EXAMINER: Prof A Marshall DEPARTMENT: EE&E
EMAIL: Alan.Marshall@liverpool.ac.uk
SECOND SEMESTER EXAMINATION REPLACEMENTS 2019/20
COMMUNICATION NETWORKS AND SECURITY
INSTRUCTIONS TO CANDIDATES
The numbers in the right hand margin represent an approximate guide to the marks available for that question (or part of a question). Total marks available are 100.
Copying any material from another source, or colluding with any other person in the preparation and production of this work will be considered suspected academic misconduct and will be dealt with according to the University’s Academic Integrity Policy.
PAPER CODE NO.
ELEC 461
Additional Information:
This is an open-book test.
Answer ALL Questions.
The use of a calculator IS allowed.
Some useful formulae attached
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M/G/1
Single Server Queues
r=+ 2A 1−
T = T + Ts A r s 1−
1 2 A= 1+ ts
2 Ts
w= 2A 1−
T = Ts A w 1−
M/M/1
2 w = 1− r = 1−
Tw = Ts Tr =Ts + Ts 1− 1−
M/D/1
w=2 r=2 + 2(1−) 2(1−)
Tw = Ts Tr =Ts(2−) 2(1−) 2(1−)
Pollaczek-Khinchine (PK) formulae:
T= w
Es 2(1−)
2m
E[s2]= Ps2
j=1
jj
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1. a)
Fig Q1a illustrates the OSI seven-layer model for communications between nodes A and 3 B. Two intermediate nodes are also shown as “?”.
Application Presentation Session Transport Network Data Link Physical
AB
Fig Q1a
Identify the type of device shown in Fig. Q1b, for the three possible configurations of the
“?” nodes in Fig. Q1a.
“?”
“?”
Fig Q1b
b) A (7,4) Hamming codeword is sent over a channel with a bit error rate of 0.05, calculate:
i) The probability of exactly 2 errors occurring in any codeword. 5
ii) The probability of >2 errors occurring in any codeword. 4
c) List three techniques used to control the flow of traffic in a network and identify the 3 protocol layer where each technique resides.
Question continues overleaf.
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Question continued.
d) Consider the periodic on-off traffic shown in Fig. Q1d, which is passing through a token bucket filter (TBF) with a token generation rate of ρ Mbps, with a token bucket capacity of C Mbits, and maximum output rate of 50 Mbps (same as the input rate).
Rate Mbps
50
100 ms 150 ms
ON
OFF
ON
***
Fig Q1d On-Off Traffic Pattern
Assuming that the token bucket is full when the first burst arrives, it is required to design
a TBF that allows the on-off traffic to pass through without shaping.
i) Find the minimum value of token bucket capacity Cmin, in terms of ρ , for the first 5
burst to pass through the TBF without shaping.
ii) After the first burst the bucket is now empty (i.e. Cmin has been drained). 5 Find the minimum token generation rate in Mbps, for the second burst (and
therefore all bursts) to pass through the TBF without shaping.
Total 25
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2. a)
A Network Provider wishes to perform traffic engineering on the network in Figure Q2a by adjusting the link weights according to the following cost function:
ci= ui (1−ui)
AB
Where ui is the link utilisation. CAB
CBE CBC
Figure Q2a
In the network all the links have a capacity of 100 Mbps.
At a certain time, the utilisation of each link is shown in the table below:
CAD
CAC
E D C CCE
CCD
link
Bandwidth used (Mbps)
Ui
Ci
AB
70
AC
50
AD
10
BC
30
CD
30
BE
50
CE
20
i) Using the cost function, determine the shortest path between nodes A and E. Do 6 not consider routes longer than 3 hops.
Question continues overleaf.
