EE450-Discussion8-Fall10

# EE450-Discussion8-Fa - Discussion#8 EE450 Sample Problems ARQ protocols(Sliding window Problem#1 Description Draw a timeline diagram for the

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Discussion #8 EE450, 10/13/2010 Sample Problems:  -ARQ protocols (Sliding  window)

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2 Problem#1: Description Draw a timeline diagram for the sliding window algorithm with  SWS=RWS=4 frames for two given scenarios using the following  assumptions: 1. The receiver sends a duplicate acknowledgement if it does not  receive the expected frame. For example it sends dupack[2]  when it expects to see frame[2] but receives frame[3] instead.  2. The receiver sends a cumulative acknowledgement after it  receives all the outstanding frames. For example it sends  ACK[5] when it receives  the lost frame[2] after it already  received frame[3], frame[4] and frame[5].  3. The timeout interval is about 2RTT.
3 Problem#1: Two Scenarios a. Frame[2] is lost. Retransmission  takes place upon  timeout (as usual). b. Frame[2] is lost. Retransmission  takes place either  upon receipt of the first dupack or upon timeout.  Does this scheme reduce the transaction time?  Note that some end-to-end protocols (e.g. variants of  TCP) use a similar scheme for fast retransmission.

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4 Problem#1 Solution: Scenario (a) Transmission  time Frame[2] Ack[1] time time Sender Receiver Frame[3] Frame[4] Frame[5] Frame[1] Dupack[2] Dupack[2] Dupack[2] Timeout=2RTT Frame[2] Ack[5] Frame[6] Ack[6] T trans  + RTT T trans  + RTT
5 Frame[2] Ack[1] time time Problem#1 Solution: Scenario (b) Sender Receiver Frame[3] Frame[4] Frame[5] Frame[1] Dupack[2] Dupack[2] Dupack[2] Frame[2] Ack[5] Frame[6] Ack[6] T trans  + RTT T trans  + RTT Transmission  time

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6 Comparing the two scenarios (a) Delay1= 2Ttrans + 3RTT (b) Delay2= 4T trans  + 2RTT Frame[2] Ack[1] Frame[3] Frame[4] Frame[5] Frame[1] Dupack[2] Dupack[2] Dupack[2] Timeout=2RTT Frame[2] Ack[5] Frame[6] Ack[6] T trans  + RTT Frame[2] Ack[1] Frame[3] Frame[4] Frame[5] Frame[1] Dupack[2] Dupack[2] Dupack[2] Frame[2] Ack[5] Frame[6] Ack[6] 2T trans 2T trans  + 2RTT Transmission  time Transmission  time T trans
7 Comparing the transaction time  (transfer delay) in the two scenarios RTT=2T prop Delay1= 2T trans  + 3RTT= 2T trans  + 6T prop Delay2= 4T trans  + 2RTT=4T trans  + 4T prop                       ? 2T trans  + 6T prop  > 4T trans  + 4T prop           ? If  Tprop  Ttrans   , in Scenario (b) the transaction time is reduced!

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8 Problem#2: Description A is connected to B via an intermediate router R. The A-R and R-B links each accept and transmit  only one packet per second in each direction (so  two packets take 2 seconds).
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## This note was uploaded on 06/25/2011 for the course EE 450 taught by Professor Zahid during the Fall '06 term at USC.

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EE450-Discussion8-Fa - Discussion#8 EE450 Sample Problems ARQ protocols(Sliding window Problem#1 Description Draw a timeline diagram for the

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