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BBN810S - BROADBAND NETWORKS - 2ND OPP - JULY 2022 |
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1 Pages 1-10 |
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1.1 Page 1 |
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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: BROADBANDNETWORKS
COURSE CODE: BBN810S
DATE: JULY2022
SESSION: 2
DURATION: 3 HOURS
MARKS: 80
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 11 PAGES {Including this front page}
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1.2 Page 2 |
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QUESTION 1
a) Give an example of an application-level requirement that might take
advantage of the drop priority field in an ATM cell? List three reasons
explaining your answer.
[2 marks]
b) Why will two ISPsat the same level of the hierarchy often peer with each [4 marks]
other? How does an IXP earn money?
QUESTION 2
Consider the logical representation in the figure below. The sources A and B are
transmitting data at the average rate of 2 Mbps each to the destination C across a
network. The data consists of 1000 bit packets. Assume that all the contribution to
the end-to-end delay comes from the store-and-forward network queue shown in
the figure, and that there are no packet losses. This queue has an average
occupancy of 12 packets.
2 Mbos
1} What's the averageend-to-end delay experienced by each packet?
[3 mark]
2) In light of your answerfor the part (1), what advanceshavemade 56 Kbpsmodems [2 marks]
possible
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1.3 Page 3 |
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QUESTION 3
The measurements available on the CAIDA AS Rank ranking site reveal the
properties of autonomous systems (Ass) studied. Take for example the information
concerning Level3, Renater and lrisa, which has the autonomous system number
3356, 2200 and 1938 respectively. On the CAIDA web page corresponding to three
ASs (see Table 1) accessible through this link (https://asrank.caida.org/), we can
identify the number of neighbours under the heading "AS degree" and the term
"global". These neighbours can be suppliers (rated provider), AS with a peering
agreement (rated peer), or customers (rated customer).
Alnumlilltifi
ASIIWl!\\ill
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a) By observing the number of neighbours of Level3 and their types, how will you [2 marks]
categorize Level3?
b) Byobservingthe number of RENATERneighbors and their types on the next page, [2marks]
how will you categorise RENATER?
c) By observing the number of neighbors of !RISAand their types on the next page, [2 marks]
how will you categorise !RISA?
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1.4 Page 4 |
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QUESTION 4
Consider the figure below. Suppose that the video is encoded at a fixed bit rate,
and thus each video block contains video frames that are to be played out over the
same fixed amount of time of 1 second. The server transmits the first video block
at tO=0sec, the second block at t=l sec, the third block at t=2 sec and so on. Once
the client begins playout, each block should be played out lsecond after the
previous block.
... 9
QJ
..0 8
E:, 7
C6
.9.a.
5
4
03
4J
l)
2
5 1-
Constant bit
rate video
transmission
by server•---...._
Video
reception
at client~-
111
to
Time
Figure 1. Video streaming transmission
a} Suppose that the client begins playout as soon as the first block arrives at
tl. In the figure below, how many blocks of video (including the first
block} will have arrived at the client in time for their playout? Explain how
you arrived at your answer.
[2 marks]
b} Suppose that the client begins playout now at tl+l. How many blocks of
video (including the first block} will have arrived at the client in time for
their playout? Explain how you arrived at your answer.
[2 marks]
c} In the same scenario at (b} above, what is the largest number of blocks
that is ever stored in the client buffer, awaiting playout? Explain how you
arrived at your answer.
d} What is the smallest playout delay at the client, such that every video
block has arrived in time for its playout? Explain how you arrived at your
answer.
[2 marks]
[2 marks]
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1.5 Page 5 |
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QUESTION 5
Suppose that a router has three input flows and one output port. It receives
packets continuously as per table 1, with all flows beginning at the same time and
queues being empty before the arrival of the first packet. Packets in each separate
flow are listed in the order they are received at the router. For example, packets
1, 3 and 6 are the first to arrive. Length represents the number of clock ticks it
takes to transmit a packet.
