## 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).