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BBN810S - BROADBAND NETWORKS - 2ND OPP - JULY 2023 |
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nAmlBIA UnlVERSITY
OF SCIEnCE Ano TECHnOLOGY
FACULTY OF COMPUTING AND INFORMATICS
DEPARTMENT OF COMPUTER SCIENCE
QUALIFICATION: BACHELOR OF COMPUTER SCIENCE HONOURS: COMMUNICATION
NETWORKS
QUALIFICATION CODE: 08BCCH
LEVEL: 8
COURSE: BROADBAND NETWORKS
COURSE CODE: BBN810S
DATE: JULY 2023
SESSION: THEORY
DURATION: 3 HOURS
MARKS: 100
SECOND OPPORTUNITY/ SUPPLEMENTARY EXAMINATION QUESTION PAPER
EXAMINER(S) PROF GUY-ALAIN LUSILAO ZODI
MODERATOR: MS ESNA MANGUNDU
INSTRUCTIONS
1. Answer ALL the questions.
2. Write clearly and neatly.
3. Number the answers clearly.
THIS QUESTION PAPER CONSISTS OF 9 PAGES (Including this front page)
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QUESTION 1 [ 10 marks]
a) Name one advantage and one disadvantage of fixed size packets (cells) [4 mark]
compared to variable length packets. Justify.
b) In Ethernet, what is the difference between a hub and a switch?
[2 marks]
c) Why does the TCP congestion control mechanism have a "slow-start" and [4 marksJ
a "congestion avoidance" phase?
QUESTION 2 [18 marks]
We consider the following set of autonomous systems, LEVEL3, GTT,
CLARANET, RENATER, INTEROUTE and IRISA as shown in figure 1. The
relationships between these autonomous systems are of the peering or transit type
(customer to provider). The letters U to Z represent any IP equipment on the
networks of each of these autonomous systems. It is assumed that only the type of
relationship between autonomous systems governs routing in this exercise: no other
decision criteria are taken into account in the routing selection .
.,__.
--+-
:Peering
: Transit
Figure 1: Autonomous Systems Relationship
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Question 2.1: Looking at the topology in Figure 1 and the service agreement
between autonomous systems.
a) Which autonomous carries the volume of traffic from Claranet to GTT?Explain. [2 marks]
b) Is the volume of traffic in a) carries free of charge? Explain.
[2 marks]
Question 2.2: Considering the topology in Figure 1, and the service agreement
between Claranet and RENATER,
a) Which autonomous system invoices the volume of traffic between Claranet and
Renater on their dedicated link?
b) Which autonomous system charges for the volume of traffic from lrisa to
Claranet?
[2 marks]
[2 marks]
Question 2.3: For a communication from Claranet to GTT, what is the path that
respect the agreements between the two autonomous systems?
[2 marks]
Question 2.4: For a communication from IRISA to Interoute, what are the paths
that respect the service agreements between the two autonomous systems?
[4 marks]
Question 2.5: For a communication from Claranet to Interoute, what are the paths [4 marks]
that respect the service agreements between the two autonomous systems?
QUESTION 3 [6 marks]
Suppose every link in the network carries two classes of traffic -telephone calls and e-
mail messages, with a separate queue for each class. When deciding which packet to
send next, the router first selects the head of the queue containing the voice traffic, and
only sends an e-mail packet if the voice queue is empty. Does the e-mail traffic have
any effect on the performance experienced by the telephone calls? If so, what can be
done to minimize the effects?
[6 marks]
QUESTION 4 [ 12 marks]
Suppose that a router has four input flows being serviced according to a Weighted
Fair Queueing (WFQ} scheduling policy. The weights given to the four queues (A,
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B, C, D) are 4, 1, 3, and 2 respectively. They are being serviced by a router at the
rate of 10 Mbps.
A
B
10
C
D
The table below gives a list of different input traffic rates (in Mbps) at the four
input queues. Fill in the resultant output rates for each these four queues. We have
filled in the first two rows to get you started. Each row [2 marks]
[12
marks]
Table 1. Weighting Fair Queuing Router
Link A
1
10
6
8
1
Input Rates
Link B Link C
1
1
10
10
6
2
0
0
5
3
Link D
1
10
2
8
5
Link A
1
4
4.8
6.67
1
Received Rates
LinkB LinkC
1
1
1
3
1.2
2
0
0
2
3
Link D
1
2
2
3.33
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QUESTION 5 [ 6 marks]
In an ATM Adaptation, what percentage of the total bandwidth do all non-payload [4 marks]
bits consume when a user transmits a cell?
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QUESTION 6 [ 12 marks]
Consider the IP network shown in Figure 1. Routers Rl to R6 belong to a MPLS
network where Rl to R6 are LSRs. These routers connect networks A, B, C and D.
The topology links, whether internal to the MPLS network or external, have a
capacity of 1 Gb/s.
We are interested in communications to C and D.
We have the following information:
Rl has announced to R2 and R3 that it can route packets but only to network D;
R2 has announced to R4 that it can route packets to C and D;
R3 has announced to R4 that it can route packets to network D;
R4 has announced to RS and R6 that it can route packets to C and D.
