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Lecture7

# Lecture7 - ECE 340 Lecture 7 Variance conditional pmf Jean...

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ECE 340 Lecture 7: Variance & conditional pmf Jean Liu Email: [email protected]

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Example to review Problem 3.18 A computer reserves a path in a network. To extend the reservation the computer must successfully refresh message before the expiry time. The messages are lost with probability ½. Suppose that it take 10 sec. to send a refresh message and receive acknowledgement. When should the computer start sending a refresh messages in order to have a 99% chance of successfully extending the reservation time.
10 min. 0 ? Refresh request

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Let X be the times that a refresh request needs to be sent till successfully received. • S X is {1,2,3,…} Let Y be the seconds that a refresh request needs to be sent till successfully received. • S Y is {10,20,30,…} Success rate of a message is 1/2 What is the set B, which begins with 1 and increases accordingly, has 99% chance of successfully extending reservation?
pmf of X? 1 1 1 1 ( ) ( ) ( ) ( ) ( ) 2 2 2 k k X k p x p x K = = = = 1 1 [ ] ( ) ( ) 2 N k k k B k P B p x = = = Set B probability ?

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