UNIVERSITY OF ESSEX
Undergraduate Examinations 2018
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 operator is proposing to use a generic routing encapsulation (GRE) tunnel as a form of virtual private network (VPN) for its customers. Briefly describe how GRE operates and discuss its appropriateness as a VPN solution.
(b) Ring protection is frequently used in WDM/SDH networks. Select a form of ring protection that gives the best form of protection and describe how it operates.
(c) The research division of a network provider has proposed a new physical layer encapsulation for wide area networking using packet switching. They ask you to design a low-cost solution for a link-layer protocol that can carry an arbitrary network layer protocol on the new system. Propose a suitable link-layer solution and describe how it operates
(d) A network operator needs a protocol to distribute the labels within its multi-protocol label switching (MPLS) network. Select a suitable protocol and briefly describe how it operates.
[10%]
[10%]
[10%]
[10%]
3 CE321-6-AU
Question 2
A network operator is using multi-protocol label switching (MPLS) as its main switching/routing technology to provide wide area networking (WAN) for a large number of companies.
(a) Describe how MPLS encapsulates protocol data units (PDUs) in Ethernet. Your description should include an explanation of how label stacking can be achieved and why label stacking might be used.
The network operator wants to provide a full layer-3 WAN routing service for its customers so that the operator handles all of the routing for each customer, including the routing protocols. Generally the customers have a large number of sites to interconnect.
(b) Propose the routing and switching solution for the layer-3 WAN service and describe how it operates. Your answer should not describe the core framing again as it is described in Part a.
[12%]
[18%]
CE321-6-AU 4
Question 3
A large company, operating on a single site, requires telephone access for a small tele-sales team of 20 staff. In the busiest hour the mean number of calls are 300 per-hour and the mean duration of each call is 2.5 minutes. The company can purchase telephone trunks from their network operator in bundles of 5 (i.e. 5, 10, 15 etc.). The manager of the tele-sales team has specified that staff should be able access the outgoing trunk lines at least 99% of the time.
(a) Using the calculation of call volume in Erlang, and the graph of Erlang-B blocking probability in Figure 1 on the following page, determine the number of trunks lines that the company should purchase to meet the required specification for the tele-sales staff. You must describe how you arrived at your answer.
The operator carries all of the company’s telephone lines in an synchronous digital hierarchy (SDH) transport network.
(b) Describe how a single telephone signal is carried in an SDH STM-1 container. Your answer should also derive the overall STM-1 bit-rate and the bit-rate that STM-1 allows for payload data.
Within the company’s internal network, the telephone signals are carried using voice over IP (VoIP) and combined with overall data traffic. The company has a complex network to serve a much larger number of users than the tele-sales team. The network mixes three types of traffic:
• general web-browsing traffic for general staff use and providing external web-services; • video for staff training and technical support information; and,
• the VoIP traffic.
(c) Select and specify the quality of service (QoS) support mechanisms for the internal company network. Your answer should specify both the mechanism used in routers/switches to give the different traffic flows an appropriate QoS and how this QoS is controlled.
[10%]
[10%]
[10%]
5 CE321-6-AU
1e+00
1e−01
1e−02
1e−03
1e−04
0 1 2 3 4 5 6 7 8 9 1011121314151617181920
Erlang E
T=5
T =10
T =15
T =20
T =25
T =30
T =35
Figure 1. Showing blocking probability P(E) for traffic volume, E, in Erlang and for a certain number of trunk lines.
END OF PAPER CE321-6-AU
Blocking Probability P(E, T)