Exam #2 for Fall 2003 - The exam and it solution are here (PDF).
Exam #1 for Fall 2003 - The exam and its solution are here (PDF).
Old exam #2 for Fall 2000 - The exam and its solution is 0x2_1sol.pdf.
Old exam #1 for Spring 2000 - The exam and its solution is 001_1sol.pdf.
Old exam #2 for Spring 2000 - The exam and its solution is 002_1sol.pdf.
Old exam #1 from Spring 1999 - The exam and its solution is 991_1sol.pdf.
Old exam #2 from Spring 1999 - The exam and its solution is 992_1sol.pdf.
EXAM #1 FOR INTRODUCTION TO COMPUTER NETWORKS Fall 1996 NAME: ______________________________________ SSN: _______________________________________ CODE NAME: _________________________________ (optional) Welcome to Exam #1 for Introduction to Networks. Read each problem carefully. There are eight required problems (each worth 12.5 points) and one extra-credit problem worth 10 points. You may have one 8.5 x 11 inch sheet of paper with you. On this sheet you may have anything you want (definitions, formulas, etc.) in handwriting or as photocopied text. Good luck!!! PROBLEM #1 Sketch the OSI protocol model and label each layer. For each of the three lower layers, provide a short one-sentence description of the function that the layers provides to its upper layer. Sketch the format of a packet sent by an OSI end node assuming that only layer 2 adds a trailer (typically, what is this trailer?) and that the physical layer provides unique start and end delimiters. PROBLEM #2 What is quality-of-service? Describe the salient differences in QoS requirements for file transfer, voice conversation, and video playback. PROBLEM #3 Show that interarrival times in a Poisson process follow an exponential distribution. PROBLEM #4 Is Little's Law useful for unstable queueing systems? Why or why not? Give the stability criterion for a queueing system. PROBLEM #5 Assume an asynchronous link that uses 8 bit words delimited by a start bit and two stop bits. The 8th bit of each word is a parity bit. Assume that this link has a probability of bit error of 0.005 (bit errors are independent and identically distributed). Answer the following: a) What is the probability that a received word is error-free? b) What is the highest possible probability of bit error if the probability that a received word is error-free must be no less than 0.80? c) What is the probability that a received word has no undetectable errors? d) What is the probability that a received word is error-free or contains undetectable error(s) or contains detectable error(s)? PROBLEM #6 Given a message, M, of 110 and a generator polynomial 101, compute the CRC and give the transmitted frame, T. Now, change any single bit in T and show that CRC checking at a receiver would detect this error. PROBLEM #7 Assume a 1-Mbps link of 1000 kilometers in length with 5 nanoseconds per meter propagation delay. Assume also constant 400-byte data frames, negligible length ACK frames, and negligible processing delay at both the sender and receiver. The sender always has data to send. Solve for link utilization (U) between a sender and a receiver assuming a stop-and-wait protocol and a Pr[bit error] = 0.0001 (bit errors are independent). PROBLEM #8 For the link described in problem #7, what window size is needed to achieve U = 100% for sliding window flow control given that Pr[bit error] = 0. EXTRA-CREDIT PROBLEM Set-up the birth-death chain for an M/M/infinity queueing system. For arrival rate less than service rate, what is a) mean number of customers in the queue, b) mean number of customers in the system, c) average customer wait time in system, and d) average customer wait time in the queue.
EXAM #2 FOR COMPUTER NETWORKS Fall 1996 NAME: ______________________________________ SSN: _______________________________________ CODE NAME: _________________________________ (optional) Welcome to Exam #2 in Introduction to Networks. Read each problem carefully. There are eight required problems (each worth 12.5 points). Please use a separate sheet of paper to start each new problem solution. Please put your name on all sheets of paper. You may have one 8.5 x 11 inch sheet of paper with you. On this sheet you may have anything you want (definitions, formulas, etc.) in handwriting or as photocopied text. PROBLEM #1 Why is TDM a good method for transporting voice? Why is TDM not a good method for transporting data? PROBLEM #2 In time-multiplexed switching time-division and space-division switch modules are combined to form one large switch. What are the motivations for doing this (i.e., implementing time-multiplexed switches)? PROBLEM #3 Compare and contrast virtual circuits as used in packet switching and "real circuits" as used in circuit switching. How are they same and how are they the different? PROBLEM #4 For a 1000-Mbps CSMA/CD Ethernet with 50 meters span, what is the minimum frame size? Assume that propagation delay is 5 nanoseconds per meter. PROBLEM #5 Consider a 200-meter 10-Mbps Ethernet. Given a collision rate of 1000 worstcase collisions per second, what is the effective remaining bandwidth of the Ethernet? Assume that propagation delay is 5 nanoseconds per meter. PROBLEM #6 Describe the mechanism used in IEEE 802.5 Token Ring for a station to attach to the "ring". PROBLEM #7 For a "typical" IEEE 802.5 16-Mbps Token Ring used as a workgroup LAN, is early token release useful? Carefully explain your answer. PROBLEM #8 Which is better, 100-Mbps IEEE 802.3 Fast Ethernet or 100-Mbps FDDI?
