Question 1

Consider the network depicted in Figure 1. Assume that each link in this network has unavailability o.4%. Calculate the end-to-end availability of the connections from A to B, A to C and B to C.

Figure 1. A network

Calculate the unavailability manually, using the perfect middle method, and also by means of the Netml system.

Figure 2. A proposed design

Question 2

Suppose the end-to-end unavailability desired for a certain network is 0.00001, i.e. 99.999% (5 nines availability). The unavailability of the links is known to be 0.001. Which of the two designs shown in Figures 2 and 3 would you choose. Explain your answer. Also estimate how much difference in cost there is between the two networks as a percentage. You may assume that the cost of each link is a fixed value X.

Figure 3. An alternative design.

Criteria

1. In Question 1, the NetML system has been used correctly and the results reported clearly, using the labels of the traffic streams, in the assignment.

2. In Question 1, a simplified diagram is provided in which all serial links are reduced to a single link, labelled with their combined unavailability.

3. In Question 1, a simplified diagram is provided, for each of the three cases (A to B, A to C, B to C), in which there are just the origin, destination, and a middle node are present, with the correct unavailabilities for each of the simplified links.

4. In Question 1, the correct answers have been obtained for all three cases by manual calculations.

5. In Q2, whether or not each design meets the desired standard has been identified.

6. In the case where one or both meets the standard, the preferred design has been selected, or, if neither meets the standard, an alternative design has been suggested.

7. A sound explanation has been provided for why the selected network meets the standard and is the best choice.

8. The cost difference between the preferred design, and the alternatives, assuming cost is proportional to the number of links, has been evaluated.