UNIVERSITY OF ESSEX
Undergraduate Examinations 2013
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NETWORK ENGINEERING
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Time allowed: TWO hours
The following items are provided:
Graph paper (available on invigilator’s desk)
Candidates are permitted to bring into the examination room:
Hand-held, non-programmable calculators (containing no textual information).
Candidates must answer ALL questions.
The paper consists of THREE questions.
All 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
Candidates must answer ALL questions.
2 CE321-6-AU
Question 1
(a) Briefly describe the point-to-point protocol and give one example of its use.
(b) A network provider wishes to protect its optical network against up to two simultaneous link failures. Propose, and briefly describe, a system that can provide this feature.
(c) Briefly describe the form of network address translation (NAT) often called port address translation or NAT overload.
(d) Briefly compare IPv4 and IPv6 and describe the primary reason that networks are moving towards using IPv6.
[10%]
[10%]
[10%]
[10%]
Question 2
3 CE321-6-AU
A company has been using traditional, circuit switched, telephony systems for a number of years as part of its core business. The company has a central office with 100 users that regularly use the telephone.
(a) If the mean call-holding time is 3 minutes and 10 new calls are made each minute, [8%] calculate the call volume in Erlang. Explain what this measurement of traffic means
and why the traffic volume in Erlang has no units.
The company purchase telephone circuits from their network provider that provides a bulk circuit trunk in size increments of 10 voice circuits, i.e. they can buy 10, 20, 30 etc. It is determined within the company that no more than 1% of calls should be blocked because the outgoing bulk circuit trunk is full.
b) Using the Erlang-B blocking probability graph in Figure 1, determine the size of the [5%] bulk circuit trunk that the company should purchase to meet the traffic and blocking requirements above. Explain how you have derived your answer.
(c) Explain how each voice circuit is represented in synchronous digital hierarchy [13%] (SDH) within a synchronous transport module level-1 (STM-1) used by the network provider. Your explanation should use a diagram to show the frame format and
relate timings within the frame format to the relevant parameters of the voice circuit.
The company is expanding and needs to move to a much larger call volume but wants to cut its costs for this service. The network provider suggests using a voice over IP (VoIP) solution where one higher-bandwidth circuit carries all the voice calls and data connections for the company.
(d) Comment on why it is more cost effective for the network operator to offer this [4%] service and thus pass on the lower cost to the company.
Question 2 continues…
Question 2 (continued)
4 CE321-6-AU
Figure 1. Blocking probability, P(E,T), against traffic volume E in Erlang, for the available trunks sizes T.
Question 3
5 CE321-6-AU
A bank is currently using Frame Relay as its WAN access technology to interconnect its central office, 100 branches and three regional centres. Nearly all of the traffic to/from the branches is between the branches and the central office. The three regional centres each require a high volume of traffic to/from the other regional centres and the central office. All of the traffic is using IP and the bank wishes to have a simple interconnectivity of its IP routers at each location.
(a) Briefly explain the difference between a star, mesh and partial mesh topology as [7%] used in a Frame Relay network. Thus determine, and explain, which of these is the
best topology for the company.
(b) Describe, with the aid of a diagram, the Frame Relay header and how it encapsulates [12%] IP packets. Use this description to explain how a router at the central office and a
network operator¡¯s switch can distinguish between Frame Relay frames that need to
be sent to different locations.
The bank¡¯s network provider is migrating towards using Ethernet as its access technology. The network operator provides this Ethernet connection using multi-protocol label switching (MPLS) that also uses Ethernet as its underlying link-layer technology.
(c) Briefly describe the main fields in the MPLS Ethernet shim label. Thus determine [6%] the maximum number of label switched paths (LSPs) that can be maintained at any
given time in any given link with this header format if no label stacking is used.
(d) Explain why MPLS is a better solution for the bank¡¯s network than Frame Relay. [5%]
END OF EXAM PAPER CE321-6-AU