EXAM #3 FOR COMPUTER NETWORKS Fall 1996
NAME: ______________________________________
SSN: __________________ CODE NAME: _______________ (optional)
Welcome to exam #3 in Introduction to Computer Networks. Read each
problem carefully. There are ten problems, you are to choose eight
problems to solve for 12.5 points each. You may do two additional
problems as extra credit for 5 points each. Circle the problem numbers
of the eight problems you choose as "required problems" and underline
the problem numbers of the two problems you choose as "extra credit
problems". You may have one 8.5 x 11 inch sheet of paper with you. On
this sheet you may have anything you want (definitions, formulas, etc.)
in handwriting or as photocopied text. If you do not understand the
above instructions, ask a question. Good luck!!!
PROBLEM #1
Give the algorithm used by an IEEE802.1d transparent bridge for learning
and forwarding of frames. The learning is used to build the bridging
tables, hence the commonly used name of "learning bridge".
PROBLEM #2
If a bridge (or router) cannot forward frames "fast enough" it is
possible that it will lose frames in high-load situations. What is the
forwarding rate that is needed for a 100-Mbps Ethernet bridge to not
lose any frames? Compute this forwarding rate for a worst-case
condition. I am not looking so much for the precise numerical answer as
for the METHOD that you use to calculate the answer. Hint: The minimum
gap between frames in Ethernet is 96 bits.
PROBLEM #3
Consider the following shortest path algorithm as used for finding a
shortest path route from S (source node) to D (destination node).
Define N as the set of nodes in the connected graph, define M as the set
of nodes in the selected path.
1) Start at a source node S, initialize set M to empty
2) Find a neighbor node not in set M with lowest link cost, add
this node to set M
3) Repeat step (2) until D is in set M
Will this algorithm correctly find least-cost routes? If this is not a
good algorithm, give an example where it fails.
PROBLEM #4
What is the purpose of the ARP protocol? Explain how the ARP protocol
works.
PROBLEM #5
Why does the TCP header have a checksum for the entire segment? Hint:
the answer "to check for errors in the segment" is "what" and not "why".
PROBLEM #6
Describe TCP slow-start. Think of an example where slow-start may not
be a good idea (e.g., an example where slow-start results in poor
performance).
PROBLEM #7
Sketch the sockets programming client/server model.
PROBLEM #8
What are the key differences between IP and ATM.
PROBLEM #9
Sketch the B-ISDN protocol model and sketch the format of an ATM cell
(identify all the fields of the cell header).
PROBLEM #10
Thesis: A 500-Gbps (gigabit per second, giga = 1e9) ATM switch can be
implemented using mid-1990's technology. Attack or defend this thesis.
Opinions, beliefs, and marketing hype will give you no points.
EXAM #1 FOR INTRODUCTION TO COMPUTER NETWORKS Fall 1995
Welcome to exam #1 in Introduction to Computer Networks. Read
each problem carefully. There are eight required problems (each
worth 12.5 points) and two extra-credit problems (each worth 5
points). Please write your answers on the exam handout in the
indicated areas, show your work on separate sheets of paper.
Please put your name on all sheets of paper. You may have one
8.5 x 11 inch sheet of paper with you. On this sheet you may
have anything you want (definitions, formulas, etc.) in
handwriting or as photocopied text. Good luck!!!
PROBLEM #1
Fill-in the blanks (indicated with a blank line) in the following
figure of the OSI reference model.
