Download Midterm 2008

Faculty of Information
Engineering & Technology
Dr. Tallal Elshabrawy
Bar Code
Communication Networks (NETW 501)
Winter 2008
Midterm Examination
Nov. 25th 2008
Please read carefully before proceeding.
1. The duration of this exam is 2 hours
2. Point value of this exam is 30%
3. Only calculators are permitted for this exam
4. This exam booklet contains 14 pages including this one. Two extra sheets of
scratch papers are attached
Good Luck!
Marks:
Problem Number
Possible Marks
Final Marks
1
2
3
Total
15
20
15
50
Problem 1:
a) List the seven layers of the OSI reference model in the table below. Give
one of the functions that are performed by each layer.
(3.5 Marks)
Layer Name
Layer
Layer
Layer
Layer
Layer
Layer
Layer
Layer Function
7
6
5
4
3
2
1
b) For the TCP/IP reference model, which layer is responsible for:
(1.5 Marks)
Point-to-Point Flow Control
Congestion Control
Data Compression
c) List two differences between TCP and UDP and provide an example for
application layer protocols that uses them
(3 Marks)
TCP
UDP
Difference 1:
Difference 1:
Difference 2:
Difference 2:
Example Application Layer Protocol:
Example Application Layer Protocol:
2
d) Suppose a router is used to interconnect four different networks: Network
1, Network 2, Network 3, Network 4
(3 Marks)
i. List the layers that would be supported by the router.
ii. How many MAC addresses does the router have?
iii. How many IP addresses does the router have?
iv. Suppose a computer is moved within Network 2. Does its IP address
need to change? (Explain your answer)
v. Name the protocol that is used to map from an IP address to a MAC
address.
e) Answer the following:
i. What is the function of port numbers used by the TCP in the TCP/IP
protocol stack
(1 Mark)
ii. The UDP in the TCP/IP protocol stack does not need to use port numbers.
State whether the statement above is true or false. (Explain your
answer).
(1 Mark)
3
iii. If two clients, one on host A and another on host B, connect to an email
server on host C to retrieve mail, how many different TCP ports on C are
used? (Explain your answer)
(1 Mark)
iv. If you want to browse a page, say http://www.guc.edu.eg, is any
application layer protocol in addition to HTTP needed? If yes name such
protocol.
(1 Mark)
4
Problem 2:
a) What are the phases of circuit switching?
(1 Mark)
b) List two points of comparison between circuit-switching and virtual circuit
packet-switching
(2 Marks)
Circuit-Switching
Virtual Circuit Packet-Switching
c) Consider transferring 65 packets between from machine A to machine B in a
communication network. Each packet transmitted by A crosses two
intermediate nodes before reaching B. Assume an ideal scenario where the
physical channel supports error free communications. Given that the packet
size should be FIXED and the total length of headers is 200 bytes, the
transmission rate and propagation time is identical for all links in the
network and the throughput efficiency over each link in the network is 90%
i. What is the total packet size?
(1 Mark)
ii. What is the total amount of information data transferred between the
two machines?
(1 Mark)
5
iii. If the transmission time of one packet (Tt) is 8 ms what is the
transmission bit rate over the communication links in the network?
(1 Mark)
iv. If the distance between the any two successive nodes is 2.5 Km, and the
propagation speed in the cable is 2.5*108 m/sec, what is the propagation
time for one packet (Tp) across one link?
(1 Mark)
v. What’s the total time for transferring the 65 packets if circuit switching is
used. Assume setup time of 200 ms? (Sketch Time Diagram) (2.5 Marks)
6
vi. What’s the total time for transferring the 65 packets if packet switching
is used? (Sketch Time Diagram)
(2.5 Marks)
7
d) Consider a code that is constructed repeating a block of 3 bits 4 times. So
for example the code word corresponding to (0 1 1) is:
011
011
011
011
i. What’s the number of valid code words in this code?
(1 Mark)
ii. If the probability of bit error is 0.01. What’s the probability of failing to
detect that an error has occurred
(2 Mark)
iii. If an error in the first two bits occur? Would it be possible to correct
that error? (Explain your answer)
(2 Mark)
e) Assuming that framing at the data link layer is achieved by using starting
and ending flags with bit stuffing. Assume that the header of the data link
frame includes only parity bits where for every seven consecutive bits in the
data to be transmitted an even parity bit is appended at the beginning of
the frame. In other words if we would like to send 28 bits of data we must
add 4 additional parity bits in the header as well as 16 bits for the starting
and ending flags. Assume that the first bit in the header corresponds to the
parity bit for the first seven bits of information data and the last bit in the
header corresponds to the parity bit for the last seven bits of information
data. What is the bit-content of the data link frame transmitted by the
source station given the information bits below?
(3 Marks)
(Indentify in the table the parity bits and the stuffed bits if needed)
1 1 1 1 0 1 0 0 1 1 1 1 1 0
8
Problem 3:
a) Consider the packet switching network shown below. Assume shortest path
routing is used in the network. Furthermore assume that the stop-andwait protocol is used for communication at the data link layer on all links.
Assume that the cost on the links correspond to the propagation time in
milliseconds over the corresponding link. Assume packet transmission times
on each link are as follows:
Link
A-B
A-C
B-E
B-D
B-F
C-E
C-F
E-F
Packet Transmission Time
4 millisecond
5 millisecond
6 millisecond
3 millisecond
7 millisecond
2 millisecond
8 millisecond
4 millisecond
B
D
5
11
9
7
F
3
A
2
7
7
6
C
E
i. Calculate the time need to transfer 15 packets between A and F. Assume
ACK time and processing times are negligible.
9
10
ii. If the link B-E fails, What kind of problems might occur in the network for
the transmission of packets between A and F
iii. If the link B-E fails at t = 30 ms. Calculate the time need to transfer 15
packets between A and F. (Assume it takes negligible time to update the
routing tables in the network after the link B-E fails. Assume ACK time
and processing times are negligible.
11
12
Extra Sheet 1:
13
Extra Sheet 2:
14