UNIVERSITY OF ESSEX
Undergraduate Examinations 2014
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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
2 CE321-6-AU
Candidates must answer ALL questions. Question 1
(a) Briefly describe the frame-relay header and thus describe how it is used to switch data-frames.
(b) A network provider is using an optical fibre to provide long-distance connectivity at approximately 10 Gbit/s using a synchronous digital hierarchy (SDH) STM-64 line- rate. The operator wishes to increase this capacity to beyond 100 Gbit/s but without having to install additional optical fibre. Propose how this may be achieved and briefly describe how the mechanism you propose operates.
(c) You are given the choice of using password authentication protocol (PAP) or challenge handshake authentication protocol (CHAP) when designing a new point- to-point protocol (PPP) system. Briefly describe and contrast PAP and CHAP and thus propose which authentication protocol should be used.
[10%]
[10%]
[10%]
(d) A small company is supplied with one IP address by an Internet Service Provider but
[10%]
wishes to have many internal PCs that can access the Internet. Propose a mechanism that allows this and briefly describe how it operates.
Question 2
3 CE321-6-AU
A company of 300 employees purchases telephone lines from a service provider in bulk trunks of units of 10 lines, i.e. it can purchase 10, 20, 30 etc. The company only needs the phone lines during the business hours of 9 am-5 pm. During business hours the mean phone call duration is 2 minutes and there are 50 new calls each minute. However, during the free hour, 5 pm-6 pm, they make the phone lines freely available for the employees to use for their own personal use and this proves to be a popular perk. During the free hour they expect calls to have a duration of 2 minutes and there to be 60 new calls each minute. During business hours the blocking rate is to be 0.01 or lower, but during the free hour the blocking rate may be as high as 0.1.
(a) Calculate the traffic volume in Erlang during both business hours and the free hour. [7%] You must show your working out and define the units used in the calculation.
(b) Using your answer from part (a) and the Erlang blocking probability graph given in [3%] Figure 1 on the next page, determine how many lines the company should purchase.
You should explain how you have derived this answer.
The service provider multiplexes the company¡¯s telephone lines, together with those of other companies that are in the same industrial zone, using a synchronous digital hierarchy (SDH) based transport system with an STM-1 line-rate of 155.52 Mbit/s.
c) Explain how SDH is used to carry telephone signals. Your explanation should [10%] include basic numerical parameters of the SDH frame with an STM-1 line-rate and
explain how lines from different companies are identified.
The service provider decides that an STM-1 line-rate is too low to serve all the companies in the growing industrial zone and decides to provision the next highest SDH line-rate. All the companies in the industrial zone use the same number of lines as you calculated in part (b).
d) Using your answers from parts (b) and (c), determine how many similar size [10%] companies can be provided with phone lines using both the STM-1 line-rate and the
next largest SDH line-rate.
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 network operator provides interconnection between multiple sites of a customer using a virtual private LAN service (VPLS). The network operator provides this service to many customers. The VPLS system uses multi-protocol label switching to switch packets between provider edge routers through label switch routers.
(a) Explain how MPLS switches packets through a label switched path. Your explanation should include a description of the MPLS shim label.
(b) Explain how the MPLS label distribution protocol (LDP) configures the MPLS switch tables in the MPLS routers.
(c) Explain MPLS label stacking and propose how it can be used to provide the VPLS service.
[10%]
[10%]
[10%]
END OF EXAM PAPER CE321-6-AU