Unit 201 Applied thermodynamics

General comments The question paper covered the syllabus, without too much emphasis on any one part. The standard was that of a final year BEng paper. Compared with previous years the candidates now have a better understanding of thermodynamic properties such as entropy and enthalpy. However, a major failing remains for some candidates in their assumption that all compression and expansion processes are isentropic. This is so even when the question indicates that that is not the case. Comments on individual questions Question 1 - Gas turbines Most candidates did not know the relative advantages of closed-cycle gas turbines, namely, the use of monatomic gases, low molar mass gases and, therefore, good heat transfer and a clean turbine so avoiding corrosion. However, most candidates understood the advantage of heat exchange. Many candidates assumed the compression and expansion processes were isentropic. Numerical answer:

(i) 135.3 kJ/kg (ii) 0.447 (iii) 33.4%.

Question 2 - Steam cycle with reheater Only a couple of candidates were able to sketch correctly the enthalpy entropy diagram and the vast majority of candidates didn’t attempt this diagram. Many candidates assumed that the dryness fraction of the steam after the low-pressure turbine was the same as that after the high-pressure turbine. Numerical answer:

heat added = 1073 MW, heat rejected = -747.5 cycle efficiency = 30.3%. Question 3 - Convergent divergent nozzle Many candidates failed to realise that it was necessary to determine the stagnation temperature and pressure at entry to the nozzle, which was essential in order to solve the problem. Note, the velocity at entry and the corresponding static temperature and pressure were given in the question. The majority of candidates were unable to sketch the temperature-entropy diagram for the flow process. Numerical answer:

inlet area = 1.794x10-3m2, throat area = 0.890 x10-3m2, exit area = 1.558 x10-3m2, thrust force = -567.7 N.

Question 4 - Air conditioning From the basic definition of specific humidity in a humid air mixture many candidates were unable to

carry out the simple proof that aa

ss

pMpM

=ω . Not withstanding this there were many good attempts at

the remaining parts of the question.

Numerical answer:

(i) 0.5757 kg/s and 0.009192 kg/s (ii) 0.005011 kg/s (iii) 9.26 ° C (iv) -21.80 kW.

Question 5 - Vapour-compression refrigeration Any liquid refrigerant entering the compressor would tend to wash away the lubricant with consequential mechanical problems. Also if the lubricant was then carried over to the evaporator it could form a film on the tube surfaces and so reduce heat transfer. Most attempts at the numerical part of this question were quite good. Numerical answer:

(i) 4.63 (ii) 97.95 kW or 92.07 kW depending on the method chosen to determine the mass flow rate.

Question 6 - Reversible air-standard cycle Most candidates sketched the pressure-volume diagram correctly but few were successful in the case of the temperature-entropy diagram. Again, most candidates were able to obtain the cycle efficiency in terms of the temperatures at positions 1, 2 and 3 and the adiabatic constant. However, few were able to take the next step and determine the cycle efficiency in terms of the temperatures at positions 1 and 2 and the adiabatic constant only. Numerical answer:

cycle efficiency = 1 5.86%, specific work output = 159.3 kJ/kg. Question 7 - Combustion products and a turbine Very few candidates were able to determine the molar or specific heat capacity the adiabatic constant of gases. This is basic thermodynamics. Those candidates who succeeded in the above generally completed the question well. Numerical answer:

Cp = 35.35kJ/kmoIK, cp =1.258kJ/kgK, γ =1.3075, W = 7.1354MW s2 – s1 = 1.131kJ/Ks

Question 8 - Reaction steam turbine Of those candidates who attempted this question most obtained good marks. However, some candidates, though few in number, still have limited knowledge of velocity triangles. Numerical answer: (i) U =157.08m/s, V1 = 46.94m/s and ΔVω =181.38m/s (ii) 28.49 kJ/kg (iii) 58.15%, the stage efficiency = 56.45 % Question 9 - Large electricity-generating power stations Few candidates attempted this question and yet there were 12 marks available in part (ii) for discussion on the environmental impact of such power stations. Throughout the world there have been many and major discussions, well documented and debated in the world media, on these issues. Thus it is surprising how few attempted this question and of those who did, how limited were their discussions.

Numerical answer:

(i) Number of power stations = 6.51, thus in reality there would be 6 power stations (ii) Discussion to cover in some detail air pollution and river pollution and the consequential impact on plant, insect, bird and fish life and also nuclear leaks and storage.

Unit 203 Fluid mechanics

General comments Both the pass rate and average mark have improved this year compared with 2005 while the number of candidates taking the examination has also increased. Although there is still a large number of candidates with failing marks, this has been offset by many of the remainder gaining good pass marks in the range of 50 to 74. The quality of answers from the passing candidates shows a firm understanding of fluid mechanics and mostly good numerical accuracy in calculations. However, even this group suffers from calculation errors caused by an inappropriate choice of units when evaluating equations. It is important that candidates take care to balance the units and dimensions of terms used in equations to avoid producing numerically wrong and often impractical answers. Failing candidates tend to demonstrate a poor understanding of fluid mechanics and give the strong impression of having made little preparation for the examination. Answers from these candidates are frequently incomplete with a concentration on the more elementary parts of questions so that the maximum marks available to them is always low. In addition, extensive arithmetic and algebraic errors reduce further the actual marks gained. To help reduce such errors, candidates should be encouraged to check their own work as they proceed through a solution since most of the errors made are clearly visible and easily rectified. One common source of error among all candidates is a failure to recognize the difference between kinematic viscosity ν and dynamic

viscosity μ. Since ρμ=ν the numerical value for the kinematic viscosity of water is about 1000

times smaller than the value for dynamic viscosity. This can have a major effect on the answer generated if a value for kinematic viscosity is mistakenly used for a value of dynamic viscosity. Candidates must take care to read each question completely and use the information given in the question. Several examples where this was not done are highlighted later in the report. Comments on individual questions Question 1 The question involves using dimensional analysis to derive the functional expression given and then applying it to a particular experimental situation. The majority of candidates attempting this question were able to utilize Buckingham’s π theorem correctly to develop the given expression in part a). However, a significant number used incorrect dimensions for the power and/or dynamic viscosity variables in the problem and as a consequence were unable to obtain the expression required. Candidates would be well advised to check the units and dimensions of problem variables before proceeding with the dimensional analysis to avoid wasting time and losing marks. In part b), very few candidates successfully determined the power required and the velocity in the working section of the tunnel when using water as the working fluid. This was because most candidates did not seem to know how to use the expression in part a) to solve the problem posed in part b). For dynamically similar operating conditions each of the non-dimensional terms in the expression has

the same constant value for both air and water operation. Thus, waterair DuDu ⎟⎟⎠

⎞⎜⎜⎝

⎛ρμ

=⎟⎟⎠

⎞⎜⎜⎝

⎛ρμ

and

waterair uDP

uDP

⎟⎟⎠

⎞⎜⎜⎝

⎛ρ

=⎟⎟⎠

⎞⎜⎜⎝

⎛ρ 3232 These two conditions can be used with the given data to solve for the

remaining unknown variables. Some candidates wrongly chose to ignore the result of part a) altogether and tried unsuccessfully to obtain a solution using non-dimensional pump coefficients. Also, careless errors by some candidates produced other problems. For example, the numerical value given for kinematic viscosity was taken as a value for dynamic viscosity yielding a value for the required power for water operation in microwatts. Even more surprising, such values were accepted without further comment. Many candidates need a lot more more practice applying the

results of dimensional analysis to reinforce their understanding of the simplicity and usefulness of the technique. The numerical answers are:

b)(i) 1.87kW (ii) 2.84 m/s

Question 2 This question concerns the application of the Bernoulli equation, the continuity equation and the momentum equation to an ideal incompressible fluid flowing in a converging elbow to determine fluid conditions at exit from the elbow and the resulting force applied to the elbow. In part a), most candidates were able to write down one of the common forms of the Bernoulli equation and many correctly listed the conditions in which it may be used. However, a significant number of candidates continue to confuse the Bernoulli equation which only applies to ideal fluid flow with the energy equation which applies also to real flows and includes loss terms due mainly to fluid frictional effects. In part b)(i) most candidates were able to correctly use the Bernoulli and continuity equations to obtain the velocity and pressure at outlet from the elbow. Other candidates introduced careless numerical and algebraic errors such as using the diameters of the elbow as radius values and getting the direction of elevation change wrong between inlet and outlet. Part b)(ii) requires a momentum analysis and the application of Newton’s third law to obtain the force on the elbow. While a few candidates correctly determined the force on the elbow the majority demonstrated a lack of clarity in their analysis. Many candidates seemed to be unaware that the momentum analysis is applied to the water flow and the forces used in the analysis are forces acting on the water in the selected control volume. The force applied to the water by the elbow is then used in conjunction with Newton’s third law to obtain the reaction force applied by the water to the elbow. It would be helpful if candidates were to show clearly in a diagram the forces assumed to be acting on the water and use this to determine the elbow force acting on the water and then the reaction force of the water on the elbow. Many candidates do not provide such a diagram nor apply Newton’s third law in their analysis causing confusion and uncertainty about the required force and its direction. In addition to this common analysis problem, many candidates introduce further careless errors. A common example in this question was using the wrong direction for the pressure force at the elbow outlet plane.

