ps3sol_ee550_fall08

ps3sol_ee550_fall08 - UNIVERSITY OF SOUTHERN CALIFORNIA,...

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Unformatted text preview: UNIVERSITY OF SOUTHERN CALIFORNIA, FALL 2008 1 EE 550: Problem Set # 3 Solution I. BOOK PROBLEM 2 . 16 (a) Consider the scenario: packet 1 is transmitted for the first time ( j = 0 ), and is successfully received. An ACK is sent to the sender and is correctly received. The sender then sends packet 2 for the first time ( j = 0 ). Suppose the packet 2 is lost and never reaches to the receiver. Then the sender times out and retransmit packet 2 ( j = 1 ). Consider another scenario: packet 1 is transmitted for the first time ( j = 0 ) and is successfully received. An ACK is sent to the sender, but the ACK is lost along the way. The sender times out and retransmits packet 1 ( j = 1 ). Compare the above two scenarios. We observe that when the receiver receives the first packet retransmission, it cannot tell whether the packet is 1 or 2 . (b) Fig. 1. Solution of 2 . 16( b ) UNIVERSITY OF SOUTHERN CALIFORNIA, FALL 2008 2 II. BOOK PROBLEM 2 . 18 Fig. 2. Solution of 2 . 18 UNIVERSITY OF SOUTHERN CALIFORNIA, FALL 2008 3 III. BOOK PROBLEM 2 . 19 Fig. 3. Solution of 2 . 19 IV. BOOK PROBLEM 2 . 20 Fig. 4. Solution of 2 . 20 UNIVERSITY OF SOUTHERN CALIFORNIA, FALL 2008 4 V. BOOK PROBLEM 2 . 21 Let RN ( τ ) be the value of RN at node B at an arbitrary time τ ; SN min is the smallest packet number not yet acknowledged at A at time t (which is regarded as fixed) and SN max- 1 is the largest packet number sent from A at time t . Since RN ( τ ) is nondecreasing in τ , it is sufficient to show that RN ( t + T m + T d ) ≤ SN max and to show that RN ( t- T m- T d ) ≥ SN min . For the first inequality, note that the packet numbered SN max (by definition of SN max ) has not entered the DLC unit at node A by time t , and thus cannot have started transmission by time t . Since there is a delay of at least T m + T d from the time a packet transmission starts until the completion of its reception, packet...
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This note was uploaded on 12/21/2010 for the course EE 550 taught by Professor Neely during the Fall '08 term at USC.

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ps3sol_ee550_fall08 - UNIVERSITY OF SOUTHERN CALIFORNIA,...

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