UNIVERSITY OF ESSEX
Undergraduate Examinations 2020
Network Engineering
Time allowed: TWO hours (exam time) + ONE hour to allow for submission time (total THREE hours)
(Please see your exam timetable or check on FASER for the deadline to upload your answers)
The times shown on your timetable are in British Summer Time (BST) (GMT+1). Please check online for a conversion to your local time if you will be undertaking your assessment outside the United Kingdom.
Candidates are permitted to bring into the examination room:
Calculator – Casio FX-83/85GT PLUS/X or Casio FX-83/85 GT PLUS/X only
The paper consists of THREE questions. Candidates must answer ALL questions. Questions are NOT of equal weight.
The percentages shown in brackets provide an indication of the proportion of the total marks for the PAPER which will be allocated.
CE321-6-AU
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Please allow at least 30 minutes at the end of your exam time to upload your work. Once you have completed the assessment do not leave it to the last minute to upload.
Please save your work throughout the examination to avoid losing your work. Please do not communicate with any other candidate in any way during this assessment. Your response must be your own work. Procedures are in place to detect plagiarism and collusion.
CE321-6-AU 2 Question 1
(a) Point-to-point protocol (PPP) is an extension of High-level data link control (HDLC). Explain the additional features provided by PPP and thus explain how PPP supports remote connections with security while HDLC does not.
(b) Synchronous digital hierarchy (SDH) is being replaced by Ethernet in core transport networks. Explain how data is carried in SDH by describing an STM-1 SDH frame and thus briefly propose why it is being replaced by Ethernet.
(c) A training company’s campus scale network requires to transmit:
• general web browsing traffic, which is not a critical service • voice-over-IP for telephone calls, which is a critical service • video delivery, which is a critical service
Propose how the network can deliver these differentiated needs and the different traffic types.
(d) A company is using generic routing encapsulation (GRE) for transmitting private addressed IP traffic between two sites that use private addresses but which are connected over the Internet which does not allow private addresses. Explain how GRE works to support this transmission by explaining the encapsulation and comment on any security issues that the company should consider.
[10%]
[10%]
[10%]
[10%]
3 CE321-6-AU
Question 2
A network operator provides an optical fibre that uses DWDM for carrying video traffic for video contribution providers. The video contribution providers rent one DWDM “circuit” for the time that they need the circuit and release it when they have finished contributing the video. There are a large number of video contribution providers but on average there are 50 requests an hour to use a circuit and on average they need the circuits for 1 hour and six minutes. The network provider guarantees that the circuits will be available for 99.9% of the time.
(a) Explain what the abbreviation DWDM refers to and briefly describe how it operates. [5%]
(b) Calculate, in Erlang, the traffic volume offered by the video contribution providers. [10%] Furthermore, using the graph in Figure 1, estimate how many circuits the network operator
will need to meet the specification, assuming that the circuits are available in multiples of 10
circuits. Explain how you have achieved your answer and what assumptions are required.
(c) The network operator has a failure in 10 of the circuits i.e. there are 10 fewer than the [5%] number you calculated in part (b). Determine the blocking probability in this failure
situation using the graph in Figure 1. If the relevant graph intercept falls between ranges
you should state the range of the answer from the two closest intercept points. You must
explain how you have come to your answer.
(d) Propose a protection mechanism to overcome failures that include either a single failure in [10%] an optical system or a single failure in an optical fibre.
CE321-6-AU
4
1e+00
1e−01
1e−02
1e−03
T =10
T =20
T =30
Blocking Probability P(E, T)
T =40
T =50
T =60
T =70
T =80
T =90
1e−04
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Erlang E
Figure 1. Showing blocking probability P (E , T ) for traffic volume, E , in Erlang and for a certain number of circuits T .
5 CE321-6-AU
Question 3
A network operator wishes to provide a wide area network (WAN) service for its customers. This service is to provide routing support for the customers and is to be sold as a L3 virtual private network (VPN).
(a) Propose a solution that the network operator should use to enable this service. Furthermore, [7%] compare the system you propose to one other system that could be used to achieve the same
aim and thus explain why you have selected the solution you have chosen compared to the
other. Note at this stage you are not explaining how the systems operate, rather you must
explain the high-level features that support your proposal of one system over the other.
(b) Explain how the forwarding of packets is achieved in the system you have chosen in Part (a) [15%] by describing the operations applied to a packet as it moves from one customer site, through
the WAN and finally to another site owned by the same customer.
(c) Design the routing for the system you have proposed in Part (a) and describe the interaction [8%] between any routing protocols. Your answer should explain how the customer will be able
to obtain routing for their remote sites.
Once you have completed your answers, please upload them to FASER
http://faser.essex.ac.uk
Remember to add your REGISTRATION NUMBER onto ALL documents that you upload. END OF PAPER CE321-6-AU