The numerical answers are:

b) (i) 12.73 m/s, 79.0 kN/m2

(ii) 25688 N in a direction 7.3° above the horizontal inlet flow direction Question 3 This question requires the development of a stream function in a potential flow field containing a uniform flow and a point source and using this to determine several features about the flow. Of the few candidates who attempted this question, only one achieved a passing mark Most candidates were able to derive an expression for the stream function for the flow in part a) and several also obtained the location for the stagnation point in part b)(i). However, in part b)(ii) very few candidates correctly developed the expression for the separating streamline although most were able to make a reasonable sketch of the flow field in b)(iii). No one successfully calculated the required air velocity and pressure difference in part c). Potential flow is not a popular topic with candidates and very few appear to make the effort to learn how to use the information contained in the stream function or velocity potential function to calculate the resulting velocities and pressures. The numerical answers are:

c) (i) 31.5 m/s (ii) 620.1 N/m2

Question 4 This question concerns the derivation of an expression for a fully developed laminar flow on the surface of a vertically moving belt and using it to determine several features about the flow. The analysis requires candidates to develop an equation for the variation of shear stress in the flow and to integrate this to obtain the expression for velocity distribution. Many candidates had difficulty developing the shear stress equation. Some included a hydrostatic pressure variation in the fluid

layer even though the pressure at the adjacent surface of the layer is constant. Others departed from the normal mathematical convention of assuming τ increases with increasing displacement y when considering a small element of fluid in the flow stream and obtained the shear stress equation with the wrong sign. Further errors occurred when applying the boundary conditions for the flow. A common mistake here was to assume incorrectly that fluid velocity is zero at the free surface of the fluid layer instead of using the correct boundary condition of zero drag force at the free surface

for which 0=dydu . Very few candidates successfully determined the conditional expression in

part b) which arises from the requirement that the fluid velocity 0≥u throughout the fluid layer on the belt. Almost no one calculated the belt velocity and oil flow rate correctly in part c), partly due to the introduction of simple numerical errors. This was also particularly evident in part d) which requires calculating a value for the Reynold’s number of the flow. Several candidates wrongly used the kinematic viscosity value as dynamic viscosity in the expression for Reynold’s number along with an assumed density for the oil, while others used the maximum velocity in the fluid layer rather than the mean fluid velocity. The numerical answers are:

c)(i) 1.226 m/s (ii) 2.04 x 10-3 m3/s d) Reynolds number is 20.4

Question 5 The question features a cone and plate viscometer for non-Newtonian fluids and requires development of some simple relationships for the viscometer along with the analysis of results obtained in a given application. Very few candidates attempted the question. In part a) the very small fluid space between the cone and the plate justifies the assumption of a linear velocity gradient (shear rate) which is proportional to rotational speed N. This in turn yields a constant shear stress τ for a given fluid. The expression in part b) is obtained by using the constant shear stress result of part a) along with integration of the elemental torque on an annular element of the cone surface. Part c) requires a simple logarithmic reduction of the given data expressed in terms of shear rate and shear stress to calculate values for the power law constants k and n. The numerical answers are:

c) (i) k = 0.043, n = 1.26 Question 6 Part a) of this question on compressible flow asked for derivation of an expression for the speed of sound in a compressible fluid and for the common form of this for a perfect gas. Very few candidates attempted this part of the question and of those that did, none completed the analysis. Several of the attempts ignored the information that the fluid was initially at rest which added unnecessary complication to the analysis. Candidates must take care to read the question carefully and use the information provided. Part b) concerns the isentropic flow of air through a convergent-divergent nozzle. However, unlike the normal form of such problems, the air enters the nozzle as a supersonic flow and leaves as subsonic flow. Quite a few solutions failed to appreciate this. A large number of candidates took the given stagnation conditions as static conditions for the air at entry and wrongly proceeded to calculate new stagnation conditions. Some candidates chose to use the density of water as the air density, resulting in nozzle areas of the order of 10-6 m2 . However, the candidates did not show surprise at the values calculated. Many other errors in the application of units in equations produced nozzle diameters of around 80 m which were also accepted without comment. Candidates should be encouraged to question their calculation results and, where these are unlikely, to check their earlier work. Other errors in calculations included assuming the air density is constant at the value given in the rubric of the paper. These candidates should realise that in compressible flow, the fluid density may vary considerably as the pressure and temperature of the fluid changes. Also some candidates wrongly persist in using temperatures in °C rather than K in calculations. This suggests a lack of understanding of thermodynamics and the gas laws. In part c) no one offered a correct explanation for the effect of falling atmospheric pressure. Indeed, most answers related to flow through the nozzle in exactly the opposite direction.

The numerical answers are: (b) (i) 0.059 bar, 143.6 K, 600.5 m/s (ii) 322.4 K, 36.0 m/s (iii) 0.1722 m, 0.1060 m, 0.2558 m

Question 7 This question concerns design calculations for an inward radial flow turbine. Part a) requires developing the equation given which will achieve maximum efficiency for the conditions stated. The key feature in the development is the need to minimise the exit velocity (kinetic energy loss) at the turbine outlet. Several candidates did this successfully although most others did not make the connection between maximum efficiency and minimum outlet velocity. In part b), candidates introduced a variety of calculation errors in their analyses. A principal source of error arose from candidates producing no velocity diagram or having a poorly-shaped velocity diagram for the turbine stage. This resulted in many errors of interpretation. Candidates would help themselves greatly if they were to draw correctly-shaped diagrams to support their analysis. Also, some candidates appeared to be using standard formulae without any clear understanding of their applicability. Using simple trigonometry with the velocity diagrams to determine the results required would be much more effective than relying on remembered formulae and applying them without due care and attention. Other errors included using the radii given in the question as diameters and treating the inward flow turbine as an outward flow turbine. Careful reading of the question would help to avoid these errors. The numerical answers are:

(b) (i) 217 rev/min (ii) 29.1º (iii) 577 kW (iv) 5%

Question 8 The question involves the analysis of flows in pipe networks. The network in part a) comprises three pipes in parallel, two 200 mm diameter pipes and one 150 mm diameter, connected between the same pair of nodes. The objective is to determine a head loss k-factor for an equivalent single pipe carrying the same total flow. Although most candidates were able to perform the correct analysis, many appeard to misread the question and used only one of the two 200 mm diameter pipes, simplifying the analysis slightly for them. In part (b)(i), the flow through the network is initially unknown and is determined only after the network flow distribution is established by utilizing the given piezometric heads at nodes in the network. The Hardy Cross method suggested for the analysis is a straightforward procedure and most candidates were able to perform the analysis correctly and accurately from an assumed initial flow distribution. Numerical errors were common in a number of cases. These included using the wrong sign for the correction, applying the flow corrections wrongly and using different values for corrected flows in pipe BD in each of the loops in the subsequent loop calculations. In part (b)(ii), a surprising number of candidates appeared not to understand the term piezometric head. Some assumed it was a pressure term, others an elevation term and only a few correctly used it as the sum of these terms. The energy equation expressed in terms of piezometric heads and including the head loss in one or more chosen pipes between selected nodes provides a means for determining the total flow Q. A small number of candidates calculated a correct value for the total flow. The numerical answers are:

a) 4.21 s2/m5 b)(i)

Pipe AB BC CD DA DB Flow m3/s 0.4832Q 0.2673Q 0.3327Q 0.5168Q 0.1841Q

(ii) 0.218 ≤ Q ≤ 0.239 m3/s

Question 9 The question concerns the use of non-dimensional performance coefficients to describe and assess the operation of pumps in a simple system application. In part a) details of a full-size pump and a geometrically similar model pump are given and calculations to determine several characteristics of the two pumps are required. The model pump head in a)(i) is obtained from the pump efficiency equation. However, a number of candidates seemed to be uncertain about the correct form of the efficiency equation, with several wrongly using the form appropriate for a turbine. Careless application of the units of power in the equation produced very impractical answers which should have been obvious to the candidate but were accepted without comment. Many candidates wrongly assumed the diameters of the full-sized and model pumps are the same and then proceeded to use the flow and/or head coefficients with equal diameters to determine the model pump speed. Only a few candidates made use of the specific speed expression given in the question to calculate the required speed. This is a further example where candidates must read the question carefully and use the information provided to perform the necessary calculations. Candidates should also note that the term geometrically-similar pumps does not mean that the pumps have to be of equal size. Almost no one correctly calculated the geometrical scale factor required in a)(iii). In part b) an experimental pump installation for the model pump is described which includes an orifice plate to help reduce the flow in the system. Most candidates correctly used the energy equation to derive the system demand equation but wrongly omitted the head loss term for the orifice plate, leading to significant errors. The required K-factor for the orifice plate is obtained from the system demand equation by applying the required head and flow for the model pump. The numerical answers are:

a)(i) 90.7 m (ii) 1751 rev/min (iii) 4.75 (iv) 8.18 MW

b) K = 1122.9 s2/m5, H = 4.0 + 2167.5 Q2

Unit 204 hydraulics and hydrology

General comments The candidates this year were not as well prepared as last year for this examination and many displayed a weakness in understanding of fundamental theory. Due to an administrative error, the answer books issued did not contain graph paper which was necessary for some of the questions. In some centres, paper was made available during the examination but may have been issued too late to be of use to the candidates. The marking scheme adopted compensated for the lack of graphically determined data. Most candidates had a good understanding of units of measurement, but some found it difficult to apply the necessary theory to the problem presented. The following report includes numerical solutions and refers to common errors observed in the candidates’ scripts.

Comments on individual questions Question 1 In general this question was poorly answered and candidates were generally unable to derive the velocity profile or the Hagen-Poiseuille equation for flow in a circular pipe. A few candidates were able to relate the equation to the Darcy equation and derive the expression for λ for laminar flow. Question 2 Few candidates recognised the conflicting demands of space requirements and the need to prevent viscous and surface tension effects dominating the model. In general there was little understanding of modelling techniques and similarity. Numerical answers to part (b) are i) scale ratio for velocity 1:5.5, ii) scale ratio for discharge 1:24648, iii) n in model = 0.025, iv) n in model = 0.033 Question 3 Most candidates attempted this question. There was a good understanding of equivalent single pipes and many candidates were able to derive the required expressions. In part (b), a number of candidates ignored the instruction to use the given flows as a starting point in the iteration. A common error was to treat the network as a single loop of 5 pipes rather than dividing it into two loops. Numerical answers to part (b) are:

Pipe 1 2 3 4 5 Flow (l/s) 151 51 239 110 60

Question 4 Some candidates were able to derive the standard expression given in part (a) but few were able show that the hydraulic jump occurred when the velocity V was zero. As a result, they could not set up the conditions needed for the finite difference solution to the gradually varied flow between the toe of the dam and the hydraulic jump. The numerical answer for the length of gradually varied flow is of the order of 40m.

