UNIVERSITY OF ESSEX
Undergraduate Examinations 2015
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 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) A company wishes to provide a virtual private network service to its workers while they are working from home. Briefly describe a suitable protocol that is an open standard. Your description should make it clear how the protocol provides integrity and confidentiality of the data and authentication of the sender.
(b) A company network requires all its clients and servers to use private IP addresses in the internal network. However, the servers need to be accessible from the public Internet and the large number of clients need to be able to access the public Internet. Describe how both internal clients and servers can be connected to the Internet.
(c) A network provider is considering using either standard high-level data link control (HDLC) or the point-to-point protocol (PPP) as the link layer protocol to carry customer traffic over a new type of optical access network. The link layer protocol needs to carry a number of layer-3 protocols (e.g. IP and some control protocols) and provide authentication of customers. Discuss whether HDLC or PPP is most suitable for this application. Your discussion should describe the key features of each protocol that influence the decision of which protocol to use.
(d) When connecting customer equipment an operator can consider a number of options of topology including:
• full-mesh;
• or, star;
• or, a combination of mesh and star.
Consider a telephone network where telephones may need to connect arbitrarily to any other telephone but with two very different scenarios:
i a large number of telephones all in the same small building i.e. a private telephone system;
ii or, a very large number of telephones distributed throughout the UK where customers are mostly concentrated close to large cities.
Taking into account switching and transport costs determine the most appropriate topology to use in each of the two scenarios. You must explain your decision.
[10%]
[10%]
[10%]
[10%]
3 CE321-6-AU
Question 2
A network operator is considering upgrading from Frame Relay to multi-protocol label switching (MPLS) over Ethernet as a mechanism to connect customer sites and support new traffic requirements. However, they are unsure if this is wise given the additional cost. You will be considering aspects of the network design that may influence this decision.
(a) Briefly describe the main difference between destination based forwarding, as used by IP, [4%] and label swapping, used by MPLS (and Frame Relay)
A 9 Gb/s to H D 10
10
10 10
10 3GH
C 10 B 2 Gb/s to H
E 10 F Figure 1
The network shown in Figure 1 is part of the operator’s network and has the available capacity marked on each edge in Gb/s. The arrows indicate the traffic to be offered from either A or B towards H, there is no other traffic in the network. The traffic will be routed using either:
• OSPF shortest-path based routing (single path);
• or, OSPF with multi-path routing extension (traffic is shared equally over two or more
paths);
• or, MPLS using manually routed label switched paths (LSPs).
(b) For each of the above routing mechanisms, determine the distribution of traffic in the [7%] network and thus describe which is the most suitable routing scheme.
The operator is considering launching a new service to carry voice-over-ip (VoIP) for its customers. This new service will require each call to be uniquely identified using either a Frame Relay data link connection identifier (DLCI) or a MPLS Ethernet shim label. If Frame Relay is used, the address field that contains the DLCI field is two-bytes long. The operator expects that in the network shown in Figure 1 it may have to carry up to 106 voice calls over the link G-H.
(Question 2 continues over the page)
CE321-6-AU 4
(c) By analysing the capabilities of Frame Relay and MPLS to uniquely identify and switch the [19%] voice calls, determine whether an upgrade to MPLS is required. Your analysis should
include a description of the relevant fields in the Frame Relay or MPLS headers and how
those fields are used.
5 CE321-6-AU
Question 3
A network operator is providing connections for experimental services using free capacity on its switched SDH network. The experimental services use an STM-1 (approx. 155 Mb/s) for each connection in a transport network that uses multiple STM-64 (approx. 10 Gb/s) bearers. The experimental services each last on average for 20 hours and there are on average 2 started in each hour. As the services are only for experimental use the network operator is only prepared to guarantee a blocking probability of P = 0.1.
(a) Calculate the traffic volume in Erlang for the experimental services. Specify the units in all [6%] parts of the calculation.
(b) Assuming that the operator has to allocate the STM-1 circuits in blocks of ten, i.e. there [6%] have to be 10, 20, 30 etc., how many circuits does the operator have to allocate to achieve
the traffic volume you have calculated to achieve the required blocking probability? You
will need to use the graph shown in Figure 1 (shown on the following page) to determine
the answer and you must explain how you have obtained your solution.
(c) By explaining the STM-1 frame format, determine the maximum payload data rate that [9%] each experimental service can carry, assuming that the overhead of the STM-1 is not used
to carry payload traffic. Your answer should be accurate to 1 kb/s.
(d) The operator network uses a number of interconnected SDH rings. By describing typical [9%] SDH network components explain how the operator will carry, switch and individually
provide the STM-1 circuits.
CE321-6-AU 6
1.000
T =10
T =20
T =30
T =40
T =50
T =60
T =70
T =80
0.100
0.010
0.001
0 20 40 60
Erlang E
Figure 1.
END OF PAPER CE321-6-AU
Blocking Probability P(E, T)