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Unformatted text preview: te representing objects, or a
Petri net. For example, consider the multimedia scenario represented as an X-Y
graph in Figure 19.15. It has four multimedia objects 0h 02, 03 and 04. St denotes the start point of object
0i. The same multimedia scenario is represented in Figure 19.16 as a petri net.
In this approach, the temporal relationships among the multimedia objects of a
presentation scenario is represented in the form of a special programming
language that has temporal synchronization functions. A few examples of
synchronization functions that such a language may have are:
1. SyncAtomicSerial (A, B): Object B starts after object A stops.
2. SyncAtomicParallel (A, B): Objects A and B start simultaneously.
3. SynElemSerial (tl, A, t2, B): Object A starts after time tl, and object B starts
after time t2 after A stops.
SyncElemParallel (tl, A, t2, B): Object A starts after time tl, and object B
starts after time t2. Based on the values of tl and t2, objects A and B may overlap.
Note that temporal specification focuses only on the temporal relationships among
the objects without explicit reference to time. Temporal synchronization
mechanisms use the temporal relationships information of temporal specification
and convert it into a set of playback schedules and retrieval schedules to enforce
the playback deadlines for various multimedia objects of a presentation scenario.
A playback schedule is the set of playback deadlines of multimedia objects in a
multimedia scenario, where playback deadline of an object is the latest starting
time of the object, for a meaningful presentation. Retrieval schedule is the set of
request deadlines to the server. It accounts for end-to-end delays and ensures that
the playback deadline is not missed.
To display an object O at time Ts (the playback deadline), the presentation site
sends a request to the corresponding media server at request time T r, which is R
time units prior to Ts. R is the response time to account for possible experienced
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- Spring '14