CS 356 (Lam), Fall 2014

Homework 3 (due at beginning of class on October 15, 2014, Wed)


Consider the maximum channel utilization of a stop-and-wait 
or sliding window protocol as analyzed in the lecture slides.  

For both problems 1 and 2 below, assume

       probability of an unsuccessful frame transmission, P = 0.01

       frame size = 10,000 bits

       channel propagation time (one-way) = 10 milliseconds


PROBLEM 1  (16 points)

For a stop-and-wait protocol, suppose

       receiver processing time (T_p) = 1 millisecond

       ack size = 500 bits

Choose the timeout duration to be 1.5 times the minimum possible value
(namely, T_a + T_p + 20 msec).

(a)  For a full-duplex channel of 64 kilobits/sec in each direction, calculate 
     the maximum utilization of the stop-and-wait protocol.  

(b)  For a full-duplex channel of 1 megabits/sec in each direction, calculate 
     the maximum utilization of the stop-and-wait protocol.


PROBLEM 2  (20 points)

For a sliding window protocol with a send window size (W) of 7,
consider the ack transmission time (T_a) and receiver processing time (T_p)
to be negligible.
   
(a)  For selective-repeat ARQ, what is the maximum utilization of the sliding
     window protocol if

     (i)  channel transmission rate is 1 megabits/sec ?

     (ii) channel transmission rate is 10 megabits/sec ?


(b)  For go-back-N ARQ, what is the maximum utilization of the sliding
     window protocol if

     (i)  channel transmission rate is 1 megabits/sec ?

     (ii) channel transmission rate is 10 megabits/sec ?


PROBLEM 3  (16 points)

Consider a TCP session whose current values of EstimatedRTT
and DevRTT are 50 ms and 10 ms, respectively.
What is the current TimeoutInterval value?

Suppose the next measured value of SampleRTT is 70 ms.
Let alpha = 1/8 and beta = 1/4 be the exponential smoothing weights.
Calculate the next values of EstimatedRTT, DevRTT, and
TimeoutInterval (in that order).