OSI Model
+----------------+
7 | ______________ | <---- Fill-in the names of the layers
+----------------+
6 | ______________ |
+----------------+ +--- Write a short description of the
5 | ______________ | | layer function
+----------------+ V
4 | ______________ | - _______________________________________
+----------------+
3 | ______________ | - _______________________________________
+----------------+
2 | ______________ | - _______________________________________
+----------------+
1 | ______________ | - _______________________________________
+----------------+
PROBLEM #2
Assume an asynchronous communications link with 5-bit words, 1
additional parity bit, 1 bit for start, and 2 bits for stop. The
data bits are numbered, i = 1, 2, ..., 5 with bit 1 being the
first bit following the start bit. Assume that bit errors are
independent and that the start and stop bits are never in error.
a) Compute the probability of a received word having no errors
if Pr[bit i in error] = 1 x 10-3.
Pr[received word has no errors] = __________
b) Compute the probability of a received word having no errors
if Pr[bit i in error] = i x (1 x 10-3).
Pr[received word has no errors] = __________
c) Compute the probability of a received word having detectable
error(s) if Pr[bit i in error] = 1.5 x 10-2.
Pr[received word has detectable error(s)] = __________
d) Compute the efficiency (utilization) of the above
asynchronous communications link assuming no bit errors
and that the sender always has data to send.
U = ______________
PROBLEM #3
In this problem you are to compute a Cyclic Redundancy Check
(CRC) code. The generator polynomial you are to use is, P = x2 +
1 (i.e., P = 101). For the following message, M, to be sent,
compute the CRC and give as an answer the bit sequence of the
transmitted frame, T.
M = 1011
T = ____________
PROBLEM #4
Assume a 100-Mbps (i.e., 100 x 106 bits per second) link of
10000-meters in length with 5 nanoseconds per meter (i.e., 5 x
10-9 seconds per meter) propagation delay. Assume also constant
400-byte data frames, negligible length ACK frames, and
negligible processing delay at both the sender and receiver. The
sender always has data to send. Is stop-and-wait flow control a
good choice for a flow control scheme? To show why, or why not,
solve for link utilization (U) between a sender and a receiver.
U = ___________
Explain if stop-and-wait flow control is a good choice for this
configuration.
_________________________________________________________________
PROBLEM #5
For the link described in problem #4, assume a bit error
probability of Pr[bit error]= 1 x 10-4 where bit errors are
independent. Compute the utilization, U. You are again to
assume a stop-and-wait protocol.
U = __________
PROBLEM #6
For the link described in problem #4, assume a bit error
probability of Pr[bit error] = 1 x 10-4 where bit errors are
independent. Compute the utilization, U. Assume a sliding
window protocol with Go-Back-N error control and a window size
(N) of 3.
U = _______________
PROBLEM #7
Assume a Time-Division Multiplexed (TDM) link with 10 one-byte
slots in a frame, each slot carrying 100-kbps, and 3 overhead
bits per frame. Solve for the bit-rate of the TDM link and the
time delay between the first bit of same numbered slot in
successive frames.
Link bit-rate = _________________
Delay between same numbered slot in two frames = _________
PROBLEM #8
Assume a Poisson process with rate of arrivals, lambda, of 6
packets per second. What is the Probability Distribution
Function (PDF) and probability density function (pdf) for the
interarrival time between packets?
F(t) =__________________________
f(t) = __________________________
EXTRA CREDIT PROBLEM #1
As discussed in lecture, what is the difference between flow
control and congestion control? Write your answer here...
Flow control is ____________________________________
_____________________________________.
Congestion control is _______________________________
________________________________.
EXTRA CREDIT PROBLEM #2
Prove the memoryless property for the exponential distribution.
That is, show that Pr[x < T + t | x > T] = Pr[x < t]. Write your
answer here...
Pr[x < T + t | x > T] = Pr[x < t]
= _________________________________________
= _________________________________________
= _________________________________________
= _________________________________________
= _________________________________________
= _________________________________________
EXAM #2 FOR INTRODUCTION TO COMPUTER NETWORKS Fall 1995
Welcome to exam #2 in Introduction to Computer Networks. Read
each problem carefully. There are ten problems, you are to
choose eight problems to solve for 12.5 points each. You may do
two additional problems as extra credit for 5 points each.
Circle the problem numbers of the eight problems you choose as
"required problems" and underline the problem numbers of the two
problems you choose as "extra credit problems". Write your
answers on the exam handout in the indicated areas, show your
work on separate sheets of paper. Please put your name on all
sheets of paper. You may have one 8.5 x 11 inch sheet of paper
with you. On this sheet you may have anything you want
(definitions, formulas, etc.) in handwriting or as photocopied
text. If you do not understand the above instructions, ask a
question. Good luck!!!
