ps4_ee550_fall08

# ps4_ee550_fall08 - UNIVERSITY OF SOUTHERN CALIFORNIA FALL...

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UNIVERSITY OF SOUTHERN CALIFORNIA, FALL 2008 1 EE 550: Problem Set # 4 Due: Monday Oct. 6, 2008 I. STOP AND WAIT EFFICIENCY Consider a Stop and Wait System with sender node A and receiver node B . The Round Trip Time is a ﬁxed constant RTT . The receiver transmits a feedback (ACK or NACK) message whenever it receives a packet from the sender (it actually sends its current RN value, having incremented this value by 1 to indicate ACK). There are never any losses. Errors can occur on both the forward and reverse channels, as follows: Forward Channel ( A B ): Errors i.i.d. with probability p Reverse Channel ( B A ): Errors i.i.d. with probability q The timeout value is set to be larger than RTT , so that the system never times out. a) Suppose that RN = i , and the sender sends an error-free packet with SN = i . Thus, the receiver responds with an ACK ( RN = i + 1 ). However, there is an error in the ACK when the sender gets it, so the sender does not know the receiver correctly received its packet. Compute the expected remaining time required for the sender to ﬁnd out that the receiver correctly received its packet (starting at the time the sender gets the ”ACK” with an error). b) Compute the exact throughput of this Stop and Wait Algorithm (assuming the sender always has data to send). II. STOP AND WAIT WITH ENERGY CONSIDERATION Suppose communication takes place between nodes A and B (where A is the sender and B is the receiver). Time is slotted

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ps4_ee550_fall08 - UNIVERSITY OF SOUTHERN CALIFORNIA FALL...

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