UCLA Electrical Engineering Department
EE132B
HW Set #4
Professor Izhak Rubin
Problem 1
Consider an ARQ stopandwait communication session between to stations A and B.
1) If station A intends to transmit a single data frame, what is the average number of retransmissions
required for the successful transport of data frames? (Hint: Let p denote the probability that the
transmit frame is received with error. Express the average number of retransmissions in terms of p.)
2) If station A intends to transmit M dataframes (M
≥
2), what is the probability that the number of
retransmissions N
R
(M) is equal to 2? (Hint: Express the probability in terms of M and p.)
Problem 2
Consider an ARQ stopandwait scheme which is similar to the scheme described in Problem 1 but a
distinct feature: suppose that an acknowledgement can be lost during its transmissions with probability q.
If an
acknowledgement is lost, station A will retransmit the dataframe after timeout. Such
retransmission process continues until station A receives an acknowledgement.
1) If station A intends to transmit a single data frame, what is the average number of retransmissions
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 Fall '09
 IzhakRubin
 Electrical Engineering, Probability theory, Telephone exchange, Local loop, Professor Izhak Rubin

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