Question 5 This was an unpopular question and those candidates who attempted this question showed in part (a) no real knowledge of pressure transients. No one was able to undertake in part (b) the derivation of the expression for the amplitude of oscillation. Numerical answers: for part (a) without friction, the head rise is 434 m; with friction it is 449.4 m, occurring after 1.2 seconds and for part (c) Z = 2.6 m, requiring a surge tank height of 54m. Question 6 This was a popular question and the majority of candidates made a good attempt at part (a). However, many were unable to derive the expression for the variation of velocity across the flow and hence solve part (b). Numerical answers to part (b) are velocities of 11.7 m/s and 9.0 m/s at the inner and outer surfaces and corresponding pressures of -65 kN/m2 and -37 kN/m2. Question 7 This was also a popular question and many candidates were able to answer part (a). The majority were able to linearise the data in part (b), but a number failed to rank the data before determining the return period from the Gringorten formula. Another mistake was to ignore the need use the expression derived in part (a) to determine return period of the required flood (100 years). Because of the error in not providing graph paper, no marks were assigned to the graph and if the return period was correct, full marks were given for part (b). The design flood was 30.5 m3/s, giving a cofferdam height of 21 m above datum. Question 8 Most candidates who attempted this question did not understand the concept of temporary storage, especially in a channel. Few realised that the reservoir needed to fill to its maximum storage capacity before spillage occurred. Again, graphs were needed in this question and so full marks were given if the method of solution was indicated. No marks were assigned to the requested comment. Numerical answer to part (b) is that the outflow starts after 24 hours and peaks at 67 m3/s at about 52 hours (where the graph of the outflow with time crosses that of the inflow). Question 9 In part (a) of the question, the majority of candidates did not understand how to use the S-curve technique to derive the 1.5 hour unit hydrograph. In part (b) few candidates realised that the net rainfall intensity and its duration are required to determine the scaling of the unit hydrograph when using constructing the convolution table. Numerical answer for the unsmoothed 1.5 hour unit hydrograph to part (a) Time hr 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Flow m3/s 0 1.4 2.9 5.8 7.2 10.1 11.5 14.4 13.4 13.9 Time hr 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 Flow m3/s 10.6 11.1 7.8 8.2 4.9 5.4 2.1 2.6 0 and to part (b) peak flow of 136.5 m3/s.

Unit 208 Materials

Comments on individual questions The list below should not be considered to be full answers to the examination questions; they are outlines only, together with a few comments on student performance. Question 1 This question attracted the least number of attempts. Part (a) required descriptions of the two common industrial polymerisation processes. For full marks, the basic mechanisms of both, and the chemical structures produced by both were required, as were the appropriate chemical formulae and chemical equations. Part (b) concerned the way side groups are arranged on a linear polymer chain and the way these affected the degree of crystallisation. Simple diagrams with accompanying explanations were required, and it was important that both of these were given to obtain maximum marks. Very few candidates attempted part (c) which asked for explanation of the concept of a co-polymer. These are linear polymers consisting of two or more mers of different kinds. The polymer chains may have the mers randomly arranged or there may be short segments (or blocks) consisting of one kind of mer only. Also required were explanations of how the properties of a co-polymer can be changed by varying the proportion of the different mers and their arrangements along the chain. Question 2 This was a far more popular question that the first. Many candidates lost marks, however, by failing to distinguish between hot work (deformation above the recrystallisation temperature giving no work hardening) and cold work (deformation at a lower temperature resulting in work hardening). This affected the answers to parts (a), (b) and (c), making it difficult for these candidates to achieve a good mark. For part (a) diagrams and brief explanatory text were required to give a good chance of high marks. Drawing is essentially a cold working processes. The others may be carried either hot or cold. For part (b) it is necessary to explain that cold work will increase the dislocation density and harden the material whereas hot working processes do not cause significant hardening but may improve the properties by, for example, replacing a coarse grained cast structure with fine equiaxed grains. The general answer to part (c) is that deformation formed components may be heat treated to modify the microstructure so as to give the required properties. Specific answers would be to recrystallise (and so soften) cold-worked material, to harden a hot-worked steel by quenching and tempering, or to solution treat and age a precipitation hardenable alloy. For part (d) machined and deformation formed components can be distinguished by their surface finish, machined components showing tool marks on flat and cylindrical surfaces. Question 3 Most candidates were able to give examples and uses of polymer matrix composites, glass fibre reinforced resins being the most common choice. Few, however, were able to give examples of metal matrix and ceramic matrix composites. An example of the former could be a high

temperature nickel based alloy reinforced with minute alumina particles to allow the operating temperature to be raised above that for the alloy alone. For the latter silicon carbide whiskers in a silicon carbide matrix is an example. The significant thing about this material is that the whiskers are not strongly bonded to the matrix and form filaments bridging the faces of any crack formed on loading. This hinders the opening of the crack (the same mechanism as operating in a glass fibre reinforced resin) preventing rapid crack propagation. For part (b) examples of selecting constituents to produce specific properties might be carbon fibres in a matrix of resin or thermoplastic polymer to produce a high modulus component of very low weight, or a strong, high modulus fibre weakly bonded to a brittle matrix to provide a component with strong resistance to crack propagation by crack bridging by the fibres. An example of controlling properties by choice of reinforcement geometry might be bonding together thin plates reinforced by long parallel fibres together in such a way that the fibres in each plate were at different angles to each other. This would give a high modulus in all directions parallel to the plates. Another example would be continuous reinforcing filaments wound in different ways around a former while being impregnated with resin to give a modulus more isotropic than with uni-directional fibres. Part (c) requested a calculation of the transverse and longitudinal moduli in a unidirectional fibre composite. The required formulas could be derived or memorised without loss of marks. Most candidates could do this but few could account for the difference between the two moduli which is simply due to the fact that the only continuous phase in the transverse direction is the low modulus matrix, the reinforcement having little effect. Question 4 To get good marks for part (a) it was important to appreciate the different ways in which the word ‘structure’ is used in different branches of engineering. At level (i) the structure consists of steel reinforcing bars plus some of the coarser aggregate in a matrix of concrete. As the scale of observation gets smaller from (ii) to (iv) we can see the fine aggregate in a matrix of sand plus hydrated cement paste, then sand particles in a matrix of needles of hydrated calcium silicates and calcium alumino-silicates. At levels (ii) and (iii) the microstructure of the reinforcing bars (ferrite and pearlite) becomes evident. Diagrams and explanation in words is needed for full marks. No more than three chemical equations for the hydration reactions are required for part (b) plus descriptions and sketches of the process and resulting microstructures at various stages of setting. For very large volumes of concrete the heat evolution in the early stages of setting can cause temperature rises sufficient to degrade the properties and some system of cooling (e.g. embedded pipes carrying cooling water) must be employed to prevent this. Question 5 Almost all the candidates attempted this question and it produced the best results. To obtain full marks for part (a) the phase boundaries on the diagram should all be correct and the phase fields and the axes of the diagram correctly labelled. The diagram should be accompanied by a list with a minimum description of the phases (e.g. � — body centred cubic iron, � — face centred cubic iron etc.) For full marks, parts (c) and (d) required schematic sketches of the microstructures at significant stages of the cooling process for two alloys annotated with the names of the phases and microconstituents present, plus brief descriptions of the changes occurring during cooling and the temperatures (taken from the diagram) at which the changes begin and are completed. A large proportion of candidates missed out the descriptions and in doing so missed obtaining a mark close to the maximum.

Question 6 This question was attempted only by a small number of candidates and had a low average mark. The key features of powder pressing required by part (a) are mining of the mineral, grinding and grading of the resultant mineral powder, followed by mixing of different size particles together to get a mixture of maximum packing density. This is followed by adding a binder (may be water) then isostatic or unaxial pressing to form a ‘green’ compact. This is then sintered. Part (b): Sintering involves heating the compact to a high temperature so that atomic transport processes take place to fill the voids (causing the compact to shrink in the process). The transport processes are diffusion of atoms (ions) through the solid, diffusion along particle surfaces and evaporation from part of a particle surface and condensation on another. The driving forces for the processes is the reduction in surface area (and therefore energy) of the compact. Large initial particle sizes require atoms/ions to diffuse larger distances to achieve densification and hence the process takes longer, and larger voids and a lower final density result. Larger initial particle sizes give large final grain sizes and as mentioned above, more, residual pores, both giving poorer properties. Diagrams as well as words are important to obtain maximum marks. Part (c): Compression tends to close cracks and tension to open them. Cracks that are opened by an applied stress have a high stress intensity factor at their tips and will propogate catastrophically if this reaches a critical value. Compression does not produce this effect and fracture occurs at much higher stresses by shear. Very few candidates were able to answer this part of the question. Part (d): (1) Add reinforcing fibres to form ceramic matrix composites. (2) Process to reduce voids and pores to a minimum, these act as sources of cracks. (3) Process to produce as low a grain size as possible. Question 7 This question also had a small number of attempts and a low average mark Part (a) required the macro- and micro-structure of wood to be described. Macroscopic features are growth rings and medullary rays with a layer of sap wood having active sap channels close to the outer surface of the limb. The microstructure consists of very elongated hollow cells axially aligned having a roughly hexagonal cross-section. The cell walls are a composite of cellulose fibres in a matrix of lignin and hemi-cellulose. There are axial fibres and helically wound fibres in different layers of the cell walls. This is an anisotropic structure with an axial strength double that along the transverse direction. Diagrams as well as descriptions in words were required for maximum marks. Part (b): Water held in the pores in wood generally make it tough, particularly in bending. However, ‘green’ timber is dimensionally unstable and changes in shape and anisotropically on drying. It has to be dried under controlled conditions before use. Rough cut timber is stacked (with spacers to allow air circulation) in a kiln maintaining a temperature a little above ambient. Part (c): Logs are plain sawn if the boards are cut by a series of parallel cuts. Such boards are subject to distortion on changing water content. Quarter sawn logs have boards cut so as to have the growth rings as nearly perpendicular to the board surface. It involves cutting the log into quarters and cutting the boards as nearly parallel to a radius as possible. This wastes more wood than plain sawing but the boards are subject to much less distortion. Part (d): (1) Impregnate wood with a suitable biocide. Creosote is an example. This will prevent or delay attack by insects, fungal growth and micro-organisms. (2) Coat wood with paint or water-proof varnish.

Question 8 Although a significant proportion of candidates attempted this question the average mark achieved was poor. Part (a) Loading of a material (within limits) is safe (i) if the loading is entirely compressive since cracks will then be closed rather than opened (this applies even if the material is brittle). (ii) if the material is ductile (even if the stresses are tensile) since plastic flow at the crack tip reduces the stress intensity factor and prevents crack propagation. Part (b) required annotated sketches of the notched bend test specimen and of the compact tension test specimen. Both of these have a notch at the base of which is a crack formed deliberately by fatigue. Specimens are loaded until rapid crack propagation occurs. Standard formulas are used to calculate the plane strain fracture toughness from the fracture load and the specimen dimensions. The tests are valid if the stress state at the crack front is predominantly plane strain. The stress state at the surface where the crack emerges must be plane stress but the test will be valid if the plane stress plastic zone size is less than 1/10 of the specimen thickness, B. The criterion is B > 2.5(KIC /��y)

2 where KIC is the plane strain fracture toughness and �y the material yield stress. Candidates were expected to know this formula. Part (c) A provisional value, KQ, for KIC , using the equation given in the question, should be calculated and substituted into the inequality above to determine whether the empirical criterion was satisfied or not. The result is that the provisional value of KIC, 70.7MPa m1/2, is valid since the right hand side of the above formula evaluates to 34.7mm Part (d): The effect of cracks in structures or components can be reduced by material selection and/or by use of a damage tolerant design philosophy combined with non-destructive testing to determine the maximum crack length present.

