimedia Synchronization

A multimedia system is characterized by the integrated computer-controlled generation, manipulation, presentation, storage, and communication of independent discrete and continuous media data. The presentation of any data and the synchronization between various kinds of media data are the key issues for this integration (Georganas, Steinmetz, & Nakagawa, 1996). Clearly, multimedia systems have to precisely coordinate the relationships among all media that include temporal and spatial relationships. Temporal relationships are the presentation schedule of media, and spatial relationships are the location arrangements of media. Multimedia synchronization is a process of maintaining these relationships by employing appropriate synchronization mechanisms and algorithms. Multimedia synchronization is traditionally challenging, especially in distributed environments. Three types of multimedia synchronization can be distinguished: intrastream synchronization, interstream synchronization, and intermedia synchronization (Crowcroft, Handley, & Wakeman, 1999). The approaches used for interstream synchronization can also be used for intermedia synchronization. The word synchronization refers to time. The easiest way of synchronizing between streams at different sites is to use a single time reference. There are several ways to provide this time reference. • The network will have a clock serve as a single reference. This approach is used in H.261/ISDN- (integrated services digital network) based systems. A single clock time is propagated around a set of codecs and multipoint control units (MCSs). • The network deploys a clock-synchronization protocol, such as NTP (the network time protocol; Mills, 1993). The time stamps of media packets will be derived from the globally synchronized clocks. The isochronous synchronization approach as described in this article heavily relies on this time reference.