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1/11
CS433/533: Computer Networks
Midterm 2
04/22/2003
7:00-9:00pm

This exam is closed book and closed notes. However, you may refer to a sheet of
8.5"x11" paper (double-sided) of your own design.

Keep your answer concise.

Keep an open mind. Some questions may require you to apply techniques you have
learned from one context to another one.

Show your reasoning clearly. If your reasoning is correct, but your final answer is
incorrect, you will receive most of the credit. If you just show the answer without
reasoning, and your answer is incorrect, you may receive no points at all.
Name:
Today’s Date:
2/11
short
questions
(41 points)
routing
MAC
(24 points)
(23 points)
layered
multicast
(12 points)
Total
3/11
1. Provide short answers to the following questions.
a) [4 points] BGP routing uses path-vector routing. Why does BGP include the whole
path in its routing advertisement?
b) [4 points] Where (input ports and/or output ports) can queueing occur in a router?
Briefly explain the conditions that lead to such queueing.
c) [3 points] Both IPv6 and IP multicast use “tunneling.” What is meant by the term
“tunneling”?
This is not covered
4/11
d. Hamming code
 [3 points] Ethernet packets can be up to 1500 bytes. How many check bits do you
need in order to correct any single bit error if you use Hamming code?

[3 points] Suppose you have an 8-bit data string 1010,1101 to transmit. Please
determine the whole string to be transmitted if you use Hamming Code. (please
number each bit and label whether it is a message bit or check bit.)
e. CRC code. Consider the generator polynomial G(x) = x8 + x2 + x + 1.
 [3 points] Assume that the data frame is 1011,0101,1010. What will be the
transmitted bit stream?

[3 points] The designer of G(x) above claims that G(x) can detect any odd number
of bit errors. Is this true or false? Please justify.

[3 points] The designer of G(x) above also claims that G(x) can detect any k
number of bit errors, if k is even and ≤ 8. Is this true or false? Please justify.
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f. [4 points] Why does a CSMA/CD based protocol such as Ethernet require a minimum
packet size (e.g., Ethernet minimum packet size is 64 bytes)?
g. [4 points] Using your GSM cellular phone, sometimes you hear the dialup tone quickly
while other times you have to wait for a longer time. What is a possible source of this
variable delay?
h. [4 points] Some vendors of 802.11 (Wireless LAN) claim that 802.11 can support
multimedia better than Ethernet does. Is this claim justified?
i. [3 points] Why does Ethernet use exponential backoff?
6/11
2. [24 points] Routing
Consider the network topology shown below. The topology consists of multiple routers
interconnected by full-duplex links. Each link has a static cost which represents the cost
of sending data over that link.
a. [3 points] The "cost" of a link can be delay, bandwidth, $cost, or congestion level. One
criticism of the Internet routing scheme is that link costs generally are static. Describe
one issue that would arise if dynamic costs based upon actual link loads were instituted.
b. [5 points] Suppose that we use the distributed Bellman-Ford (distance-vector)
algorithm. For simplicity we only consider the routes to destination F in this sub-problem.
Please calculate the distance and next hop from each node to F using the distance vector
algorithm. To get partial credit, you may want to show your steps.
7/11
c. [4 points] Assume that the distance vector protocol runs in rounds. In each round, a
router sends its distance table to each of its neighbors. Is there an upper bound on the
number of rounds it takes for the distance-vector algorithm to converge (i.e. until the
routing tables stop changing), after a link changes status? Assume that your network is
always connected.
d. [4 points] Similar to (c). What if your network becomes partitioned?
e. [4 points] Please draw a source tree for source F if we run a source-based multicast
routing protocol.
f. [4 points] Please draw a shared tree if we run a shared tree protocol and C is the core
(center).
8/11
3. [23 points] Media Access Control
Suppose there are N stations on a LAN that has a capacity (transmission rate) of C. We
studied a number of multiple access protocols to share the LAN: (1) TDMA where the
channel is equally partitioned among the N stations, (2) Slotted Aloha with access
probability 1/N, (3) Ethernet, and (4) Token Passing in a ring where a station will always
release its token after each transmission is finished (i.e., the last bit of its transmission has
returned to it).
Assume all packets have a fixed length L and the one-way end-to-end propagation delay
of the channel is P. Please answer the following questions:
a) [6 points] Suppose only one station ever has packets to send (i.e., the other N-1
stations generate no traffic). What is the maximum possible throughput achievable by
this single station under each of the protocols? Which one performs the best?
TDMA:
Slotted Aloha:
Ethernet:
Token Passing:
b) [6 points] Suppose now that each station always has traffic to send. For each of the
protocols, is it possible to fully utilize the channel? If not, indicate how/why the
protocol limits the maximum throughput.
TDMA:
Slotted Aloha:
Ethernet:
Token Passing:
9/11
c) [6 points] Similar to (b). What is the worst-case amount of time a station has to wait
(between each send) under each of the protocols?
TDMA:
Slotted Aloha:
Ethernet:
Token Passing:
d) [5 points] If you want to implement multimedia applications with guaranteed delay,
which protocol(s) do you want to use? Please justify.
10/11
4. [12 points] Layered Multicast, Multimedia, and Application Adaptation.
[This is not covered]
One issue facing multicast is heterogeneous receiver capacity: some receivers have high
bandwidth (i.e., TCP-friendly share) while others have low bandwidth. As a result, sending to all
receivers at a single rate may not be desirable. One approach to addressing this issue is to use
layered multicast (don’t get confused with the layered network architecture). In layered multicast,
the sender sends related contents concurrently to multiple layers, where each layer is just a
different IP multicast group. A receiver subscribes incrementally from layer 0 to layer i, where i
is the largest number such that the aggregated bandwidth of layers 0 to i does not exceed the
receiver’s available bandwidth.
a. [3 points] Consider a source tree of node S below. The receivers are the leaf nodes. The
number below each receiver node is its available bandwidth. Assume that the sender sends to 3
layers: base layer 0 at 1Mbps, layer 1 at 2 Mbps, and layer 2 at 4 Mbps. Please label the layers
each receiver joins and draw the multicast source trees of the three groups for the three layers.
S
.
3 Mbps
Layer 0
7 Mbps
4 Mbps
2 Mbps
Layer 1
1 Mbps
Layer 2
b. [3 points] Please describe how a receiver can estimate its fair share of bandwidth.
11/11
In the next two questions, we study how to determine the content of each layer.
c. [3 points] For non-multimedia multicast such as software distribution, the sender can
send a file (e.g., a CD image) concurrently to different layers (i.e. to different groups) so
that a receiver with higher bandwidth can receive the complete file faster. Please describe
a simple encoding scheme (i.e., how the contents of different layers are generated) for
layered file transfers. Hint: a receiver should not receive (too much) duplicate content
from different layers.
d. [3 points] For multimedia content such as Internet TV, the sender can send the media
to different layers so that a higher layer is a refinement of the layers below it. Please
describe an encoding scheme for layered multicast of video so that receivers with higher
bandwidth can receive a higher quality of video.