Homework Assignment #1
Mobile Computing – CSE40814/60814
Due Date: September 28th 2012
This assignment is to be completed individually!
Question 1: Why is TCP called a reliable transport protocol and UDP an unreliable
transport protocol? Explain the purpose of “sequence numbers” (why are they
needed to provide reliability?) and “ACK” messages. TCP is also called an “end-toend” protocol; what does this mean?
Question 2: The explanations in class have focused on communications between
two computers; this is often referred to as “unicast communications”. In contrast to
that, explain both “broadcast” and “multicast” communications. In LANs,
broadcasting is sometimes used to distribute packets, but this is not typically the
case in WANs, e.g., IP routers on the Internet do not support broadcasting. Instead,
all broadcast packets are simply “dropped”. Why do you think this is the case?
Question 3: Consider the network topology below, where circles indicate the
communication and interference range of each node A..F, i.e., each node can hear its
immediate neighbors to the left and right. Assume that RTS/CTS is not being used.
Answer the following questions.
A
B
C
D
E
F
1. Node B currently sends to node A and node C wants to send to node D. Is
node C allowed to do so (i.e., can it do so without causing a collision) and will
it decide to do so?
2. Node C sends to node B and node E wants to send to node D. Is E allowed to
do so and will it do so?
3. Node A sends to node B and node D sends to node C. Which other nodes are
allowed to send at the same time?
4. Node A sends to node B and node E sends to node F. Which other nodes are
allowed to send at the same time?
Question 4: CSMA, as discussed in class, belongs to the category of “contentionbased” MAC protocols, i.e., nodes must contend for access to the medium. This is in
contrast to protocols in the “contention-free” category, where these protocols
ensure that there will always be only one node accessing the medium, thereby
eliminating collisions. TDMA is an example of such a protocol; explain how TDMA
works and explain the meaning of “frames” and “slots”. Name at least one
disadvantage of TDMA compared to CSMA.
Question 5: In CSMA/CA, nodes use a random delay before accessing the medium.
Why is this being done? Also explain the purpose of “exponential backoff”.
Question 6: Read up on the IEEE 802.11n protocol and describe one of its features
that allow it to achieve higher bandwidth than previous 802.11 protocols.
Question 7: Explain what LOS (Line of Sight) stands for and how cell phones can
communicate with each other even if there is no LOS between them (or their cell
towers).
Submission Instructions:
 Submit your homework either as word or pdf file. The file name should be
Homework1-Lastname.[docx|pdf].
 Submit your homework by emailing it to nyadav@nd.edu before the
deadline.
 The deadline for submission is September 28th, 11.59pm.