Unit 209 Mechanics of Solids

Comments on individual questions Question 1 Relevant part of syllabus: Axi-symmetric components This question requires knowledge of stresses in thick cylinders, a topic that regularly occurs in this paper. Part a) asks for an explanation of a practical way of assembling a bush with an interference fit onto a solid shaft. Lamé’s equations, together with appropriate boundary conditions, are used in part b) to calculate the interference pressure of 80 N/mm2 for the specified maximum hoop stress of 100 N/mm2 in the bush, which occurs at the surface of the bore. Consideration of the hoop strain resulting from this interference pressure leads to the calculation of the change of diameter in each component, noting that different physical properties are used appropriate to the material of the component. The total radial interference gives the exact bore of the bush as 49.928 mm. Candidates should be careful that they are using the dimension of radius or diameter from the given data. The compatibility of the strains at the interface caused difficulty for some candidates who reached this stage of the calculation. Question 2 Relevant part of syllabus: Finite element analysis. a) Shape functions can be determined from the fact that they must equal unity at the particular node, and zero at the other nodes. E.g. N1 should include the factor (s) so that it is zero at s=0, and also the factor (s-1) so that it is zero at s=1. But to make the result involving the product s(s-1) equal to unity at s=-1, we need to divide by 2 leaving the result as s(s-1)/2. Similar reasoning can be applied to the other 2 nodes, giving – (s+1)(s-1) and s(s+1)/2 resp.

b) The displacement equation u = N1u1 + N2u2 +N3u3 requires that at node 1, where u = u1, N1 = 1 and N2 and N3 are zero. Also for rigid body motion, i.e. when u = u1 = u2 = u3, then N1 + N2 + N3 = 1.

c) Substituting the shape functions and the given nodal displacements into the displacement equation quoted in b) above, and collecting terms gives u = s2/2 + 5s/2 +5 d) The strain is obtained by differentiating the above equation w.r.t. “x” bearing in mind that s = x/5 and evaluating for s = 0.5 (the required strain point), giving a value of 6 x 10-4

Several candidates started from first principles and therefore took much longer to answer than was necessary. Question 3 Relevant part of syllabus: Torsion a) Bookwork to get J= �D4/32 b) Max. shear stress (Tr/J) occurs where r/J is a maximum, i.e. at the small end of the shaft, and is 50.9 N/mm2. To determine the angle of twist, it is necessary to derive an expression for the twist of a thin disc, ‘dx’ thick located at a distance ‘x’ from one end. Its diameter can be expressed in terms of its position ‘x’ and the given shaft dimensions. This expression must then be integrated to give the twist of the whole shaft, which was 5.973 x 10-3 radians.

Many candidates assumed that the maximum shear stress occurred at the large end of the shaft. For part b. many invented what they considered to be an equivalent shaft, e.g. a stepped shaft of 2 parallel sections, or a uniform shaft of ‘average’ diameter. These gave reasonable, but not accurate, answers. There was some misunderstanding of J the polar second moment of area. This is a property of an area, not of a volume such as the complete shaft. Question 4 Relevant part of syllabus: Prerequisite understanding of equilibrium and compatibility. a) Remembering that both components expand, it is necessary to find the temperature rise at which the copper rod has expanded 0.05mm more than the steel cylinder.

Answer 83.30C. b) There is a further temperature rise of 46.70 C. The components are now subject to thermal expansion and mechanical strain due to the force acting between them, the copper being compressed and the steel stretched by the same force magnitude. This force can be determined as 9786 N, which produces stresses of -31.15 N/mm2 in the copper and 6.18 N/mm2 in the steel. Some candidates ignored the expansion of the steel sleeve in the first part. It was apparent that some substituted into remembered equations without sufficient understanding what was happening. Question 5 Relevant part of syllabus: Beams (Centre of shear) An understanding of the stresses in thin-walled open tubes was being tested in this question. Most candidates were able to say what the shear centre means but were then unable to calculate its position for the circular tube with a longitudinal slit. By inspection in this instance, the position of the section centroid and the axis of symmetry through the slit are identified. It is easier to work with polar coordinates to determine the second moment of area of this section, which is required in the expression for shear stress or shear flow that is integrated around the section to find the torque from the internal shear force. This torque is then equated with the torque from the externally applied load at the shear centre, which shows that the shear centre is at twice the mean radius from the centroid in the direction away from the slit. Question 6 Relevant part of syllabus: Mohr’s circle (strain gauge rosettes) Part a) asks for a brief description of the practical measurement of surface strain using an electrical resistance strain gauge. In part b) the observations from three strain gauges in the form of a rosette on a flat plate are given for an associated tensile load. The readings from the gauges with their known orientation have to be combined to determine the principal strains. The principal strain values may be calculated using the strain transformation equations or determined graphically by constructing a Mohr’s strain circle. The tensile stress (load/area) is a principal stress since it results from the sole load. Hence Young’s modulus is given by this stress σI (σII = 0) divided by the strain εI measured by gauge 1 because it is orientated in the direction of the applied load. The transverse strain is εII and Poisson’s ratio is εII/ εI. The answers are E = 222 GN/m2 and ν = -0.3. In part c) the shear modulus may be calculated from the values just obtained as G = E/2(1 + ν) or graphically as shear stress (τmax)/shear strain (γmax) from the Mohr’s stress circle and Mohr’s strain circle respectively. Answer is G = 85.4 GN/m2. Most candidates were able to give the basics of operation of an electrical resistance strain gauge, but half were then unable to make much progress towards finding the principal strains. A significant number of the more successful half plunged straight into substituting numbers into correct equations to establish, after a lot of algebra, that gauge 1 is orientated in the principal stress direction; whereas a careful reading of the question and a look at the physical problem would give

this result by inspection. For some candidates there was uncertainty about the relation between Young’s modulus and shear modulus. Question 7 Relevant part of syllabus: Beams (asymmetric cross-section) For a beam in bending, the neutral axis passes through the centroid of the cross section, the position of which in this case is determined by inspection due to the anti-symmetry of the Z-section. The second moments of area are then calculated with respect to a convenient axis system with origin at the centroid. For principal axes the cross product moment of area, Ixy, is zero. The rotation of the chosen axes for this to occur may be determined from the transformation equations or graphically by Mohr’s circle, and the principal second moments of area follow in similar manner. The answers are α = 22.4

O anti-clockwise from the web, I1 = 1,651x103 mm4 and I2 = 137x103 mm4.

The applied end load is resolved into components along the principal axis directions. Then, using the given equation for the deflection of a cantilever, substitute these calculated second moments of area and the given length and elastic constant of the beam to obtain the deflection in the principal axis directions. These are combined to give the magnitude and direction of the end of the cantilever as 1.18 mm at 33.8

O from the horizontal downwards and to the left.

Question 8 Relevant part of syllabus: Buckling a) There are 2 possible buckling planes to consider. Horizontal plane (with respect to diagram in question paper) with a high column stiffness but pinned ends, and Vertical plane with lower column stiffness but with effectively built-in end conditions. The horizontal plane gave a buckling load of 123.4 kN, while the other gave 113.7 kN, so the vertical plane is the worse case and the required answer is 113.7 kN. b) This requires a circular section column with pinned ends to withstand a load of 113.7kN, and results in a required minimum wall thickness of 3.185 mm. Some candidates made the correct calculations in part a) but selected the higher load value as the correct answer. For part b) there was some confusion over which loads and which end conditions were appropriate and some difficulty in solving for the tube’s inner diameter which appears as a fourth power. Question 9 Relevant part of syllabus: Fatigue Details of the topics can be found in numerous Solid Mechanics and Design text books. The marker was not expecting complete coverage of all the topics, but an indication that the candidate knew and understood the subject. Typical mistakes included : a) not mentioning that the S in the S-N curve stands for stress amplitude, rather than stress. b) and not realising that the S-N curve relates to zero mean stress, and so not appreciating the

need for a Goodman ( or other) diagram to compensate for non-zero mean stress cycles. c) no problems. d) no mention of electro-plating as a poor surface finish for resisting fatigue. e) This specifically relates to changing stress levels during the life of a component, but many

included environmental factors.

Question 10 Relevant part of syllabus: Beams (Elastic/plastic behaviour) This question requires a basic knowledge of the stress-strain behaviour of materials in the elastic and plastic ranges of stress and the internal stress distribution for a beam in bending. For the purpose of the calculation in this question the material is represented by idealised stress-strain behaviour. Part a) required linear increases of strain with increasing stress to the elastic limit and from this limit no increase in stress for unlimited increase in strain. The unloading curve is a straight line, parallel to the loading line, from a point beyond the elastic limit back to zero stress to indicate that on removal of the load there is a residual strain or permanent deformation. For part b), at initial yield the internal stress increases linearly from zero at the neutral axis to the yield stress at the outer surface where the stress is a maximum. Using the ETB with the values for this condition corresponds to an applied moment, M = 2.4 kNm. For a plastic hinge, the stress is uniform, equal to the yield stress, from the neutral axis to the outer surface. For this condition M = 3.6 kNm. The applied moment in part c) is a value between that for initial yield and that for a plastic hinge and so the stress distribution will be part elastic and part plastic. Consideration of the moment of the internal force about the neutral axis equilibrating the externally applied moment enables the position of the transition from elastic to plastic behaviour to be calculated. The residual stress distribution after the load has been removed may be obtained from the superposition of the elastic-plastic stress distribution and the stress distribution that would have existed had the material been wholly elastic up to the maximum stress. Answer: residual stress at the surface, y = 15 mm, σ = 100 N/mm2, at y = 12 mm, σ = 0, at y = 10.6 mm, σ = -46.7 N/mm2, at y = 0, σ = 0.

There seemed to be widespread confusion on the meaning of ‘elastic – perfectly plastic’ in the description of the idealised behaviour of a material. Many candidates sketched a stress-strain curve for mild steel showing roughly linear elastic behaviour, the typical yield and non-linear plastic characteristics of this material and failed to show the correct path for unloading from a point beyond the elastic limit.

