EE511-Homework 1
Note: In all written assignments, please show as much of your work as you can. Even if
you get a wrong answer, you can get partial credit if you show your work. If you make a
mistake, it will also help the grader show you where you made a mistake.
Problem 1
Consider an application that transmits data at a steady rate (for example, the sender
generates an N-bit unit of data every k time units, where k is small and fixed). Also,
when such an application starts, it will continue running for a relatively long period of
time. Answer the following questions, briefly justifying your answer:
a. Would a packet–switched network or a circuit-switched network be more
appropriate for this application? Why?
b. Suppose that a packet-switched network is used and the only traffic in this
network comes from such applications as described above. Furthermore, assume
that the sum of the application data rates is less than the capacities of each and
every link. Is some form of congestion control needed? Why?
Problem 2
This elementary problem begins to explore propagation delay and transmission delay,
two central concepts in data networking. Consider two hosts, A and B, connected by a
single link of rate R bps. Suppose that the two hosts are separated by m meters, and
suppose the propagation speed along the link is s meters/sec. Host A is to send a packet
of size L bits to Host B.
a. Express the propagation delay, dprop, in terms of m and s.
b. Determine the transmission time of the packet, dtrans, in terms of L and R.
c. Ignoring processing and queuing delays, obtain an expression for the end-to- end
delay.
d. Suppose Host A begins to transmit the packet at time t=0. At time t=dtrans,
where is the last bit of the packet?
e. Suppose dprop is greater than dtrans. At time t=dtrans, where is the first bit of the
packet?
f. Suppose dprop is less than dtrans. At time t=dtrans, where is the first bit of the
packet?
g. Suppose s=2*109, L=1024 bits, and R=256 kbps. Find the distance m so that
dprop equals dtrans.
Problem 3
Perform a Traceroute between your computer and www.ucsd.edu at three different
hours of the day. (Add screenshots of the three Traceroutes to your answer!)
a. Find the average and standard deviation of the round-trip delays at each of the
three hours.
b. Find the number of routers in the path at each of the three hours. Did the
paths change during any of the hours?
c. Try to identify the number of ISP networks the Traceroute packets pass
through from source to destination. Routers with similar names and/or
similar IP addresses should be considered as part of the same ISP. In your
experiments, do the largest delays occur at peering interfaces between
adjacent ISPs?
d. Repeat the above for a destination on a continent different then the source.
Compare the intra- and inter-continent results.
Problem 4
Suppose you would like to urgently deliver 400 terabytes of data from Boston to Los
Angeles. You have a 10 Gbps dedicated link for data transfer available. Would you
prefer to transmit the data via this link or instead use FedEx overnight delivery?
Explain.
Problem 5
Suppose there is a 10 Mbps microwave link between a geostationary satellite and its
base station on Earth. Every minute the satellite takes a digital photo and send it to the
base station. Assume a propagation speed of 2.4 * 108 meters/sec.
a. What is the propagation delay of the link?
b. What is the bandwidth-delay product, R * dprop?
c. Let x denote the size of the photo. What is the minimum value of x for the
microwave link to be continuously transmitting?
