UNIVERSITY OF ESSEX
Undergraduate Examinations 2016
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) Point-to-point protocol (PPP) is used in a wide range of network connection scenarios. With reference to the features of PPP, analyse why it is so widely used.
(b) Internet protocol security (IPsec) and generic routing encapsulation (GRE) are two protocols that can be used for tunnelling. Briefly compare the two protocols and give one example tunnelling application for each of the protocols.
(c) A company wishes to have private addresses allocated to a large number of clients and two servers. The clients are required to access HTTP services in the Internet and the two servers are required to serve content to clients hosted in the Internet. The company considers both static network address translation (NAT) and port address translation (PAT) as NAT solutions. Briefly contrast both static NAT and PAT and thus propose which is suitable for the company clients or servers respectively.
(d) A network operator provides an optical network to interconnect a number of hospitals in a single geographical region. The optical network requires very high reliability as it will be used by surgeons assisting remote surgery through live video conferencing. Propose a suitable optical protection mechanism and describe how it operates.
[10%]
[10%]
[10%]
[10%]
3 CE321-6-AU
Question 2
A network operator provides customers with virtual connections at a standard line rate of 2.048 Mb/s (E1) using Frame Relay.
(a) Briefly describe how a network operator identifies customer traffic in a Frame Relay frame. Your description should refer to any important fields in the Frame Relay header.
Although the customers’ access to the network is provided using Frame Relay, the operator transports these 2.048 Mb/s (E1) Frame Relay connections using a synchronous digital hierarchy (SDH) network.
(b) Describe the SDH network STM-1 container and thus propose how Frame Relay connections can be carried in an SDH network.
The network operator provides a guaranteed bit rate as a committed information rate (CIR), but allows customers to offer higher traffic rates up to the excess burst size for free. However, traffic offered above the CIR may, sometimes, be dropped; all traffic beyond the excess burst size is dropped.
A customer using the Frame Relay connection has two types of traffic:
(i) an essential video conference stream that has a traffic rate below the CIR (ii) general data traffic using TCP/IP that can use any available bandwidth.
(c) Propose how both the operator and customer can manage this traffic using Frame Relay as the access technology.
[5%]
[10%]
[15%]
CE321-6-AU 4
Question 3
A network provider offers 100 Mb/s Ethernet connections to some business customers as a virtual private LAN service (VPLS). The VPLS service is delivered using the operators multiprotocol label switching (MPLS) network which operates between provider edge routers. Multiple customers connect to each provider edge router.
(a) Describe the format of an Ethernet frame used within the operator’s MPLS network. Thus, [10%] explain how frames are switched between provider edge routers.
(b) Explain how MPLS provides label stacking. Thus, by referring to the VPLS service, [5%] explain why label stacking is necessary in this scenario.
A set of 16 business customers each connect to a provider edge router which has a total of
2 Gb/s link to the main operator network, leaving 4 × 100 Mbs spare as four circuits that can be shared. The operator offers the customers shared use of these additional 4 circuits for overnight database backups between midnight and 8 am. These database backups take an average of two hours and, on average, two customers an hour start a database backup at a random time during the night. The customers get access to the “spare” circuits on a first-come-first-served basis.
(c) Calculate the traffic volume for the overnight database backups. Specify the units in all [4%] parts of the calculation.
(d) Using the Erlang blocking probability given to you in Figure 1 (shown on the following [4%] page) estimate the blocking probability that the customers will observe using these “spare” circuits. You must explain how you have obtained your solution.
(e) If the customers wanted to achieve a blocking probability of 0.001, how many “spare” [4%] circuits would be needed?
(f) Consider the scenario and your answers in (c)–(e). Propose a change to the customers [3%] behaviour so that the customers can achieve the database backups overnight without any
blocking. Thus, briefly explain the essential assumption that Erlang blocking probability
makes with regard to the customers behaviour.
5 CE321-6-AU
1e+00
T=1
T=2
T=3
T=4
T=5
T=6
T=7
T=8
T=9
T =10 T=
11
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Erlang E
Figure 1. Showing blocking probability P (E , T ) for traffic volume, E , in Erlang and a number of circuits, T .
END OF PAPER CE321-6-AU
Blocking Probability P(E, T)