Assessment Code Any calculator
Calculators may be used in this examination but must not be used to store text. Calculators with the ability to store text should have their memories deleted prior to the start of the examination.
Special Requirements: None
School of Engineering
Department of Electronic, Electrical and Systems
Engineering Level M
04 21483
Computer and Communication Networks
Summer Examinations 2019
Time Allowed: 1 hour 30 minutes
Answer ANY TWO questions out of THREE
The allocation of marks within each question is stated in the right-hand
margin.
Page 1 of 6 TURN OVER
Calculators may be used in this examination but must not be used to store text. Calculators with the ability to store text should have their memories deleted prior to the start of the examination.
1. (a) (i)
(ii)
(iii)
(b) (i)
SECTION A
(ANSWER ANY TWO QUESTIONS)
Convert the binary IPv4 address [2] 11000000101010000000011100011010 into the dotted decimal notation. What class of network (A – E) does this address belong to?
An IPv4 address in CIDR notation is given by 147.188.146.24/16. What [3] are the associated:
• Subnet mask
• Subnet address
• Subnet broadcast address
A network uses the subnet mask 255.255.255.240. A message arrives [5] with address 192.168.34.27, what is the target subnet number? How many useable addresses exist on the subnet?
The designer of a low-power network for use with sensors in the field [5] intends to use Dijkstra’s algorithm as the basis for routing decisions.
The basic topology of the sensor network is shown below. Using Dijkstra’s algorithm, complete the skeleton table to determine the shortest routes from all nodes to node A (which is linked to the backhaul network). From your table, explicitly state the shortest path from node E
to node A.
E
F 3
C 2
7
B6
2
H
2
3A5 3
D
1
3
G
V
Distance [V]
Previous [V]
Path to A
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Assessment Code
Page 2 of 6
TURN OVER
Calculators may be used in this examination but must not be used to store text. Calculators with the ability to store text should have their memories deleted prior to the start of the examination.
(ii) Initial field trials have shown that battery life on some of the sensor nodes [2] is a serious issue, and so the designer proposes that a gradient based routing strategy should replace the existing shortest paths-based approach. Explain how gradient based routing differs from shortest path routing, and why this is of advantage in low power wireless sensor networks.
(iii) Wireless sensor networks often use flood routing as a way of overcoming [3] issues caused by the mobility of nodes. Flood routing approaches can cause an exponential growth in the number of packets circulating in a network, and lead to problems such as broadcast storms. Describe three approaches that can be used to mitigate these issues, explain your answers.
Assessment Code Page 3 of 6 TURN OVER
Calculators may be used in this examination but must not be used to store text. Calculators with the ability to store text should have their memories deleted prior to the start of the examination.
2. (a) (i)
Open-loop congestion control approaches work by attempting to prevent [2] congestion occurring on the network. One common open-loop approach
is admission control. Explain how admission control works and give examples of two key traffic descriptors that might be used as part of the process.
(b) (i)
A traffic shaping system based on a token bucket algorithm is being used [2] to manage congestion in a packet-switched network. Explain how the token bucket algorithm is used to determine if traffic conforms to the agreed policy.
(c) (i)
Network congestion leads to delays in the processing of packets, and [2] poor overall network performance. The delay performance of a network
can be determined using Little’s formula. State Little’s formula in its blocking form.
(ii) TCP uses a closed-loop congestion window to regulate traffic flow [4] between senders and receivers. Assuming that the network is initially uncongested and that the default segment size is 1, explain the four phases of control the window may apply to the sender.
(ii) The traffic shaping algorithm determines that packets are being delivered [2] to the network from an end-user at a rate which doesn’t conform to the traffic policy, suggest two actions that the system may take to maintain
the performance of the network.
(ii) The performance of blocking networks is often discussed in terms of the [2] long-term arrival rate (𝜆), and the long-term service rate (𝜇). If the arrival
rate is greater than the service rate then the network will drop packets. What will happen if the arrival rate is lower than the service rate?
(iii) A network gateway has a service rate of 40 Mbps. The packet size on [6] the network is 20KB, and the arrival rate of packets at the gateway is 125 packets/sec. If the gateway corresponds to an M/M/1 queuing model, and
has 10 buffers, what is the probability of an overflow?
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3. (a)
(i) Ethernet networks use collision detection to mitigate the problem of [2] frames colliding during transmission. Explain why wireless networks cannot use collision detection.
(ii) In place of collision detection, many wireless network standards exploit [3] short messages known as handshakes to protect longer data transmissions. What are the roles of the RTS and CTS messages, and
why are they an effective approach to managing network capacity?
(iii) In CSMA-CA wireless networks, three different inter-frame spacings are [4] commonly used as part of the MAC algorithm. List the three spacings in terms of duration from longest to shortest and explain how they enable efficient access to the available bandwidth to be managed by the point controller.
(i) A student living in a block of flats is configuring his home wireless access [3] point as part of a 2.4 GHz 802.11b network. Explain why he may be better choosing one of the overlapping channels rather than one of those guaranteed to not overlap with each other (i.e. why there may be an advantage to the student in avoiding channels 1, 6, & 11).
(ii) 802.11 networks allow nodes to enter sleep states, thus conserving [4] power. Explain the role of the power management bit and beacon frames
in this process.
(i) An 802.15.1 (Bluetooth) network is being used in a kitchen to wirelessly [4] connect a set of speakers. Explain why signal quality may be a problem
in this environment, and how the division of the 2.4GHz spectrum by the 802.15.1 standard is used by devices to mitigate this risk.
Calculators may be used in this examination but must not be used to store text. Calculators with the ability to store text should have their memories deleted prior to the start of the examination.
(b)
(c)
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Assessment Code + Computer and Communication Networks
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