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Question continued.
ii) If the bandwidth used on link CD increases to 80 Mbps and the bandwidth 4 used in link AB reduces to 20 Mbps, what is the new shortest path between
nodes A and E?
b) The routers in the network shown in figure Q2b are configured to run link state 3 routing. At some time T0, the link between nodes D and F fails. How many link
state packet (LSP) updates will be sent if reliable flooding is used to update the network? How many steps (cycles) will this take?
A
BE
CD
F Link break at T0
Figure Q2b
c) In a subnetwork consisting of 50 nodes, every node has an average of 3 2
neighbours. If link state routing is employed, what is the average size of the link state database in each node?
d) Two problems encountered in wireless networks are the “Hidden node” and the 10 “Exposed Node”. With the aid of diagrams explain each of these.
Total 25
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3. a)
Sensor devices transmit their data to a base station using an uplink shared radio channel. Each device transmits packets for exactly 25 bytes (200 bits) and the radio channel has a rate of 2 Mbps and exhibits a bit error rate of 4×10-3. Slotted Aloha is used as the medium access protocol.
i) Determine the maximum time period when packets are vulnerable to collisions 1 on the channel.
ii) In one second an average of 50 devices attempt to transmit onto the channel with 4 one packet each, and their transmission attempts follow a Poisson distribution. Calculate the normalised offered load to the network (G), the probability () of
no collisions on during the vulnerability period, and from this the probability of
a collision on the radio channel.
Layer 3 IP packets of size 4000 bytes (32000 bits) are delivered to a layer 2 HDLC frame protocol with the structure shown in figure Q3c. The data field (payload) can hold up to 200 bytes.
Figure Q3c – HLDC frame structure
The HDLC frames are sent over a radio link that exhibits a bit error rate of 5×10-5. The frames are reassembled into the IP packet in the receiver.
i) If a “go-back-N” link protocol is used whereby all HDLC frames have to be resent 7 if the IP packet is found to be in error. Determine the probability of successfully sending an IP packet.
ii) If a selective repeat ARQ protocol is used calculate the new probability of 3 successfully sending an IP packet. Assume that no more than 2 frame errors can
occur in any sequence of transmissions.
b)
Question continues overleaf.
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Question continued.
c) A radio data link connects two sites 30 km apart. Data is sent across the link in 500 byte frames and the radio link operates at a data rate of 100 Mbps. Assuming the link has a bit error rate of 4 x 10-5, and a velocity of propagation v = 3 x 108 m/s.
Compare the determine the link utilisation efficiency using:
i) a Stop and wait ARQ protocol and, 6
ii) a selective repeat protocol with a send buffer k=4 frames. 4
Total 25
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4. a)
In a particular TCP session, the maximum segment size is set to 1024 bytes. Initially 5 the congestion window was set to 64 Kbytes, however a timeout occurred and so
the threshold is set to 32 Kbytes and the congestion window
set to 1 Kbyte for transmission number 0.
Draw a graph of the congestion window size against time (i.e. transmission number sequence), whenever the slow start algorithm is used. Label your graph and annotate the relevant points on it for each successive TPC transmission. Assume that a timeout occurs whenever the congestion window reaches 38 Kbytes. Continue your graph for 20 transmissions.
The packets arriving to a router are analysed and it is observed that 25% are voice packets of length 48 bytes, and 75% are data packets of length 1200 bytes. The packet arrival rate is maintained at 1000 packets per second and the router has a single outgoing link at 12 Mbps.
i) Calculate the average time taken to transmit a packet. 2 ii) Using the PK formulae, calculate the average time a packet spends in the 5
Router.
In wide area networks, hot and cold “potato routing” strategies are used by 4 commercial network providers to route traffic between different autonomous systems belonging to different operators that are interconnected in multiple locations. Explain (in less than 200 words) the differences between each strategy
and why each is used.
i) Describe how public-key encryption can also be used to encrypt and decrypt 5
messages. Give an example to illustrate your answer.
ii) Briefly (less than 200 words) explain the two components of firewalls. 4
Total 25
b)
c)
d)
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