Digital Communications and Network Security Exam

Candidates are reminded that the major steps in all arithmetical calculations are to be set out clearly.

You should submit your answers as a single Word document via Canvas. Indicate the questions you have answered on the first page of your document.

If you include graphics in your answer, please embed these into the Word document (e.g. a photograph of a hand-drawn graphic). The source of any copied and pasted figures should be cited. If you have embedded any scanned/photographed diagrams and graphs, then your submission file cannot exceed 100MB.

This is an open book assessment so you may consult your notes, textbooks and the Internet.

You must not collaborate with someone else on this assessment, it should be wholly your own work. Your work will be checked for evidence of plagiarism and/or collusion using Turnitin.

Please make sure you regularly save your work and leave plenty of time to upload your work before the deadline. Late submissions will score zero.

Candidates are permitted to use a calculator in this examination.

SECTION A

1. Fundamental security aspects of the TLS protocol heavily rely on the concepts of digital signatures and certificates.

(a) Explain the aim of digital signatures as a cryptographic technique in general, and for their use in public key certificates more specifically.

(b) Alice uses the Google Chrome web browser in order to establish a secure TLS session to the BBC's website bbc.co.uk. The BBC's web server stores a certificate, provided by its Certificate Authority (CA). Draw and annotate a diagram, showing the interaction between all four parties when creating, distributing and validating the certificate using TLS.

(c) Start up a web browser in order to view the BBC's website and its certificate. Use this to retrieve the name of the BBC's CA, briefly research its history online and write down your findings. Furthermore, what are the values of the BBC's and its CA's public keys (it is sufficient to give the last 5 hexadecimal digits of these values)?

2.(a) Describe and explain the main components of a Digital Communications system. Your answer should include diagrams and explanations of each stage from message signal input to processing of the received signal.

(b) Explain with the aid of diagrams why windowing functions may be applied on signals prior to evaluating the spectral content of the signal.

If you use MATLAB or Simulink to help answer this question you should explain the MATLAB code and/or Simulink model as well as also uploading the .m, .mlx and/or .slx file(s).

3. (a) Draw and explain constellation diagrams for amplitude shift keying (ASK) on off keying, binary phase shift keying (BPSK), and quadrature phase shift keying (QPSK) signals and explain how these modulation schemes can be implemented.

(b) Discuss the likely sources of noise in communications schemes and with the aid of constellation diagrams, explain how noise affects each of the modulation schemes in part (a).

If you use MATLAB or Simulink to help answer this question you should explain the MATLAB code and/or Simulink model as well as also uploading the .m, .mlx and/or .slx file(s).

SECTION C

4. (a) A PC is browsing a website on a server. The TCP window size is 4000 Bytes. The server is sending the website information using 1000-Bytesegments. How many segments will the server send before it requires an acknowledgment from the PC? Explain what will happen if one of the segments is lost due to congestion in the network?

(b) Describe why an application developer might choose to run an application over UDP rather than TCP.

5. (a) Suppose an ISP assigns the following block of addresses: 172.16.20.0/24. You want to create four (4) subnets from this block, with each block having the same number of IP addresses. What are the addresses of these subnets (give your answer in the form of a.b.c.d/x)? What are the maximum number of IP addresses you can have in each new subnet?

(b) Describe four main important benefits of migrating from IPv4 to IPv6.