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BBN810S - BROADBAND NETWORKS - 1ST OPP - JUNE 2022 |
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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: JUNE 2022
SESSION: 1
DURATION: 3 HOURS
MARKS: 70
EXAMINER(S)
FIRST OPPORTUNITY EXAMINATION QUESTION PAPER
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 8 PAGES (Including this front page)
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QUESTION 1
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 [2 marks]
a "congestion avoidance" phase?
QUESTION 2
We consider the following set of autonomous systems, LEVEL3, GTT,
CLARANET, RENATER, INTEROUTE and IRISA as shown in figure I. 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
Provider
(RISA
Figure I: Autonomous Systems Relationship
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Question 2.1: Looking at the topology above, the agreement between Claranet and [2 marks]
Level3 implies that (select all correct answers):
Claranet is billed for the volume of traffic sent and received from
Level3.
Level3 is billed for the volume of traffic sent and received from
Claranet.
Traffic exchanged between Claranet and Level3 is not billed.
Level3 routes traffic from Claranet to the rest of the Internet.
Question 2.2: According to the proposed topology, the agreement between
Claranet and RENATER implies that (select all correct answers):
Claranet is billed for the volume of traffic exchanged with RENA TER.
RENATER routes traffic from Claranet to the rest of the Internet.
RENATER routes traffic from Claranet to IRISA.
The exchange of traffic between Claranet and RENATER is not
invoiced.
[2 marks]
Question 2.3: For a communication from Y to V, what are the paths that respect
the agreements between the autonomous systems? (select all correct answers)
Claranet -> RENATER-> Interoute -> OTT
Claranet -> RENATER -> OTT
Claranet -> Level3 -> OTT
Claranet -> RENATER-> Interoute -> RENATER-> OTT
[2 marks]
Question 2.4: For a communication from X to W, what are the paths which
respect the agreements between the autonomous systems? (select all correct
answers)
IRISA -> RENA TER -> Interoute
IRISA -> RENA TER -> Claranet -> Level3 -> OTT -> Interoute
[2 marks]
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IRISA -> RENA TER -> GTT -> Interoute
IRISA -> RENA TER -> Claranet -> Level3 -> GTT -> RENA TER ->
Interoute
Question 2.5: For a communication from Y to W, what are the paths that respect
the agreements between the autonomous systems? (select all correct answers)
Claranet -> RENATER -> Interoute
Claranet -> Level3 -> GTT -> RENATER -> Interoute
Claranet -> RENATER -> GTT-> Interoute
Claranet -> Level3 -> GTT -> Interoute
QUESTION 3
Suppose eve1y link in the network can-ies 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?
[4 marks]
QUESTION 4
Suppose that a router has three input flows and one output port. It receives packets
continuously as per table below, with all flows beginning at the same time and
queues being empty before the arrival of the first packet. Packet order in each
separate flow is listed in the table (packets 1, 5 and 7 are the first to an-ive). Length
represents the number of clock ticks it takes to transmit a packet.
Packet id Length Flow
1
200 1
2
200 1
3
160 2
4
120 2
5
160 2
6
210 3
7
150 3
8
90
3
Determine the order in which packets are transmitted by the router if:
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(i) Fair queuing is used.
[3 marks]
(ii) Weighted fair queuing is used, with flow 2 having weight 2, and the
other two with weight 1.
[3 marks]
QUESTION 5
In an ATM Adaptation, what percentage of the total bandwidth do all non-payload [5 marks]
bits consume when a user transmits a cell?
QUESTION 6
Consider the IP network shown in Figure 1. Routers Rl to R6 belong to a network
using MPLS (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
RI
-
R2
R2
R3
R3
R4
R2
RS
R2
R6
R2
A
R2
B
R2
C
R2
D
D
Default R2
Cost
-
JO
10
10
JO
JO
10
10
10
10
10
R2 RoutinoTable
Dest. NH Cost
RI
RI
10
R2
-
-
R3
R4 20
R4
R4 10
RS
R4 20
R6
R4 20
A
R4 30
B
R4 30
C
C
JO
D
RI 20
Default R4 10
R3 Routin°Table
Dest. NH Cost
RI
RI
10
R2
R4 20
R3
-
-
R4
R4 10
RS
R4 20
R6
R4 20
A
RI 30
B
R4 30
C
R4 30
D
RI 20
Default R4 JO
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R4 RoutingTable
Dest. NH Cost
RI
R3 20
R2
R2 10
R3
R3
10
R4
-
-
RS
RS 10
R6
R6
10
A
R6 20
B
RS 20
C
R2
20
D
R3 20
Default R2
10
RS Routin Table
Dest. NH Cost
RI
R4 30
R2
R4 20
R3
R4 20
R4
R4 10
RS
-
-
R6
R4 20
A
R4 20
B
B
10
C
R4 30
D
R4 40
Default R4 10
R6 RoutinoTable
Dest. NH Cost
RI
R4 30
R2
R4 20
R3
R4 20
R4
R4 10
RS
R4 20
R6
-
-
A
A
10
B
R4 30
C
R4 30
D
R4 40
Default R4 10
We have several information on the labels used:
R4 switching table:
(FEC)
(FEC A)
(FEC B)
(FEC C)
(FEC D)
Label
IN
6
2
7
3
Label
OUT
I
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 I 0.
When RI receives a packet with an MPLS label equal to 6, it is bound for D.
Complete the commutation table of R6.
[12 marks]
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QUESTION 7
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.
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TransmissionRound
a) Give two reasons why slow start is used, and explain why it does a better
job than congestion avoidance for that fimction.
[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 TCP fast recovery is operating. What
does fast recovery do and explain why is it beneficial?
[2 marks]
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_r
t) Identify the interval(s) of time whenfast 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
Suppose you are designing a sliding window protocol for a 1 Mbps p-to-p link to
the moon, which has a one-way latency of 1.25 seconds. Assuming that each frame [2 marks]
carries 1 KB of data.
a) Find the window size of the protocol based on the bandwidth-delay [2 marks]
product.
b) What is the minimum number of bits you need for the sequence
number field?
QUESTION9
Explain the main difference between an ADSL and VDSL used for broadband [4 marks]
communication. Which of these two technologies give better performance in terms
of speed?
============End
of Examination==========
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