PROBLEM #1
Explain the key difference between datagram and virtual circuit
packet switching.
___________________________________________________________
___________________________________________________________
PROBLEM #2
Explain the difference between physical and logical topology in a
LAN.
___________________________________________________________
___________________________________________________________
PROBLEM #3
Label and define the fields in an IEEE802.3 frame (fill-in the
blanks in the figure below). For each field, indicate how many
bytes it contains. Recall that the Ethernet and IEEE802.3 frame
types have a subtle difference.
+-----------------------------------------------------+
| ___ | ___ | ___ | ___ | ___ | _____ | ___ | ___ |
+-----------------------------------------------------+
_____ = ____________________________________________
_____ = ____________________________________________
_____ = ____________________________________________
_____ = ____________________________________________
_____ = ____________________________________________
_____ = ____________________________________________
_____ = ____________________________________________
_____ = ____________________________________________
PROBLEM #4
Explain why Ethernet (CSMA/CD) has a maximum span. Or, why
cannot an Ethernet LAN be made arbitrarily long in span? Your
answer should focus on the MAC layer operation of CSMA/CD.
Assume that there are no electrical (or physical layer) issues
preventing an arbitrarily large Ethernet.
____________________________________________________________
____________________________________________________________
____________________________________________________________
PROBLEM #5
Give a pseudocode description of the Ethernet (CSMA/CD)
1-persistent algorithm.
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
PROBLEM #6
Explain why in an IEEE802.5 Token Ring LAN, it is unlikely that a
lobe failure will cause the entire ring (i.e., all stations) to
be affected
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
PROBLEM #7
Consider a 16-Mbps IEEE802.5 Token Ring with total ring
propagation time of 130 microseconds. Assume two stations with
200-byte frames to transmit and that no other stations have
frames to send. The two stations have saturated transmit queues
and are capable of transmitting on each available token. What is
the ring utilization assuming (case 1) no early token release,
and (case 2) with early token release. State any and all
assumptions.
U = ______________________ for case 1
U = ______________________ for case 2
PROBLEM #8
Explain why 100-Mbps Fast Ethernet might be less suitable than
FDDI for use as a backbone.
____________________________________________________________
____________________________________________________________
____________________________________________________________
PROBLEM #9
For the transparent bridged network below, show the final address
tables. Assume that all stations communicate with all other
stations. The station source addresses are A, B, C, and D as
shown.
Ethernet
|
station | | |
+---+ | | +----------+ |
| A +---+ +---+ Bridge 2 +---+
+---+ | | +----------+ | +---+
| +----------+ | +---+ D |
+---+ Bridge 1 +---+ | +---+
+---+ | +----------+ | +---+ |
| B +---+ +---+ C |
+---+ | | +---+
| |
Table for Bridge 1: +------------+--------------+
| Address | Direction |
+------------+--------------+
| A | <- |
| | |
| | |
| | |
| | |
+------------+--------------+
Table for Bridge 2: +------------+--------------+
| Address | Direction |
+------------+--------------+
| | |
| | |
| | |
| | |
| | |
+------------+--------------+
PROBLEM #10
For the source route bridged network below answer the following
questions:
a) Give all possible source routes from A to C, A to D, and D
to B. The ring numbers are 001, 002, and 003 (as shown)
and the bridge numbers are 1, 2, and 3 (as shown). All ring
and bridge numbers are shown in hexadecimal. As an example,
the route from D to C is 0003300020.
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
b) If bridge 3 were to have the bridge number 1 assigned to it,
would this be legal? Explain your answer here...
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
Token Ring
/\ /\
station | | | |
+---+ | | +----------+ | | +---+
| A +--+ +--+ Bridge 1 +--+ +--+ C |
+---+ | | +----------+ | | +---+
| | | |
| | | | /\
+---+ | | +----------+ | | +----------+ | | +---+
| B +--+ +--+ Bridge 2 +--+ +--+ Bridge 3 +--+ +--+ D |
+---+ | | +----------+ | | +----------+ | | +---+
| | | | \/
\/ \/ ring 003
ring 001 ring 002
EXAM #3 FOR INTRODUCTION TO COMPUTER NETWORKS Fall 1995
Welcome to exam #3 in Introduction to Computer Networks. Read
each problem carefully. There are ten problems, you are to
choose eight problems to solve for 12.5 points each. You may do
two additional problems as extra credit for 5 points each.