Table 1. Queuing Router
Packet id
1
2
3
4
5
6
7
8
Length
200
200
160
120
160
210
150
90
Flow
1
1
2
2
2
3
3
3
Determine the order in which packets are transmitted by the router if Weighted [Smarks]
fair queuing is used, with flow 2 having weight 2, and the other two with weight 1:
QUESTION 6
a} What percentage of an ATM link's total bandwidth is consumed by the [4 mark]
payload bits in ATM?
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1.6 Page 6 |
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QUESTION 7
You are hired to design a reliable byte-stream protocol that uses a sliding window
{like TCP). This protocol will run over a 100-Mbps network. The RTTof the network
is 100 ms, and the maximum segment lifetime is 60 seconds.
(a) How many bits would you include in the AdvertisedWindow of your protocol
header?
(b) How many bits would you include in SequenceNum fields, assuming a
minimum packet size of 40 bytes?
[2 marks]
[2 marks]
QUESTION 8
The Delta operator implements an IP/ MPLS / Ethernet network whose topology
is given in figure. 1. Networks A, 8, C, D and E are IP's networks.
Figure 2. MPLSNetwork
The routing tables of the routers Rl to R7 are given in the Table 2.
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1.7 Page 7 |
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Table 2. Routing Table
RI Routin Table
Dest.
NH
Cost
R2
RS
I
R3
R7
2
R4
R7
2
RS
RS
I
R6
RS
2
R7
R7
I
Network RA
I
A
Network RS
3
8
Network R7
4
C
Network R7
3
D
Network R7
3
E
R4 Routino Table
Dest.
NH
Cost
RI
R7
20
R2
R6
10
R3
R6
10
R4
R7
-
RS
R6
10
R6
R7
10
Network R7
20
A
Network R6
20
B
Network R6
20
C
Network RD
20
D
Network RE
10
E
R7 Routin Table
Dest.
NH
Cost
RI
RI
I
R2
RS
2
R3
R3
I
R4
R4
I
RS
RS
I
R6
R6
I
Network RI
2
A
Network RS
3
8
Network R3
2
C
Network R4
2
D
Network R4
2
E
R2 Routin Table
Dest.
NH
Cost
RI
RS
2
R3
R6
2
R4
R6
2
RS
RS
I
R6
R6
I
R7
RS
2
Network RS
3
A
Network RB
I
B
Network R6
4
C
Network R6
3
D
Network R6
3
E
RS Routino Table
Dest.
NH
Cost
RI
RI
30
R2
R2
20
R3
R6
20
R4
R6
10
R6
R6
-
R7
R7
20
Network RI
20
A
Network R2
10
B
Network R6
30
C
Network R6
40
D
Network R6
10
E
R3 Routino Table
Dest.
NH
Cost
RI
R7
2
R2
R6
2
R4
R6
2
RS
R6
2
R6
R6
I
R7
R7
I
Network RI
3
Network R4
3
B
Network R4
I
C
Network RI
3
D
Network R4
3
E
R6 Routin Table
Dest.
NH
Cost
RI
RS
30
R2
R2
20
R3
R3
20
R4
R4
10
RS
RS
20
R6
R7
-
Network RI
10
A
Network R2
30
B
Network R3
30
C
Network R4
40
D
Network R4
10
E
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1.8 Page 8 |
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It is assumed that the Delta network administrator has enabled MPLS on his
network. Equipment Rl to R7 are Label Switch Routers (LSP).(They switch packets
using label. LSPsare built on demand; that is, we wait until the path is necessary
to build it. The Delta network does not implement quality of service.
An IP packet is sent from a computer in Network A to another computer in Network
D. The packet is forwarded by router RAvia the Delta Network Router Rlwith a TTL
of 40. Router Rl is the Border Router receiving the packet. It will trigger the
creation of the LSPthat will route the packets to network D. It is assumed that this
LSPis the first created in the network.
a) Which router will choose the label to use on the LSPlink at the exit of Rl
(LSPRl to R7)towards Network D?
