Midterm Exam
Computer Networks
CSC522
Spring 2007
Answer any five questions from the following
Policy: Take-home exam. Send your answer by email to kababs@gmail.com
Due date: March 18, 2007 (noon time).
1. (a) You want to measure the approximate available bandwidth between your host and a distant
station during a time when the packet-switched network is lightly loaded. Indicate how you would
approach the task. (b) Suppose you have a choice of sending messages either by packet-switching
or by circuit-switching. How would you decide?
2. Suppose that
x
bits of user data are to be transmitted over a
k
hop path in a packet switched
network as a train of packets each containing
p
data bits and
h
header bits with
hpx
+
>>
.
The bit rate of the lines traversed by the packets is
b
bps and the propagation delay is negligible.
What value of
p
minimizes the total delay?
3. Ethernet and wireless networks have some similarities and some differences. One property of
Ethernet is that only one frame at a time can be transmitted on an Ethernet. Does 802.11 share this
property with Ethernet? Discuss your answer.
4. Consider a network system with a link bandwidth
5.1
=
B
Mbps. The buffer capacity is infinite
and the arrival process is Poisson with mean average rate of
=
λ
1000 packets/sec. Assume that
packet lengths are exponential with a mean of
L
= 1000 b/packet.
a. Compute mean number of packets in the system.
b. Compute mean delay for a packet
c. Assume now that mean arrival rate triples and the bandwidth is increased to 4.5 Mbps.
What would be the mean number of packets and mean delay in the new case? How would
justify your answers both computationally and intuitively?
5. Compare and contrast the advantages and disadvantages of standard ALOHA vis-à-vis slotted
ALOHA. How would they compare with something like Reservation ALOHA?
6. As we know, CSMA works as follows: When a host wants to send a frame he checks if the
channel is busy. If not, the host sends its frame (possibly at the end of a mini-slot). If it is, it waits
a random time before sending its frame. In either case if there is a collision, the sender waits a
random back-off time (from the end of the epoch) and try to re-send.
In one modification of this protocol, called "persistent CSMA", the sender that finds the channel busy,
waits until it is not busy anymore, and then sends it frame.
(a) Under what condition will this protocol be better than then regular CSMA? Give example
and try to analyze the pros and cons of each protocol.
In another CSMA modification a compromise between these two protocols may be attempted. If the
host finds he channel busy it waits till the end of that epoch, and then with probability P it immediately
