UNIVERSITY OF ESSEX
Undergraduate Examinations 2019
Network Engineering
Time allowed: TWO hours
Candidates are permitted to bring into the examination room:
Calculator – Casio FX-83GT PLUS or Casio FX-85GT PLUS 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.
Please do not leave your seat unless you are given permission by an invigilator.
Do not communicate in any way with any other candidate in the examination room.
Do not open the question paper until told to do so.
All answers must be written in the answer book(s) provided.
All rough work must be written in the answer book(s) provided. A line should be drawn through any rough work to indicate to the examiner that it is not part of the work to be marked.
At the end of the examination, remain seated until your answer book(s) have been collected and you have been told you may leave.
CE321-6-AU
CE321-6-AU 2 Question 1
(a) An optical network is to be provided for an experimental remote surgery application where the surgeon will remotely control a robot used for surgery. Select a suitable protection mechanism for this network and explain how it operates.
(b) Explain how synchronous digital hierarchy (SDH) carries both voice and data traffic. Your answer must show how the frame format of a STM-1 SDH frame is organised and consequently derive both the full STM-1 bit-rate and the useful bit-rate for carrying data/voice traffic.
(c) Point-to-point protocol (PPP) and high-level data link control (HDLC) are widely used protocols. Explain the key similarities and differences between them. Consequently, explain which is most appropriate for use in customer access connections.
(d) Domestic broadband access often uses one of two technologies: • asynchronous digital subscriber line (ADSL)
• cable
Contrast the two technologies and thus explain the key differences in performance between the two systems.
[10%]
[10%]
[10%]
[10%]
3 CE321-6-AU
Question 2
A network operator provides 25 data circuits, each with a capacity of 100 Mb/s as an on-demand service to customers at a business park. These data circuits are designed to provide extra capacity on a first-come-first-served basis to customers that need them; they are in addition to permanent network connections that the customers use. Customers are billed per-hour of usage for these on-demand circuits as an incentive to only use them as needed. Only one customer at a time can use a single data circuit.
(a) If customers demand the data-circuits with a mean holding time of 2 hours and mean arrival rate of 6.5 per hour, calculate the traffic volume in Erlang. Furthermore, using the graph in Figure 1, estimate the blocking probability that customers will observe. Explain how you have achieved your answer and what assumptions are required.
The network operator notes that there is a plan to double the size of the business park and aims to maintain the on-demand service with the same blocking probability as you determined in Part (a).
(b) Determine how many circuits are needed for the increased business park. You should assume that the operator can only choose the number of circuits as a multiple of 5 as shown by the T values on the curves of Figure 1. You must explain how you have come to your answer and any assumptions you have made.
One particular customer has the following requirements for external traffic:
• voice-over-IP only during weekdays 8am to 6pm
• data for a daily offsite backup of the customer’s database
• non-essential web-browsing for office workers during weekdays 8am to 6pm
• web-traffic for the main company sales HTTP server at the customer’s premises.
The customer does not want to pay for one of the extra data circuits but finds that voice-over-IP service is poor and that visitors to the sales HTTP server report it is slow to respond even though it is a server with plenty of CPU/disk capacity.
(c) Propose how the customer should solve the reported service problems through changes to router configurations.
[10%]
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CE321-6-AU 4
1e+00
1e−01
1e−02
1e−03
1e−04
T=5
T
=10
T =15
T
=20
T =25
T =30
T =35
T
=40
T =45
T =50
0 2 4
6 8
10 12 14 16 18 20 22 24 26 28 30
Erlang E
Figure 1. Showing blocking probability P (E , T ) for traffic volume, E , in Erlang and for a certain number of data circuits T .
Blocking Probability P(E, T)
5 CE321-6-AU
Question 3
A company is growing from a single campus to extend to two other new locations. The campus uses a number of VLANs and the company wishes to extend these VLANs to the two new locations. Low cost is a factor in the decision for the wide area networking (WAN) solution that the company wants to implement. The two new locations are too far away to put in their own fibre so the company approaches a network provider that sells the following three services:
• leased lines between every pair of sites over a SDH network • MPLS L3 VPN
• MPLS L2 VPLS
(a) Briefly explain the capabilities of three services above in simple terms, as you may explain to a high-level technical manager who is not familiar with the terms. Thus, select which is most appropriate for the company to use. You should expand the abbreviations as part of you explanation.
(b) Explain how the system you have chosen in part (a) operates. Your answer must describe the frame/packet format and the components required in the network.
END OF PAPER CE321-6-AU
[10%]
[20%]