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4, a larger threshold in P2Cast usually leads to the admission of more clients. However, a smaller threshold would help to provide better quality of the played out video - it is more likely that clients get continuous playback without a glitch. A smaller threshold makes the patch size smaller, thus the patch disruption is less likely. Furthermore, a small threshold reduces the number of clients in a session. Hence the probability that the clients' base stream gets disrupted decreases. For instance, Fig. 17 depicts the probability that the base stream encounters at least one disruption during the playback if the base tree is a balanced binary tree. We assume that a node can leave with probability , and a departure will affect all its descendant nodes. We note that the curve is concave and the probability decreases as the number of nodes decreases. In the extreme, when the threshold is zero, the P2Cast reverts to the unicast service model, where clients' early departures do not affect each other and the continuous playback is guaranteed once a client is admitted. Therefore the threshold in P2Cast gives the service provider a knob to adjust the balance between the scalability and the clients' viewing quality.
Next: Related Work Up: Failure Recovery - Providing Previous: Disruption effect on continuous Yang Guo 2003-03-27