Unit 210 Structural analysis

General comments The performance of the candidates was relatively disappointing with an overall aggregate mark of 37% and half of the candidates failing to achieve the pass mark. The spread of the marks indicates that a significant number of candidates failed to make a serious attempt. Comments on individual questions Section A – Elastic Analysis of Structures Question 1 The question comprised three parts and centred on the use of influence lines in the analysis of a simple, singly statically indeterminate, two-span continuous beam. About half of the candidates attempted the question. Many were able to provide an adequate definition of the principle but were unable to demonstrate an understanding for the application to the solution of the problem. Very few candidates attempted the third part (awarded only 3/20 marks). The solution involved the use of Muller-Breslau’s theorem to resolve for the vertical reaction force at one of the simple supports. This was achieved by the application of a unit load to the released structure and use of simple beam theory to resolve the equation for the deflected shape of the beam. By observation, the influence line for the corresponding reaction force could be constructed and substitution of the externally applied loads to yield the value of the reaction force. Candidates generally demonstrated a valiant attempt to resolve for the deflection but were unable to complete the solution. Marks were awarded to reward those who demonstrated an understanding for the route to the solution even if they had failed to arrive at a reasonable (feasible) numerical answer. Question 2 Candidates were asked to solve for the bending moments and shear forces carried by a 3-span continuous beam. Most of the candidates attempted this question. About half of the candidates were able to display a sound approach to this and to produce a sensible numerical solution. a) The most straightforward approach is to use the moment distribution method to resolve the

member-end moments by successive approximation. The shear forces and bending moments in the spans could then be solved using simple statics. Some candidates attempted to solve for the joint rotations by use of the slope-deflection equations. This caused difficulties because of the four unknown and as a result many failed to solve the complete the whole question.

b) The frame is propped (prevented from sway) and the moment distribution solution allows

solution for the propping force from the solved member-end moments. A second analysis is then performed for the frame with the released prop and the propping force only applied. The two sets of internal forces are then superimposed. Candidates who attempted this part generally showed that they understood the moment distribution approach to solution of sway.

Question 3 The question involved the solution, using the force method, of a 2-pin parabolic arch rib. About half of the candidates attempted this question and most demonstrated a very good understanding for the approach and were able to give a correct answer. The approach involves the release of one of the 4 supports and the solution for the displacement of the arch at that point. A unit load is then applied to the release point and the corresponding displacement found. By superposition it is then possible to find the value of the reaction force to cause no displacement of the arch in the direction of considered reaction. The geometry causes the

internal force to be that of simple axial force, the bending and shear being negligible. An observation of shallowness of the arch helps to simplify the calculation. Question 4 A majority of the candidates attempted this question. However, most candidates failed to progress with the solution beyond the initial elementary stages. Many attempted to solve assuming the frame to be statically indeterminate – using a slope-deflection approach. This question caused difficulties for most. The displacement of the statically determinate frame can be solved by considering the bending actions only. The internal bending moments are solved using simple statics. A strain energy approach can then be applied, integrating to sum for the total strain energy in bending for the frame. Displacements of the loads are then given by differentiating the strain energy equation with respect to the particular load. Alternatively, virtual strain energy can be calculated and the dummy unit load approach used to find the displacements. Question 5 Very few candidates attempted this question. It involved the formation of the stiffness matrix for a two-bay plane frame structure. The stiffness coefficients being found from use of the slope-deflection equations. Question 6 About half of the candidates attempted this question. Unfortunately, as printed, the figure contained a typographical error. This caused the calculation to be more extensive/complex than had been intended. The marks scheme was therefore adjusted to allow candidates to gain full marks for the question by solving the first stages of the problem and showing that they understood the approach to resolving the forces using the force approach, even if the numerical solution was not entirely correct. In spite of the error in the figure, candidates managed to gain reasonable marks in the question. The solution of the statically indeterminate truss involved the need to release an internal member and solve for the displacements using a unit load approach – for the externally loaded structure and then for the truss with a unit load replacing the released element. Compatibility could then be applied by observing that the released element will not change in length and the information can then be used to find the internal forces. This is a fairly straightforward approach and is given in most of the standard textbooks. Section B – Plastic Analysis and Ultimate Load of Structures Question 7 Very few candidates attempted the question on the plastic collapse of the frame. However, some of those who did, and demonstrated a valiant attempt, provided a reasonable/sensible solution. a) The description of approach is fairly straightforward and published in many of the standard

textbooks which include coverage of plastic analysis. b) There are 2 possible plastic collapse mechanisms: beam failure, with plastic hinges forming

under the beam load and at the beam-column connections and; a combined sway/beam failure in the columns, with some alternative plastic hinges forming near to the column bases. The critical case is found to be the beam failure case.

Question 8 The question on yield line analysis was a popular choice for the majority of candidates and most performed well. This was the highest scoring question. The approach used is to calculate the work done by the applied loads for a chosen yield mechanism and to equate this with the internal energy for plastic rotation on the chosen yield lines. This results in an equation with a single unknown dimension, x, thus giving the position of the yield line for minimum energy and thus the critical value of M.

Section C – Mechanics and Deformable Bodies Question 9 a) The solution involved the derivation of the stiffness matrix for a triangular element in a state of

plane stress. This involved the observation of the relationship between strain, and in turn stress, and node displacements and equating the internal and external work. The matrix can then be defined by observation of the displacement function.

b) Construction of the overall stiffness matrix for the example structure involved the simple

observation of connectivity of the structure and planting of the local (element) stiffness coefficients in appropriate rows and columns of the matrix. This is a very straightforward process for a candidate who understands the matrix form of the stiffness equations.

Question 10 Very few candidates attempted this question. However, some of those who did were able to provide a reasonable solution. The biharmonic equation is obtained by differentiating the stress function and the value of the constants are found by observation of the boundary conditions. Section D – Stability Question 11 Only a few candidates attempted solution to the question on stability of the simple rigidly jointed frame. a) The position of the load to cause complete collapse of the frame by simultaneous buckling of

both columns can be found by equating the critical elastic buckling loads for the columns (considered as slender struts) in relation to the position of the load on the beam – noting the need to consider the sway modes for the columns.

b) Explanation as to the change in behaviour of the frame if it is prevented from swaying is a

straightforward observation of change in effective length of the column BC. The derivation of the effective length of the fixed-fixed end condition is achieved by writing equilibrium equation for first buckling and solving the differential equation of flexure with consideration for the boundary conditions. This is covered in most textbooks.

Question 12 Very few candidates attempted the question on the derivation of buckling load of a simple strut with a uniform transverse load. The solution involved the observation of moment equilibrium and use of simple beam theory to derive the equilibrium equation for the strut. The solution of the equation is then found assuming a simple harmonic deflected shape and the constants are found by observation of the boundary and load conditions.

Unit 211 Structural design

General comments This year’s examination resulted in a higher pass rate than previous years. General comments however should still include advice as to the habit some candidates have of copying out information given with the examination material, such as steel section sizes, onto the answer book. This is a waste of time, surely, and falls into the same category of those candidates who write down what they would have done in a question had they bothered to work out the numbers. Comments on individual questions Question 2 For question 2 which was the design of a steel column subjected to axial load and bending, a disappointing number of candidates mixed up the Mxx and Myy directions of bending due to loads on those axes. It is suggested that care be taken in the future and advice sought if this comment is not understood. Question 3 This question was, unfortunately, a contentious question as only one sheet of beam section tables was included in the examination material instead of two. The majority of candidates, fortunately, either were told to or made the decision to use the smallest of the beams provided instead of those suggested in the question. Those who attempted the question and gave up were awarded marks relative to the stage they reached and the examiner is confident that no candidates who failed the examination failed because of this question alone. In future, beam (or column) sections will still be suggested to save the candidates’ time, but extra care will be taken that all steel sections available will be included in the examination material. Finally on this question, the deflection limit for floor beams, to BS 5950, is generally span/360 using unfactored live loads. Question 7 This question was a straightforward concrete design of a slab and supporting beam. Many candidates got confused in working out the moments on the slab (shear not being critical). The best advice is to think of a 1 metre width of slab and work out the loading, w, as a UDL in kN/m and insert into the suggested formula. Most multiplied in the span twice and thus had a slab with compression reinforcement. All questions were from the main body of the structural steel, timber, masonry and concrete syllabi, and it was obvious, to the examiner, that more preparation had been done for the 2006 examination than in previous years.

Unit 213 Geotechnical engineering

General comments As in previous years a number of scripts were barely legible; candidates are encouraged to print if they know their handwriting is difficult to read. The candidates are reminded that examiners are looking at the quality of the answer not the quantity of text it contains. When drawing graphs candidates should always choose a suitable scale for the axes and label them with titles and units where appropriate. Comments on individual questions Question 1 Most candidates understood the reasons for ground investigation however, the logic for the design of an investigation was not appreciated by most. This is clearly shown by the answers to Q1b), where a knowledge of ground investigation had to be applied to a retaining wall; this was answered poorly. Most students’ spoke of ground investigation in terms of foundations – few had any other example to offer. Question 2 Candidates are unused to converting what they can see from a figure to its meaning for geotechnical engineering. No candidate commented that the limestone might contain Karst and many did not appreciate that black pyritic shale is likely to be the weakest member of the materials excavated. All noted the presence of water. The structure of the ground went largely unnoticed; the view was looking North and the strata was dipping 40° @ 085°, ie it was dipping at angles greater than that expected for the sliding friction of these materials and across the tunnel; joints would therefore run almost along the axis of the tunnel. Candidates were not able to reconstruct the 3D character of the ground. Question 3 This question was attempted by few candidates. Candidates should have taken into consideration the tropical nature of the site, the age of the rock, jointing, weather conditions in the first part of the question. They should have then gone on to explain how the excavation could be analysed reflecting the actual nature of the rock. In parts c) and d) the candidates were required to provide methods of supporting and improving the rock mass strength; various solutions were appropriate and each solution was marked on its own merits. Question 4 Hydrogeology remains a subject with which most candidates struggle. Much could probably be appreciated if the candidates recognised that the principles used are the same as those they have used in hydraulics. They should have used hydraulics principles to establish answers for the first parts of the question. The latter parts of the question required construction of a flow net together with knowledge of its properties. Question 5 The three parts of this question addressed three very different topics; Q5(i) Soil classification; most candidates ignored the use of consistency limits and none mentioned the presence of odd soils such as peat.

