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UNIVERSITY OF SOUTHAMPTON ELEC3222W1


SEMESTER 1 EXAMINATIONS 2015/16

COMPUTER NETWORKS

Duration 150 mins (2.5 hours)


This paper contains 4 questions

Answer ALL questions.







Only University approved calculators may be used.

A foreign language dictionary is permitted ONLY IF it is a paper version of a
direct ‘Word to Word’ translation dictionary AND it contains no notes, additions
or annotations.

5 page examination paper

ELEC3222W1

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Answer ALL questions

1 (a) State one advantage and one disadvantage of protocol
layering.
[2 marks]

(b) Using Ethernet/IEEE 802.3 as an example, explain the
difference between a network hub, switch, and router.
[3 marks]

(c) Alice has many uncompressed digital photographs
taken at a resolution of 1920 x 1080 pixels with 3
bytes/pixel. She needs to send some to Bob, and
considers two options: 1) using a modem with a data-
rate of 56 kbps, or 2) using a courier to deliver a large
USB flash drive. The courier is available immediately,
and travels at an average speed of 60km/hr. Assuming
that Bobby lives 100 km from Alice, calculate how
many photos Alice must send to Bobby in order for the
courier to offer a higher data rate than the modem?
[7 marks]

(d) A network is formed from two battery-powered wireless
nodes: A and B. Node A very infrequently transmits
data to Node B. Describe and compare the impact
that the MAC layer has on the delay/latency and the
energy consumption of A and B when:

(i) using the ALOHA MAC protocol
(ii) using the B-MAC protocol
[7 marks]

(e) A LAN contains 20 stations, a fixed frame size of 100
bits, and has a data-rate of 1 kbps. Calculate the
worst-case (longest) time that a station might have to
wait before it can begin transmitting its frame when:

(i) using the basic bit-map MAC protocol
(ii) using the binary-countdown MAC protocol
[6 marks]
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TURN OVER

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2 A bit string, 110111111012, is passed from the network layer
to the data link layer for transmission.

(a) The data link layer first applies a (15,11) Hamming
code (using even parity). Calculate the bit string that is
produced.
[8 marks]

(b) Next, the data link layer frames the data using bit
stuffing (the flag byte is 011111102). Using your answer
from (a), calculate the bit string that is produced. If you
were unable to answer (a), assume that the original bit
string was unchanged by Hamming coding.
[5 marks]

(c) On receiving the bit string from the data link layer, the
physical layer applies Manchester coding. Using your
answer from (b), sketch the waveform that is produced
for the first 10 bits transmitted by the physical layer. If
you were unable to answer (b), assume that the
original bit string was unchanged by bit stuffing.
[5 marks]

(d) Briefly state the purpose of applying each of the above
three steps (a)-(c), instead of just directly transmitting
the bit string passed from the network layer?
[3 marks]

(e) Calculate the maximum overhead that the data link
layer may add through the above steps (a)-(b), in the
general case where any 11-bit string could be passed
from the network layer. The overhead is equal to the
number of bits that the data link layer passes to the
physical layer minus the number of bits it had originally
received from the network layer.
[4 marks]

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3 (a) Explain why, In cryptography, it is necessary to have
redundancy in messages? State a method of
protecting against replay attacks and evaluate your
solution.
[7 marks]

(b) Suppose that a system uses Public Key Infrastructure
(PKI) based on a tree hierarchy of Certification
Authorities (CAs). Alice wishes to communicate with
Bob and gets a certificate from Bob signed by CA X.
If Alice has never heard of X, explain what can she do
to verify that she is talking to Bob?
[7 marks]

(c) Table 1 shows a mutual authentication process using a
PKI directory server (PKIDS). Describe how it works
and identify all the symbols used. At step 7, RB is
encrypted. Is this necessary or would plaintext do?
Explain your answer.

[11 marks]



Step Direction Contents
1 Alice → Bob Request for EB
2 PKIDS → Bob EB
3 Alice → Bob EB(A,RA)
4 Bob → PKIDS Request for EA
5 PKIDS → Bob EA
6 Bob → Alice EA(RA,RB,KS)
7 Alice → Bob KS(RA)
Table 1




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END OF PAPER

Copyright 2016 © University of Southampton Page 5 of 5
4 (a) Explain how Network Address Translation (NAT) and
Classless Interdomain Routing (CIDR) work and what
their purpose is.
[9 marks]

(b) An IPv4 router has the routing table as shown in
Table 2. Describe what the router does with a packet
addressed to each of the following destinations:
128.96.39.10, 128.96.40.12, 128.96.40.151,
193.4.153.17 and 192.4.153.90.
[5 marks]

Subnet number Subnet mask Next hop
128.96.39.0 255.255.255.128 IF0
128.96.39.128 255.255.255.128 R1
128.96.40.0 255.255.255.128 R2
128.4.153.0 255.255.255.128 R3
R4
Table 2

(c) A set of transport layer system calls and the messages
that result when they are called is given in Table 3.
Draw a diagram or a table showing the different state
transitions involved when a client requests a file from a
server. Distinguish between messages initiated by the
client from those initiated by the server.
[11 marks]

System call TPDU sent Meaning
LISTEN None Block until a process tries to
connect
CONNECT CON. REQ. Attempt to get a connection
SEND DATA Send information
RECEIVE None Block until a TPDU arrives
DISCONNECT DCON. REQ. Wait until a DCON. REQ. arrives
Table 3