Circle the problem numbers of the eight problems you choose as
"required problems" and underline the problem numbers of the two
problems you choose as "extra credit problems". Write your
answers on the exam handout in the indicated areas (unless stated
otherwise), show your work on separate sheets of paper. Please
put your name on all sheets of paper. You may have one 8.5 x 11
inch sheet of paper with you. On this sheet you may have
anything you want (definitions, formulas, etc.) in handwriting or
as photocopied text. If you do not understand the above
instructions, ask a question. Good luck!!!
PROBLEM #1
List and briefly describe five criteria for a "good" least-cost
path algorithm.
1) _____________________________________________________
2) _____________________________________________________
3) _____________________________________________________
4) _____________________________________________________
5) _____________________________________________________
What is the key difference between RIP and OSPF?
________________________________________________________
________________________________________________________
PROBLEM #2
Use Dijkstra's algorithm to find the shortest path from node A to
all of the other nodes in the undirected graph below. Show each
step of the algorithm (i.e., show the order in which edges are
selected). Give the final path costs. Link costs are as shown
on the figure below. Use a separate sheet of paper to show your
work and your solution.
A 1 B 2 C 5 D 3 E
x----------x----------x----------x----------x
| | |
| 4 | 4 | 4
| 1 | 4 |
x----------x----------x
F G H
PROBLEM #3
For the network shown below, fill-in the ARP cache for station A.
Also fill-in the two blanks to indicate what are the contents of
the cache. Assume that station A has communicated with all
stations on subnetwork 0001 and subnetwork 0002 within a "recent"
period of time.
Station Router
1111 2222 3333 4444 5555 <- hostid
+---+ +---+ +-------+ +---+
| A | | B | | 1 2 | | C |
+-+-+ +-+-+ +-+---+-+ +-+-+
| | | | |
---+-----------+-----------+- -+-----------+---
0001 0002 <- netid
ARP Cache for station A
____________ ____________
MAC for A = 400011111111 +--------------+--------------+
B = 400022222222 | . |
C = 400033333333 | . |
port #1 = 4000AAAAAAAA | . |
port #2 = 4000BBBBBBBB | . |
| . |
| . |
| . |
| . |
| . |
+------- ------+--------------+
PROBLEM #4
Why does TCP use an adaptive retransmission algorithm? Describe
how the "original" TCP calculated a timeout value based on a
sample Round Trip Time measurement. Also, briefly describe
Karn's algorithm and explain what problem it solves in the
original TCP adaptive retransmission algorithm.
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
PROBLEM #5
Sketch the client/server model we used for sockets programming.
Show your answer on a separate sheet of paper.
PROBLEM #6
List five data communications standards bodies. For each
standards body, give one standard that we have covered in class.
1) ________________________________________________________
2) ________________________________________________________
3) ________________________________________________________
4) ________________________________________________________
5) ________________________________________________________
PROBLEM #7
Give two key goals of Frame Relay and describe the primary
difference between X.25 and Frame Relay.
1) ________________________________________________________
2) _______________________________________________________
Key difference is__________________________________________
_____________________________________________________________
PROBLEM #8
Give five goals for Frame Relay congestion control and sketch a
graph showing offered load (x-axis) versus network throughput
(y-axis) for a Frame Relay network for the three cases, ideal
congestion control, actual congestion control, and no congestion
control. Label the points at which congestion avoidance and
congestion recovery should occur. This problem is continued on
the next page.
1) ________________________________________________________
2) ________________________________________________________
3) ________________________________________________________
4) ________________________________________________________
5) ________________________________________________________
PROBLEM #8 (continued)
Effective |
network |
throughput |
|
|
|
|
+--------------------------------
Offered load
PROBLEM #9
Draw and label the protocol model for B-ISDN. Briefly describe
the functions of each layer. Show your answer on a separate sheet
of paper.
PROBLEM #10
Why does ATM use a small fixed-length cell?
____________________________________________________________
____________________________________________________________
What is the goal of AAL1 (i.e., what kind of traffic is it
intended to carry)?
____________________________________________________________
What is the goal of AAL5 (i.e., what kind of traffic is it
intended to carry)?
____________________________________________________________
What is the purpose of ATM LAN emulation?
____________________________________________________________
____________________________________________________________
Briefly, how does ATM LAN emulation work?
____________________________________________________________
____________________________________________________________
____________________________________________________________