P2. Recall the simple model for HTTP streaming shown in Figure 7.3. Recall that
ID: 3534562 • Letter: P
Question
P2. Recall the simple model for HTTP streaming shown in Figure 7.3. Recall that B denotes the size of the client’s application buffer, and Q denotes the number of bits that must be buffered before the client application begins playout. Also r denotes the video consumption rate. Assume that the server sends bits at a constant rate x whenever the client buffer is not full.
a. Suppose that x < r. As discussed in the text, in this case playout will alternate
between periods of continuous playout and periods of freezing.
Determine the length of each continuous playout and freezing period as a
function of Q, r, and x.
b. Now suppose that x > r. At what time does the client application
buffer become full?
Explanation / Answer
a)During a playout period, the buffer starts with Q bits and decreases at rate r - x.
Thus, after Q/(r - x) seconds after starting playback the buffer becomes empty. Thus,
the continuous playout period is Q/(r - x) seconds. Once the buffer becomes empty,
it fills at rate x for Q/x seconds, at which time it has Q bits and playback begins.
Therefore, the freezing period is Q/x seconds.
b) Time until buffer has Q bits is Q/x seconds. Time to add additional B - Q bits is (B
- Q)/(x - r) seconds. Thus the time until the application buffer becomes full is
seconds.
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