We know the routing tables ofrouters Rl to R6.
RI Routin; Table
Dest. NH Cost
RI
-
-
R2
R2
10
R3
R3 10
R4
R2 10
R5
R2
10
R6
R2
10
A
R2
10
8
R2
10
C
R2
10
D
D
10
Default R2 10
R2 RoutingTable
Dest. NH Cost
RI
RI
10
R2
-
-
R3
R4 20
R4
R4 10
R5
R4 20
R6
R4 20
A
R4 30
8
R4 30
C
C
10
D
RI
20
Default R4 10
R3 RoutingTable
Dest. NH Cost
RI
RI
10
R2
R4 20
R3
-
-
R4
R4 10
R5
R4 20
R6
R4 20
A
RI
30
8
R4 30
C
R4 30
D
RI
20
Default R4 10
R4 RoutingTable
Dest. NH Cost
RI
R3 20
R2
R2
10
R3
R3 10
R4
-
-
R5
R5 10
R6
R6
10
A
R6 20
8
R5 20
C
R2
20
D
R3 20
Default R2
10
R5 Routin, Table
Dest. NH Cost
RI
R4 30
R2
R4 20
R3
R4 20
R4
R4
10
R5
-
-
R6
R4 20
A
R4 20
8
8
10
C
R4 30
D
R4 40
Default R4
10
R6 Routin; Table
Dest. NH Cost
RI
R4 30
R2
R4 20
R3
R4 20
R4
R4 10
R5
R4 20
R6
-
-
A
A
10
8
R4 30
C
R4 30
D
R4 40
Default R4 10
We have several information on the labels used:
R4 switching table:
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(FEC)
(FEC A)
(FEC B)
(FEC C)
(FEC D)
Label
IN
6
2
7
3
Label
OUT
1
5
8
10
Next Hop
R6
RS
R2
R3
Hypotheses : It is sought to minimize the number of labels used and to group streams having
the same FEC as much as possible by assigning them the same label when possible.
The edge LSRs do the POPs.
The packets destined for A received by R2 carry a label equal to 12.
The packets destined for B received by R2 carry a label equal to 5.
The packets destined for C received by R2 carry a label equal to 8.
The packets destined for D received by R2 carry a label equal to 10.
When Rl receives a packet with an MPLS label equal to 6, it is bound for D.
Complete the commutation table ofR6.
[12 marks]
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QUESTION 7 [ 14 marks]
The Transmission Control Protocol uses a method called congestion control to
regulate the traffic entering the network. The behavior of TCP congestion control
can be represented as a graph in which the x-axis indicates the time, and the y-axis
indicates congestion window size. Please use the graph shown below to answer the
following questions. Note that the graph does not explicitly show timeouts, but you
should be able to figure out when timeouts happened based on the events shown.
35r ~T-1
C., 30
E
25
.., .
iii 20
"O
.E 15
C
0
1., ;; 10 t
00
I
u5 5 II
--1
i
·-1-·. Ii . . •••'
I--!- '
+I •,
'
.- :- __; __;j_ I : j'
TransmissionRound
a) Give two reasons why slow start is used, and explain why it does a better
job than congestion avoidance for that function.
[2 marks]
b) Identify the intervals of time when TCP slow start is operating. For each
interval, identify which of the above reasons apply and do not apply and
explain why.
[2 marks]
c) Identify the intervals of time when TCP congestion avoidance is operating.
Why congestion avoidance should be used instead of slow Start during
these intervals. Please clearly identify one specific reason
[2 marks]
d) Identify the intervals of time when TCP fast retransmission is used. Please
explain what fast retransmission does and how it is triggered.
[2 marks]
e) Identify the intervals of time when TCPfast recovery is operating. What
does fast recovery do and explain why is it beneficial?
[2 marks]
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f) Identify the interval(s) of time when fast recovery could have happened,
but did not. Identify one specific example of a circumstance that may
prevent fast recovery from happening.
g) Which version of TCP is represented in this Figure?
[2 marks]
[2 marks]
QUESTION 8 [ 10 marks]
Suppose you are designing a sliding window protocol for a I Mbps p-to-p link to
the moon, which has a one-way latency of 1.25 seconds. Assuming that each frame
carries I KB of data.
a) Find the window size of the protocol based on the bandwidth-delay [6 marks]
product.
b) What is the minimum number of bits you need for the sequence [4 marks]
number field?
QUESTION 9 [10 marks]
Consider a VC network with a 2-bit filed for the VC number. Suppose that the
network wants to set up a virtual circuit over four links: link A, link B, link C and link
D. Suppose that each of those links is currently carrying two other virtual circuits,
and the VC numbers of these other VCs are as follows:
Link A
Link B
Link C
Link D
00
01
10
11
01
10
11
00
Assume that each of the existing VCs may only traverse one of the four links.
1) If eachVCis required to usethe sameVCnumber on all the four links along its
path, what VC number could be assigned to the new VC?)
[5 marks]
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2) If each VC is permitted to have a different VC number in the different links
along its path, how many different combinations of four VC numbers (one for
each of the four links) could be used?
[5 marks]
=============End
of Examination============
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