[2 marks]
b) The switching table in Rl contains the following line:
In case the IP packet is to leave the Delta network, which router pops the
MPLS header?
c) Which LSRsent the packets to router R4 for the communication from Rl
to D?
d) Which next hop is associated with these packets in the switching table of
R4?
e) If packets leaving router R7 are labelled 9, draw the switching table of
router R7.
[2 marks]
[2 marks]
[2 marks]
[3 marks]
QUESTION 9
a) Consider 10 flows passing through a Fair Queue (FQ) router with an
outgoing (4) link running at 100Mbps. Five of the flows are part of a file
backup service and can each fill the link if they are allowed to. The other
five are video streams running at 2Mbps. Given that the router is the
bottleneck for all the flows, how fast do the flows operate?
[3 marks]
b) Assume a link of capacity 10 Mbps that is traversed by four flows with
arrival rates of 6, 4, 2, and 1 Mbps, respectively. How much bandwidth will
each flow get? (Show all your calculations.)
[2 marks]
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1.9 Page 9 |
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QUESTION 10
Using the network in Figure 2, give the virtual circuit tables for all the switches
after each of the following connections is established. Assume that the sequence
of connections is cumulative; that is the first connection is still up when the second
connection is being established and so on. Also assume that the VCI assignment
always picks the lowest unused VCI on each link, starting with 0.
[12
marks]
llmtD
S-2-- I
Ho<r C
Hu,1 E
3 r•w•-a..._.:.. _,,.
0
Ho,t I'
H,,st H
Figure 2. Virtual Circuit Network
1) Host D connects to host H {1 mark)
2) Host B connects to host G {1 mark)
3) Host F connects to host A {1 mark)
4) Host H connects to host C {1 mark)
5) Host I connects to host E {1 mark)
6) Host H connects to host J {1 mark)
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1.10 Page 10 |
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QUESTION 11
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 Figure 4 shown below to answer the
following questions (Note that the Figure 4 does not explicitly show timeouts, but
you should be able to figure out when timeouts happened based on the events
shown).
45
I
I
I
I
I
40
:::::::i::_:-~--: ::::::::::::: Q)
35
.t:!
UJ
30
o-
'O Ill
C:
·-
...
CQ):
25
j_m _J________ g> E
C:
;0; Ill
20
II)-
Q)
en
15
C:
0
----~-/---------+---!-------+!---------- (.J
10
___l_?____l __!___~_-_____!______!_______
------~-/_,: ____t____
I
I
I
I
I
--:- -:- --:- } I
I
I
I
- -- - -- . -- -- - - -~ - - - - - - -- -- - -- -- - - -- - -
5 ----L-~-------~-------~------~----
--~
I
I
I
I
/
0
I
I
I
I
I
I
I
I
0
5
10
15
20
25
30
Transmission Round
Figure 4. TCP Congestion Window Size
a) Give two reasons why slow start is used, and explain why it does a better
job than congestion avoidance.
[2 marks]
b) Identify the time intervals when the TCPslow start is operating. For each
interval time, identify which reasons apply and do not apply and explain
why.
[2 marks]
c) Identify the time intervals when the TCPcongestion avoidance is operating.
Why congestion avoidance should be used instead of the slow start during
these intervals. Please clearly identify one specific reason
[2 marks]
d) After the 16th transmission round, a loss of segment 3 is detected by a
triple duplicate ACK or by a timeout? Explain your answer.
[2 marks]
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2.1 Page 11 |
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e) What is the value of ssthreshold (slow start threshold) at the 18th [2 marks]
transmission round?
f) Assuming a packet loss is detected after the 26th round by the receipt of a
triple duplicate ACK, what will be the values of the congestion window size [2 marks]
and of ssthresh?
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