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 Supplementary 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 7 TURN OVER
1. (a)
(i)
(ii)
(iii)
Within autonomous systems, network routing is commonly based on distance- vector routing algorithms.
Describe how, in distributed implementations of distance vector routing algorithms, the most appropriate route for packets to cross a network is determined. [2]
Routing protocols, such as the Routing Information Protocol (RIP), make use of these to determine the best path to a given destination. Explain how the RIP protocol makes use of UDP messages to gather the information needed to build
the local routing table. [2]
Figure 1 shows an example of a network; the initial steady-state routing to node
D, derived using the Bellman-Ford algorithm, is shown in Figure 2. Taking the steady state information shown in Figure 2 as the starting point, continue the table to show how the Bellman-Ford algorithm allows the network to recover from the failure of the link between A and E, and hence find (and state) a new path between nodes A and D. [6]
Figure 1
Figure 2
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.
Source Node
Iteration
A
B
C
E
F
G
H
5 – steady state at failure
E,5
E,5
A,8
D,3
A,6
E,8
F,8
Assessment Code Page 2 of 7 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.
(b)
(i) (ii)
(iii)
(iv)
Across autonomous systems, Interior Gateway Protocols like RIP are replaced by Exterior Gateway Protocols, such as the Border Gateway Protocol (BGP).
What are the three main types of autonomous system? Give a brief
description of each. [2]
Autonomous systems are identified by AS numbers. Until the year 2007 these were expressed purely using asplain notation, but since then this has been replaced with asdot notation. By describing the two notation forms, explain why
it was necessary to introduce asdot notation. [2]
BGP is a policy-based path vector routing protocol. Explain how path vector routing works, and why it, and the use of routing policies, are the preferred approach to routing at the level of autonomous systems. [2]
In Figure 3, A, B, and C are ISP backbone networks, and V, W, X, and Y are customer networks. Suppose that B and C have a peering relationship, and A
is a customer of both providers. Network A would like traffic destined for network
W to come only via network B, but will allow traffic destined for network V to come via either route. How should A advertise its routes to B and C? Explain
the route rules that should be provided to C by B. [4]
B
VX
A
Assessment Code
Page 3 of 7
TURN OVER
W
C
Figure 3
Y
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)
(ii) (iii)
The 802.11 family of wireless networks have several notable physical differences that impact on their performance characteristics. 802.11a networks operate in the 5 GHz band, while 802.11b networks operate at
the much lower 2.4GHz band. Give one advantage and one disadvantage to 802.11a of operating in the higher frequency band, [2] relative to 802.11b.
Explain the use of channels in 802.11b networks and why they are important to ensuring good transmission rates. [3]
Figure 4 shows a typical 802.11 frame. On infrastructure networks the first three address fields (address 1, address 2, and address 3) are used.
Figure 4
Explain, by describing the sending and receiving of messages via an infrastructure network, why three addresses are used by 802.11 when standard Ethernet only requires two. [5]
Figure 5 shows four laptops and their associated wireless ranges. Assume node Y is transmitting to node Z. Node W wants to transmit to node X but has to defer resulting in a failure to exploit bandwidth fully. Explain why this is the case, which common problem in wireless networking it is an example of, and why the problem is not seen in wired Ethernet network environments. [3]
Length (bytes)
2
2
6
6
6
2
6
0- 2312
4
(b) (i)
Figure 5
Assessment Code Page 4 of 7 TURN OVER
CRC Payload
Address 4
Seq. Control
Address 3
Address 2
Address 1
Duration
Frame Control
Field
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) Handshaking is used in wireless networking to reduce the likelihood of collisions, explain why handshaking is an efficient way to achieve this goal. [2]
(iii) In congested wireless environments the probability of collisions between handshakes is comparatively high. By explaining exponential backoff, show how 802.11 wireless MAC further reduces the risk of collisions occurring. [2]
(iv) The designer of a wireless network is given the option of using an infrastructure-based or an ad-hoc architecture. The network will be used
for the transmission of video in real time, and a requirement of the system
is that the risk of collisions should be minimised. Which architectural model (infrastructure or ad-hoc) should the designer choose based on
this requirement? Explain your answer. [3]
Assessment Code Page 5 of 7 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.
3. Mobile Ad-hoc Networks (MANETs) are a class of wireless networks that typically display a high degree of mobility. Two routing approaches commonly used in these environments are Dynamic Source Routing (DSR) and Ad-hoc On-demand Distance Vector (AODV) routing.
(a) (i)
MANET routing protocols fall into one of two main classes: proactive and reactive. Describe the two classes and give an example of each. [2]
(b)
(ii) DSR and AODV both use flooding for route discovery rather than for the delivery of data; this reduces the network traffic overhead associated with flood routing and also reduces the risk of broadcast storms developing. Name two other control measures that can be applied when using flood routing to avoid broadcast storms. [2]
(iii) How do optimised forms of DSR and AODV attempt to further limit the spread of flooded control packets within the network? [1]
An engineer is developing a mesh network that will be used to control a swarm of drones during a New Year’s Eve event. The drones will carry torches and fly into formations based on coordinate-based configuration data sent to each drone individually during the show.
(i) The engineer has selected DSR as the routing method being used by the swarm. By making reference to the RREQ and RREP messages, and to
the content of the data packet headers, explain how the controller on the ground can send a message to an individual drone within the swarm. [3]
(ii) The swarm displays a very high degree of mobility, suggest two problems may the engineer encounter with the use of DSR in this context. [2]
(iii) DSR is not performing well in trials of the swarm, and the engineer opts instead to adopt a simple flooding strategy to the routing of instructions. Why might flooding prove more effective than DSR in this case? [2]
(iv) Would AODV routing have been likely to have performed significantly better than DSR in this context? Justify your answer. [3]
AODV is used as the routing protocol in ZigBee, a communications standard for low-power, low-data rate devices, such as wireless light switches.
(i) Why would DSR’s approach to storing route information likely be inefficient in the case of this type of device, and how does AODV attempt
to address the problem? [2]
(ii) Explain how AODV makes use of sequence numbers to reduce the risk
of stale routes being used to direct messages. [3]
(c)
Assessment Code Page 6 of 7 TURN OVER
Assessment Code + Computer and Communication Networks
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