UNIVERSITY OF ESSEX
Undergraduate Examinations 2017
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 following items are provided:
Graph paper (available on the invigilator’s desk)
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) Frame relay can provide information for flow control to help with quality of service. Describe how frame relay provides this function.
(b) A company wants to provide highly secure transport between two branch offices over a wide area network ensuring both confidentiality and integrity. The company has a requirement that any IP service may be carried and that no specialist network security software is installed on any branch servers or clients. The company wants to ensure that the packets, including the IP headers, are not changed in any way during the transport. Propose a solution and briefly describe how it operates.
(c) A company has been issued with six public IP addresses by a network operator. The company has a large number of internal IP clients and four servers that all use private IP addresses. The company wants all clients to be able to access the Internet and to allow the Internet to access the four servers. Propose a solution and describe how it operates.
(d) A new physical layer circuit switched system is to be used by an operator as an experimental customer service. It does not yet have a link-layer design. The link-layer protocol must be capable of carrying a number of different higher layer protocols and must provide authentication. Propose a suitable solution and describe how it operates. Your solution should state which parts of the protocol would need to be modified to work with the new physical layer system.
[10%]
[10%]
[10%]
[10%]
3 CE321-6-AU
Question 2
A company runs a number of call centres using telephone lines provided by a network operator. The operator provides the circuits in E3 circuits that are multiplexed within an STM-1 SDH container.
(a) Even though modern networks are dominated by high-volume data traffic, much of the [5%] underlying circuit-switched network data-rates are derived from multiples of the 64 kb/s
telephony channel bit-rate. Explain how this bit-rate was derived.
(b) Explain how a voice channel is carried within a SDH STM-1 container. Thus, derive both [10%] the total bit-rate of an STM-1 container and the useful throughput of traffic within an
STM-1 container.
The tele-sales call centre has 2000 staff making sales calls. The company wants to make sure these calls get through to potential customers, thus they require a maximum blocking rate of 0.5%. The mean number of sales calls per-minute is 700 and the mean duration of these calls is 2 minutes. Each E3 circuit provided by the operator carries 630 voice calls.
(c) Calculate the traffic volume in Erlang and thus determine how many E3 circuits the [7%] company needs to rent to meet the required blocking rate. You will need to use the Erlang
blocking probability graph shown over the page in Figure 1. You must explain all of your
working out and how you have used the graph.
The complaints division is located at a different site to the tele-sales division and has 1000 staff answering calls. The mean number of complaint calls is 50 per-minute and the mean call length is 50 minutes. The company has purchased 2 × E3 circuits for telephone connectivity to the complaints division. If a member of staff is not available to answer the call, the call is immediately terminated.
(d) Comment on the blocking probability in the complaints division in particular describing the [8%] limiting factors on calls being able to get through.
CE321-6-AU
4
1.0
0.5
0.1
0.05
0.01
0.005
0.001
0.0005
1 E3
2 E3
3 E3
4 E3
5 E3
6 E3
Blocking Probability P(E)
0.0001
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Erlang E
Figure 1. Showing blocking probability P(E) for traffic volume, E, in Erlang and for a certain number of E3 circuits.
5 CE321-6-AU
Question 3
A company wishes to connect a number of branch offices and the company headquarters (HQ) using a service provided by an operator. The company has a number of requirements for this connectivity:
• the branch offices will be connected in a partial mesh but all branches will be connected to HQ.
• a cost effective solution should be used, so leased lines are not appropriate, i.e. the underlying operator network should share different customer’s traffic on the same network using packet/frame switching of some kind.
• bit-rates between the branches vary between 100 Mb/s and 10 Gb/s.
• there are 2500 branches to be interconnected.
• each connection requires differentiated traffic for three different classes of service: best-effort IP data traffic; voice-over-IP; and, video (for training and meeting videos).
• each branch office has between 50-2000 connected IP clients and servers and a number of different internal LANs.
• the operator should provide segmentation of traffic between different customers as the company does not want the overhead of providing their own security solution.
(a) Determine how many virtual paths or connections are required at HQ and thus propose an [4%] operator connection solution that matches the requirements.
(b) Propose a network design for the above network. The design must specify and briefly [26%] describe major network protocols used by both the company and the operator to control the
network and carry the data traffic. Your answer must describe the devices used to connect
the customer and the operator.
END OF PAPER CE321-6-AU