Q5(ii) Some did this very well, sketching appropriate stereonets and indicating how they could be used to analyse rock failures Q5(iii) No candidate understood the implication of “laminated” materials; they have a transverse anisotropy making the definition of their strength a function of direction.

Question 6 This question required candidates to analyse results from a laboratory oedometer test. They should have related change in void ratio to change in height of the sample. Some made errors at this stage but generally this part of the question was well done. After plotting the results the candidates were then required to evaluate the coefficient of volume compressibility mv, many made errors with the units. Some of the plots were poorly labelled and difficult to follow. In the second part of the question candidates should have explained that mv was used primarily to evaluate the magnitude of primary consolidation settlement and explained the parameters in the formula sc = mv.h.Δσz

Question 7 This question required the candidates to evaluate the flow beneath a concrete weir through an anisotropic soil. Candidates should have initially drawn the weir to a transformed scale and then gone on to draw a conventional flow net and then evaluate the hourly flow beneath the weir. Some were able to do this, others failed to redraft the weir and some quoted the flow rate per second not per hour. In the second part of the question candidates should have explained how the pressure head could be evaluated at points below the weir by subtracting the elevation head from the total head and then how to move on to evaluate uplift force from the individual values of pressure. In the final part candidates should have used the flow net they had produced earlier to evaluate the hydraulic gradient over the last curvi-linear square and compared that to the critical hydraulic gradient (the critical hydraulic gradient can be assumed to be unity). Question 8 Candidates are referred to standard texts/ websites to identify the difference between the techniques in the first part of this question. Few candidates used diagrams to help with their explanation of the techniques. Many failed to describe the mechanics of the processes and their effects on the soil structure. In the second part of the question many described only how cement grout worked and failed to recognise the roles of other types of grout. Question 9 Few attempted this question and, of those that did, the standard of answer was variable. This question required the candidates to analyse the results of the liquid and plastic limit tests. Candidates should have plotted penetration versus moisture content data in order to determine liquid limit, then after determining the plastic limit, evaluate the plasticity index and hence classify the soil using Casagrande’s A-line chart which was included in the question paper. For the second part of the question candidates are referred to standard texts. Generally the standard of answer for this part was better than the first. Question 10 Candidates were required initially to discuss 3 options that were appropriate to retain a 4 metre high cutting in addition to evaluating the appropriateness of reinforced earth. Knowledge of reinforced earth and its advantages and disadvantages varied widely. Knowledge of other methods of retaining the cutting was better, appropriate methods included reinforced concrete retaining walls, masonry retaining walls, crib walling, anchored sheet piling, reducing the angle of the cutting etc. Of those that described the methods of failure many were not able to list more than two. The examiner was looking for a description of four failure modes. These may have included tie/geogrid pull out, tie/geogrid breaking, overturning, sliding and bearing capacity failure.

Question 11 This question was answered by relatively few candidates. Of those that did attempt it, most were able to derive an answer for the first part although some did take very indirect routes and some derivations were not logical. Most were not able to describe how to measure residual shear strength (shear vane, shear box etc) although most were able to describe what residual shear strength was and to provide a shear strength/strain plot to show how it developed Question 12 This question required the candidates to analyse the foundation proposed. They should have established the short and long term ultimate bearing capacity and, in their comments recommended which was critical. Additionally they should have suggested that settlement be checked in both cases to ensure that this was not a critical factor. Some calculations were long, rambling and difficult to follow. Candidates are encouraged to provide neat, well laid out solutions, annotated where appropriate. Many described all four foundation types not three as required by the question. Some clearly had little idea of the fundamental types of foundation and how they functioned. Question 13 This question was attempted by many with a significant proportion scoring well. Candidates should have evaluated the change in stress by Fadum’s method at the point 5 metres below ground level due to the foundation/ excavation after evaluating the net increase/decrease in vertical stress due to the foundation/excavation. They should then have gone on to list the change in vertical stress at the two dates. For the second part of the question the candidates should have listed reasons why they would carry out the calculations. These might have included checking loads on buried structures, checking settlement, checking on overstressing of strata. They should have recognised that other stresses at other locations would need to be determined to get a comprehensive picture of what was happening to the soil and the significance of time on settlements depending on soil type.

Unit 215 Fields and circuits

General comments The examination scripts submitted by some candidates were extremely untidy, sometimes with the solution to part of one question even being embedded in the solution to a quite different question. The handwriting of some candidates was so bad that, at times, it was difficult to understand what was intended and consequently to award any marks. Candidates should pay much more attention to the organisation and presentation of their material, to ensure that they obtain appropriate credit for correct and relevant answers. Many candidates answered more than one question in an individual answer books and some even used one answer book for the whole paper. Many candidates did not fill in the cover sheet, indicating the questions that they had attempted. As with points 1) and 2) above, this is simply a matter of developing a good and sensible technique before even attempting the examination. With a significant number of candidates failing to achieve double figures, it appears that many are attempting the examination without any prior study of the topic whatsoever (see for example comments to questions 1,2, 5 and 6 below). Comments on individual questions Question 1 Although this was a popular question and should have provided easy marks, many candidates did not appreciate the simple concept that the original charge on the system must be conserved throughout. In addition, a very common mistake was to neglect the vector properties of the electric field. Question 2 Another question which should have yielded easy marks, but again many candidates did not appreciate the vector nature of the magnetic flux density. Question 3 The solutions to this question mainly involved written explanations of straightforward phenomena, but many students were far too brief with their descriptions (sometimes simply yes or no). This may imply an unfortunate lack of understanding of the basic physics of the situation. Question 4 Perhaps because the unusual topic of magnetic flux compression may not have been encountered previously, very few candidates had sufficient confidence to attempt this question. It was however framed in such a way as to make the solution quite straightforward. Question 5 A very surprisingly large number of candidates attempted to determine the inductance of the system using electrostatic rather than electromagnetic field concepts ie D,E and εo instead of B,H and μo. Only a couple of candidates remembered to calculate the contribution made by the magnetic field within the centre conductor to the overall inductance.

Question 6 This question was very popular and often well attempted. A number of candidates achieved reasonable marks. However a significant proportion attempted the question y domain (jω) analysis, whereas since it is a transient problem an analysis in the s-domain is obviously appropriate. Few candidates sketched correctly the required waveforms. Question 7 Only very few candidates attempted this problem and no reasonably correct solutions were received Question 8 This was a popular question, but many candidates either attempted to determine Z-parameters or Y-parameters for the circuit. This is a classic example of time being wasted and marks being lost through failure to read the question carefully and to answer what is actually being asked. Question 9 Again, many candidates failed to read the question correctly and did not draw the required line voltage. Consequently they applied Fourier analyse to the wrong waveform. Question 10 This was another popular question with many candidates successfully completing the analytic parts. However, the required sketches of the waveforms often produced results that appeared to have no relationship with the analytical work.

Unit 216 Electrical machines and drives

General comments The standard of answers this year has generally improved over those of last year. The notable areas of weakness are in variable speed operation of machines and in questions requiring a qualitative answer. It was generally observed that the students had difficulty clearly expressing themselves in English and that poor hand writing made some answers confusing to the point where the examiners could not be sure of the point being made. Comments on individual comments Question 1 This was a fairly standard question on ‘DC machine’ types and their equivalent circuit representation and similar questions are found in text books. Almost every candidate attempted the question and most performed well. Areas of weakness were as follows. Inductances were omitted from the motor circuit representation in part (a). Some students could draw the constant power and torque regions but did not provide the explanation required in part (b). Braking techniques were discussed by majority of students as required in part (c) without properly describing “Regenerative braking” or providing a correct diagram. Though many students have attempted part (d) few managed to arrive at the correct solution. Numerical answers:

Part d (i): Braking resistor = 4.0 Ohms Part d (ii): Rate of deceleration= 91 rpm/s

Question 2 This was a numerical question concerning a DC shunt motor. It was attempted by more than 80% of candidates. Many successfully solved part (a) which used standard formulae but few were able to solve part (b) which required derivation of an equation in order to calculate the additional resistance. Some candidates managed to form the quadratic equation but could not solve it for the right answer. The number of completely correct solutions to (b) was very low. Numerical answers:

Part a (i): Voltage across the armature = 490 V Part a (ii): Torque = 75 Nm Part a (iii): Efficiency = 92 % Part (b): Additional resistance = 329 Ohms

Question 3 This question was about 3-phase induction machines and covered standard material. One would expect well prepared candidates to perform well. This was the case in parts (a), (b) and (c). However part (d) required obtaining an analytical expression to estimate magnetising reactance for the given circuit. It was clear that many candidates lacked the analytical skills to make a success of this. In part (e), the students knew the star-delta starter principle and the relationship with the current but failed to mention the start-delta starting torque relationship (approximate factor by which these are either increased or decreased).

Question 4 This question required calculation of variables from the equivalent circuit of a 3-phase delta connected induction motor. Part (a) appeared like a relatively difficult section for many students as they were not able to arrive at the right expression for torque in terms of slip and phase voltage. The following parts of this question were dependent on this expression from part (a). This had a cascading effect and as a result, only few students answered the subsequent parts accurately. Numerical answers:

Part (b): Starting torque = 455 Nm Part (c): Slip = 0.39 Part (d): Safe torque during voltage sag = 227 Nm

Question 5 The question concerned operation of a 3-phase induction motor from a variable-voltage/frequency inverter. In part (a) many candidates simply listed benefits rather than describing those benefits (in terms of why they arise or why they are useful). In part (b) majority were not aware of the importance of connecting the low-pass filter between the power converter and the motor. The sketches of typical waveforms required in parts (c) and (d) were generally good. Many candidates showed a lack of understanding of soft starting strategies for induction machines. Question 6 This question related to single-phase machines and universal motors. Although the candidates recognised the concepts concerning single-phase machines, their explanation were not supported by appropriate diagrams (e.g. production of pulsating magnetic field or torque-speed curves of two equal and oppositely rotating magnetic fields) in part (a). Most answers to part (b) were incomplete and few included comments on techniques such as shading the stator poles. Part (d) was widely attempted but approximately half of the attempts did not obtain the expression of torque developed by a universal motor in terms of the supply voltage amplitude and frequency. Question 7 It was clear that the topic of brushless DC machines is still not well known to candidates despite its appearance in previous exams. Relatively few candidates tackled this question. Of those that did, most gave reasonable answers to parts (a) and (b) on the brushless DC machine’s basic constructional features and circuit topology. In the final tow parts two different comparisons were called for but many candidates confused or mixed-up the issues and this lack of clarity seemed to betray a lack of understanding of why certain benefits or features arose. As has been observed in previous years and in other questions this year, many candidates list advantages and disadvantages without the reasoning or description required. Question 8 In this question on power electronics circuits for machine drive systems most candidates could draw the circuit diagrams as asked in the 3 sections of part (a) but few could support the drawings with clear explanations of the operating principle of power converters (particularly the DC-DC chopper with a step-down characteristic). Part (b) concerned ‘snubber circuits’ and candidates provided a reasonable explanation but very few could go on to illustrate the disadvantages that arise from their use. Question 9 This question concerned motoring and braking operation of a DC motor. It is a standard question and the topic has been included in the examinations of previous years. Some candidates were well prepared and could answer the question easily but others had not prepared. There were satisfactory answers to parts (a) and (b) but in part (c) answers were incomplete. Reasonable attempts were made at describing a 2-quadrant chopper although some students also drew and described a 4-quadrant chopper. Some candidates drew the circuit without explanation of its operation and forfeited half the marks.

