Media and Synchronization Granularities
 Granularity of synchronization within a set of media streams is the number of synchronization points that must be identified. The size of synchronization granule plays an important role in the performance of the whole presentation. The closer the synchronization points, the better quality is the overall presentation.

However, more synchronization points also introduce additional system overhead. Thus, an appropriate synchronization granularity must be defined to achieve simultaneously satisfactory multimedia presentations and tolerable system overhead.

As the media streams are represented as objects, synchronization points should at least be specified at both beginning and end of each object. However, the synchronization granularity can be defined more finely. Additional synchronization points can be defined in the midst of objects to permit finer synchronization control among media streams.

Preliminary experiments have been conducted to examine the effect of the number of synchronization points in the presentation of continuous media streams. In particular, we experimented with speech streams which is applicable to educational learning applications. We measured the average delay, the percentage of the stream that is not played (tolerance), and the maximum continuous portion of the stream that is dropped at any given time. The acceptable values of these QoS parameters can only be determined experimentally.

A sample experiment of speech data with various sizes of synchronization granules is illustrated below.

Granule(msec)
Tolerance(%)
Avg. Delay (sec)
Dropped Data Time (sec)
Avg. Drop Time (msec)
Tolerated Avg. Delay (sec)
1
8
5.278
83.978
19
3.6
5
8
3.378
24.727
86
3.6
10
8
3.553
15.021
85
3.6
20
8
3.571
17.906
100
3.6
40
8
3.626
20.604
107
3.6
80
8
3.643
18.309
110
3.6
160
8
3.626
11.830
111
3.6
250
8
3.901
19.111
113
3.6
 

 

The experimental results help us to conduct analytical analysis on synchronization granularites. For example, let the total delay that can be tolerated in the presentation of S be tol fraction of S, the average time spent at each synchronization point be d t, the network delay for transmitting S be N fraction of S, D be the fraction of S that can be dropped, and d be the maximum continuous data that can be dropped at any given time. We can then determine the lower and upper bounds on the value of S:

                                    S>=    and    S<= 
 

We can then determine the lower and upper bounds on the media granule size of each stream. The synchronization granularity of the presentation of the streams can then be determined.

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