Solve the following Questions giving clear Answers.

Which situations cause Silly Window Syndrome (SWS)? What is the impact of SWS on network throughput?

(i) Objects: Files in a commercial, distributed, Internet-based le service. Policy: The owner may authorise other principals to download the le. (ii) Objects: Sales data relating to a company. Policy: Those employed in the Sales Departments of all branches of the company worldwide may read the data. (iii) Objects: Electronic health records (EHRs) in a nationwide service. Policy: The owner (patient) may read from its own EHR. A qualied and employed doctor may read and write the EHR of a patient registered with him/her. (iv) Object: The solution to online coursework. Policy: The coursework setter has read and write access. A candidate has no access until after the marks have been published.

A network-based service manages persistent objects. The service must enforce an access control policy to protect the objects. (a) Discuss how this access control might best be implemented for the following example of objects and policy components: Objects: Files in a University Department's le service, operating behind a rewall. Policy: The owner may specify read, write and execute rights in terms of principals and groups. [4 marks] (b) Discuss how this access control might best be implemented for two of the following examples: [8 marks each] (a) Describe the way in which a problem should be represented in order to allow its solution using a heuristic search technique. [5 marks] (b) Dene what it means for a search algorithm to be complete, and to be optimal. [2 marks] (c) Dene what it means for a heuristic function to be admissible, and to be monotonic. [2 marks] (d) Describe the operation of the A? heuristic search algorithm. [5 marks] (e) Prove that the A? heuristic search algorithm is optimal when applied in conjunction with a monotonic heuristic. State the conditions under which the algorithm is also complete, and explain why this is the case. [6 marks] (a) Dene formally the semi-structured data (SSD) model. [5 marks] (b) Show how SSD expressions can be expressed in XML. [2 marks] (c) What are the main dierences between the SSD and XML models? [2 marks] When viewed graphically, simple SSD expressions denote trees. Consider a variant, d-SSD, where the edges emanating from any node in the tree must have a unique label, but where the labels may themselves be d-SSD expressions. (You may disregard object identities (oids); hence d-SSD expressions always denote trees.) (d) Dene the syntax of d-SSD expressions. [3 marks] (e) Give a d-SSD expression to represent the following: (i) the array ["Do","Re","Mi"]; (ii) the set {11,52,44}; (iii) the bag {{10,10,13,42,13,10}}. [1 mark each] (f ) Dene the syntax for path expressions in the d-SSD model. [2 marks] (g) Hence describe precisely how the d-SSD model can be extended to represent graphs.

Explain what impact each of the following can have on reliability, development costs, programmer productivity and software eciency: (a) top-down renement; [4 marks] (b) the Z specication language; [4 marks] (c) loop invariants; [4 marks] (d) structural induction; [4 marks] (e) the choice of programming language for use in a project. [4 marks] You may wish to explain what the alternatives or opposites to the concept discussed are, or give brief examples. (a) Prove that if L is a regular language, its complement is also regular. [6 marks] (b) For each of the following languages over the alphabet {a,b}, state whether or not it is regular and justify your answer. (i) {w | w is not a palindrome} (ii) {ak | k is a multiple of 3} (iii) {ak | k is prime} [14 marks] A faulty component is drawn at random from a pile of rejects. Use Bayes's Theorem to determine the probabilities that the faulty component was fabricated by machines A, B and C respectively. Express your answers as fractions. [9 marks] (c) Six faulty components are drawn at random from a pile of rejects. What is the probability that two were fabricated by machine A, two by machine B, and two by machine C? Your answer should be written as an expression which may incorporate the values determined in part (b). [6 marks]