In part (d) little attention was paid to current limit so most answers were a diagram of a single control loop based on a tacho-generator feedback. Most did not explicitly show a difference block and error signal. Some candidates described the action of the control loop. Numerical answers: Part (a): Duty cycle = 0.66 Part (b): Efficiency = 95.4%

Unit 217 Electrical energy systems

General comments A recommended textbook for this paper is Electrical Power Systems by B M Weedy and B J Cory, Fourth Edition, Revised 1998. To prepare for the examination students are advised to study the relevant materials contained in the book in addition to the material in the unit syllabus. Comments on individual questions Question 1 This was a question on load flow and it was simpler than an equivalent load flow question in last year’s paper. It was a test of basic understanding of the background theory and calculation method. The question was probably the least popular on the whole paper and was attempted by only few candidates. The theory and the calculation method are clearly presented in the recommended textbook. Question 2 The question was on asymmetrical fault calculation and was divided into two parts, with the former being on the derivation of the connection of the sequence networks for a phase-phase-earth fault. The second part was an entirely numerical calculation. The question was popular and the average mark for the second part was high. The first part was poorly attempted, although the derivation is well covered in most standard power systems textbooks. Question 3 The question examined the student’s basic understanding of the transient stability of synchronous machines. It was a very popular question and was attempted by about 90% of the students. Those who attempted it answered well except for the second half of part b), with and the main reason being that candidates had not read the question carefully. In that part only few students explained how it should be solved and none of them finished the numerical calculation completely. Question 4 The question tests the basic understanding of system economics and system operation. It was a very popular question and was attempted by over 70% of the students. The question was divided into three parts and the students had difficulties when they came to the pricing of electricity. The material for answering the questions is well covered in the recommended text. Question 5 This was a standard question that tested the theory and calculation of electric field strength of an underground cable with two different layers of concentric dielectric. About 50% of the students attempted the question and the majority made a reasonable attempt. The average mark was also good. The theory and calculation method of the question are covered in most standard power system textbooks. Question 6 The question was a traditional one on the loading, reactive compensation and operation of overhead transmission lines. The question was popular and was attempted by about 60% of the students. However the average mark was low. Candidates did not appear to understand the relationship between overhead transmission line loading and reactive compensation, although the theory is well covered in standard power system textbooks.

Question 7 The question was on the simple equivalent circuit of single phase transformers and the implementation of the circuit model to calculate, in per unit form, the transformer input/output voltages. It was a very popular question and was attempted by about 85% of the students. The average mark would have been much higher if the theoretical part had been better attempted. Again the theory and method of calculation is covered in most standard power system textbooks. Question 8 The question was designed to test the basic theoretical knowledge of representing the parameters of a synchronous machine in phasor form, and then using these basic relationships to calculate the machine output quantities. The question was very popular and was attempted by over 85% of the students. Again the average mark would have been much better if the theoretical part had been better attempted. Question 9 Only about 14% of the students attempted this question and the majority of those appeared to have come from the same examination centre. They also made a reasonable attempt. The three topics in the question, from which the students were asked to write a technical report on one, covered three different areas: electricity market, system automation and network design.

Unit 218 Electronic systems engineering

General comments Overall the results achieved by the candidates in this years examination showed a considerable improvement over the results achieved in 2004 and 2005. 50% of the candidates sitting the examination achieved a pass grade, which is up from the 33% pass rate achieved in 2005 and the 39% pass rate achieved in 2004. It is however disappointing to report that the examination performance of many candidates was poor with roughly 40% of the candidates obtaining a Grade F. Many of the candidates obtaining the fail Grade of E and F continue to show poor exam strategy and time management. Candidates should be guided to read the paper thoroughly, and concentrate carefully on keeping to the subject matter requested. Many scripts contained answers to only three or four questions and where five questions were attempted the answer provided to the fifth question was often of negligible value. Candidates should be encouraged to make sure that they leave at least 20 minutes for their fifth question. On a positive note it can be reported that many of the candidates were well prepared for the examination with many papers showing well thought out responses to questions. On an additional positive note three analogue questions concerning amplifier design, filter analysis and the Wien bridge were generally well answered with many candidates obtaining good marks for these questions. The combinational digital logic question that until last year had normally been successfully answered by a majority of the candidates again produced many poor responses. Comments on individual questions Question 1 This question tests a candidate’s ability to design combinational logic circuits given simple specifications. The question also tests the ability: -

• to implement logic circuits using AND and NAND gates, • to apply Karnaugh map reduction techniques, • to design a logic circuit using a PLA.

In section (a) many candidates produced solutions which did not utilise a single 3 line to 8 line decoder as required by the question. The design of the simple decimal-to-BCD encoder of section (b) produced very few correct responses with many candidates making no attempt at a solution and many others demonstrating that they had no appreciation of the logic circuit’s function. The combinational multiplier circuit specified in section (c) is a four input, four-output circuit and although a proportion of the candidates were able to derive the correct Boolean equations for section (c) i) many of the equations were not minimal. A significant proportion of the candidates produced faulty truth tables and Boolean equations for functions, which were unrelated to the specified multiplication operation. In the PLA design of section (c) ii) many candidates employed a PLA with four outputs and more than eight-product terms, which was a larger PLA than specified in the question. Several candidates did however provide good answers and obtained full marks for section (c). The combinational logic section of the syllabus has been covered by a question in each of the last three years papers. Question 2 Approximately 30% of the candidates attempted this question which tests the knowledge and understanding of MOSFET transistors. The candidates responses were very variable with total marks showing an unusually wide spread across the full range.

Section (a) which tests understanding of the operating principles of a MOSFET transistor circuit produced satisfactory responses from most candidates. Section (b) which tests a candidate’s ability to perform simple D.C. analysis of a NMOS transistor circuit, was in the main poorly answered. Many candidates failed to sketch the correct circuit with the transistor being connected through the 1.0kΩ resistor to the 18 V supply. A number of candidates did not appear to understand the difference between operation within the current source action area and operation outside the current source area. There was a complete lack of comment when calculations threw up impossible drain-source voltages in excess of –20 V and currents measured in Amps. Section (c) tests a candidate’s ability to derive a MOSFET transistor operating point given the transistor transfer characteristic and only about one in two of the candidates attempting the question gave an answer to this section. It is regrettable that few candidates demonstrated the ability to correctly draw an appropriate bias line on the transistor transfer characteristic so few candidates were able to determine the required quiescent operating values. The operation of a NMOS transistor was examined in the 2005 paper and section (c) is identical to a section in the 2004 paper. Numerical answer: (b) i) VDS = 6.9V, ii) VDS = 0.7 V, iii) 10kΩ (c) IDQ = 0.19mA, VDSQ = 6.45V, VOQ = 7.4V Question 3 Section (a) of this question tests the ability to analyse a feedback circuit employing an operational amplifier and a particular feedback network. Sections (b), (c) and (d) tests candidate’s knowledge of second order low pass filters and stability criteria. This was one of the most popular questions in the paper and the first three sections of the question were quite well answered by many candidates. Most candidates were able to correctly state the appropriate nodal equations and the majority of these candidates were then able to derive the required voltage transfer function. A few of the candidates stated the appropriate nodal equations but lacked the mathematical manipulative ability to derive the given expression. Section (b) was answered satisfactorily with candidates correctly identifying appropriate functions for fo and Q, deriving the correct transfer function and sketching an appropriate response for a 2nd order low pass filter. In section (c) most candidates were able to obtain the correct value for fo, however a significant number of candidates quoted the expression Q = ωo/bandwidth and used it to determine fo. This expression is an accurate expression for band-pass filters but cannot be used in a low-pass filter situation. Few candidates were able to derive the correct maximum value for A in section (d) and obtain full marks. Questions on operational amplifiers have appeared in each of the last three years papers and second order high pass filters were examined in the 2005 paper. Numerical answer:

(b) fo = 1/2πkRC2 Q = k/2 (c) fo = 5.3 kHZ (d) The maximum value of A is 1.5.

Question 4 Approximately two out of three candidates attempted this question which tests a candidate’s knowledge of and ability to design small signal amplifiers. The question produced many good responses and a significant number of these gained more than 14 marks. Section (a) is standard bookwork which tests a candidate’s understanding of a transistor current mirror circuit and many candidates obtained full marks for this section. Given a basic specification for a single stage audio amplifier, section (b) tests a candidate’s ability to take decisions and design an amplifier to meet specific parameters. This section produced a wide range of responses from excellent to very poor. In recent years similar questions have in general not been well answered so it is gratifying to note that a significant number of candidates have mastered the single stage transistor amplifier design process. Most candidates provided an appropriate single stage amplifier circuit diagram showing all the relevant components. They also recalled equivalent amplifier circuits and some unnecessarily produced several pages of theory relating the transistor parameters with gain and input impedance etc. A number of the candidates appeared reluctant to make any assumptions on component values so that they could get a realistic design started. Even the basic task of estimation and calculation of suitable transistor base biasing resistors proved to be too difficult for many of the candidates. A significant number of the candidates ended up with a suitable amplifier design. The operation of a current mirror was covered in the 2003 paper while the ability to design a single stage amplifier was tested in the 2004 paper. Question 5 This question tests a candidate’s knowledge of flip-flop operation and the ability to analyse and design simple synchronous circuits using J-K flip-flops. Approximately 75% of the candidates attempted the question and more than a third of these answers gained 12 or more marks for the question. Section (a) tests a candidate’s practical knowledge of the operation of an edge-triggered J-K flip-flop. Although many candidates produced accurate Q output waveforms many other candidates produced faulty waveforms showing a lack of understanding of the operation of the asynchronous inputs. Section (b) tests a candidate’s understanding of a state diagram and the ability to derive state transition and excitation tables and then the ability to design a simple sequential synchronous circuit using JK flip-flops. A satisfactory number of candidates showed that they both understood the design process and could exercise it by producing accurate logic diagrams and showing that the circuit was self-correcting. However many candidates who had correctly completed the design process lost marks in section (b) for failing to show on their logic diagram that the variables A, B and C which were shown on their diagram as inputs were in fact identical to the flip-flop outputs A, B and C. In addition many candidates failed to consider output Y in their analysis or show the output Y anywhere on their logic diagram. Generally the Karnaugh map work of the candidates was acceptable. However many candidates made errors in the minimisation process and did not employ ‘do not care’ states. JK flip-flops were a topic in the 2004 paper and some aspect of sequential circuit design has been covered in each of the last three years papers. Question 6 Section (a) is bookwork and tests a candidate’s understanding on the necessary conditions for stable sinusoidal oscillations while section (b) tests a candidate’s understanding of how a Wien bridge circuit functions. This was one of the most popular questions in the paper with approximately 75% of candidates attempting the question. Many gained good marks with half of the attempts attracting a mark of 12 or more.

Most candidates did well in section (a) although many lost marks for making no mention of the gain condition for constant amplitude. Section (b) i) was in general very well answered with most candidates giving the correct derivation of the frequency of oscillation in terms of circuit components. Rather surprisingly section (b) ii) which required only a simple calculation provided difficulty for quite a few candidates who could not calculate the maximum and minimum values of the frequency of oscillation. Lack of basic arithmetical and manipulation ability was a notable disappointing feature in many cases. Only a few candidates successfully calculated the minimum gain to sustain oscillations asked for in section (b) iii. Sinusoidal oscillators have not been covered in any of the past three year’s papers, however other types of waveform generators were examined in the 2004 and 2005 papers. Numerical answer:

(b) ii minimum frequency = 15.9 Hz, maximum frequency = 1.59 kHz (c) minimum gain = 3 Question 7 This question tests a candidate’s understanding of feedback, and the ability to calculate the effect of feedback on an amplifier so it is not unexpected that it was attempted by around 70% of the candidates. In a similar manner to question 4, the question produced many good responses and a significant number of responses gained more than 14 marks. There were however also many poor responses from which the candidates gained only two or three marks. The candidates who gained good marks had no trouble with the gain and impedance calculations of section (a) while the candidates who performed poorly were generally unable to correctly derive or state the expression linking open-loop and closed-loop impedance. Approximately half of the candidates attempting the question correctly indicated that the amplifier of section (b) was a voltage follower/unity gain buffer and provided a suitable application for the amplifier. A few candidates obtained full marks for section (c), which tested knowledge of feedback theory and its effect on the gain and frequency response of an amplifier. Many candidates correctly calculated the mid-band gain but were unable to calculate the closed-loop frequencies. Generally these candidates were able to correctly sketch Bode diagrams so this illustrated that they knew what the effect of the feedback would be on the frequency response. This section also tested the ability to understand the concept of decibels and perform gain calculations when values are expressed in dB. In recent years dB calculations have caused considerable difficulty. It is pleasing to be able to report that the number of candidates who inserted the dB values directly into the gain formulae, without any prior translation of the dB value into an actual numerical value for the gain, was much reduced on recent years. The effect of feedback on amplifier performance has been covered in each of the last three years papers. Numerical answer: (a) closed-loop gain = 20, input impedance = 12000MΩ output impedance = 0.017Ω (c) i) gain = 36.3dB, ii) 0.51 Hz and 24.2Mz Question 8 This question tests a candidate’s knowledge and understanding of Class A and Class B power amplifiers. Less than 20% of the candidates attempted the question and in general these candidates did quite well and obtained good marks. However a number of candidates responded quite poorly to the descriptive section (a) by providing a response that emphasised the differing amplifier application areas but made no mention of the different periods of current flow throughout a full cycle of the supply. Section (b) tests candidate’s understanding of a Class B push-pull amplifier and candidate’s ability to perform power dissipation and efficiency calculations. A few candidates lost marks when their

calculations produced unrealistic values like 50kW and then made no mention of possible error in their calculation. Power amplifiers have not been covered in any of the past three years papers. Numerical answer:

(b) iii max. power output = 83.3W, max. power dissipation = 16.9W iv efficiency at max. power output = 78% efficiency at max. power dissipation = 50% Question 9 Less than a third of the candidates answered this question, which covers standard bookwork, on two different parts of the syllabus. Few of the candidate’s responses produced good marks for all three sections of the question with most candidates being awarded marks in a 6 to 10 band for the question. Section (a) which tests a candidate’s understanding and knowledge of the CMOS and TTL digital logic families produced good responses from a number of candidates. However many other candidates spent their time drawing detailed circuit diagrams of different logic gates and did not provide any appropriate information on the characteristics or application area of the two families. Sections (b) and (c) which test knowledge and understanding of basic D-to-A and A-to-D converters produced some good but also some very poor responses. It was clear that some candidates had covered the material while others clearly had not. The 2004 paper had a section of a question on the characteristic features of TTL gates and A-to-D converters featured in both the 2003 and 2004 papers. Numerical answer:

(c) Final state = 101101

Unit 219 Telecommunication systems engineering

Comments on individual questions Question 1 This was a reasonably popular question although the answers were, on the whole, rather disappointing. The advantages being looked for in answer to part (a) included increased demand for digital data transmission, easy source coding, easy channel coding, easily traded resources (power/bandwidth/time), standardisation of signals allowing a single network to be used for disparate applications (voice, video data etc.). Other credible answers (even when unexpected) were given appropriate credit. Transmitter blocks which were given credit in part (b) included ADC (when split into its component parts of sampling, quantisation and PCM coding credit was given for each element separately) source coding, encryption, channel coding, multiplexing, line-coding, pulse shaping, modulation and multiple accessing. The converse processes in the receiver (plus equalisation, matched filtering and the decision process) also earned credit. Other blocks are possible and were given credit on merit. For part (c) many students described any process of mapping source information to a bit stream as source coding. This was not accepted; some reference to redundancy removal or data compression was required to earn the allocated marks. Most students described security coding in terms of privacy only. This scored half the allocated marks. Reference to at least one other security function such as authentication or data integrity was required to score all the available marks. The function of error control coding was generally well described. Question 2 The formula I = log2[1/P(m)] or equivalent was correctly given by many students attempting part (a) and correctly applied to find the information content of the message specified. Part (b) was not well answered in general. It had been complained that no graph paper had been supplied. The examiners assume that this was due to not understanding the distinction between an instruction to sketch and an instruction to plot in the context of a function or relationship. Part (c) was generally well answered by those attempting it, many students earning full marks for a numerically correct answer. This was occasionally (and surprisingly) the case even when the candidate had apparently struggled with part (a), part (b), or both. Some candidates misinterpreted part (d) assuming that ‘half its maximum’ referred to half the value for equiprobable symbols. (It was intended to refer to half the value for statistically independent symbols with the probabilities stated in part (c).) Credit was given, however, where this misinterpretation had been made and its consequences followed through correctly. Question 3 Most candidates answering part (a) could, at least roughly, describe what was meant by a periodic signal. (Significant leniency was shown to rather vague descriptions - candidates are reminded to strive, as far as possible, for precision in their use of language as well as in numerical work. The characteristic feature of a periodic signal’s spectrum asked for in part (a) is its discreteness (i.e. line character).

Few candidates competently tackled part (b). The correct entry to use in the table is exp[-π(t/τ)2]thus identifying τ = √(π/1000). The resulting pulse spectrum is then given by substituting this value into the table entry τ exp[-π(τ f)2]. The special property of the Gaussian function under Fourier transformation is that the shape remains Gaussian. The 3 dB bandwidth in part (c) corresponds to the bandwidth at which the (voltage) spectrum has fallen to 1/√2 of its peak value. In part (d) candidates were expected to justify the spreading of the pulse in the time domain due to the reduction in pulse spectrum width that occurs as a result of filtering. Question 4 Many candidates attempted this question. A disappointing number had difficulty in distinguishing the four categories of noise listed in part (a). (Many candidates, for example, incorrectly stated - or implied - that Gaussian noise had a Gaussian spectrum rather than a Gaussian probability distribution.) Almost no candidates could correctly supply a convincing proof of part (b) but nearly all candidates attempting the question scored very well on part (c). Question 5 This question covers the syllabus topics relating to A-D conversion for telephone signals with particular reference to the PSTN and the related techniques for transmission, switching, control and multiplexing. This was the most popular question on the exam paper and one for which a majority of candidates were able to display good knowledge of the multiplex structure of the SDH and hence score well. a) The basic channel parameters, and thus the entire subsequent multiplex structure of the SDH, are based upon the sampling and log-quantisation requirements of a 3.4KHz telephone channel to produce a basic 64Kb/s bit stream. Several candidates neglected to mention the need for the important anti-aliasing filter prior to sampling. b) A large majority of responses correctly described the 9 row by 270 byte frame structure of an STM-1 frame and calculated that STM-1 SDH can carry (270-9) x 9 = 2349 telephone channels. Similarly most responses identified the specific benefits of SDH by comparison with legacy techniques – synchronous working; hence no data loss through slippage; the ability to extract single channels without having to de-construct a multiplex mountain; the possibilities for more sophisticated system functions. Question 6 Another popular question here covering an understanding of the characteristics of transmission lines with particular reference to telephone lines and with particular reference to the need for broadband digital Internet use. a) Some candidates were aware of the attenuation and frequency response limitations of copper twisted pair lines: few quoted dispersion and crosstalk as major problems limiting bandwidth and very few noted that reactive compensation, adaptive equalisation and cable design were available to alleviate these limitations. b) Almost all responses were able to quote the channel parameters of basic rate ISDN and many were aware that ADSL uses frequency-division multiplexing to allocate bandwidth to POTS, a wideband uplink and a narrower-band downlink. This is particularly suited to internet use since the majority of bandwidth is required for uploading. c) Although most responses could quote bandwidths for ISDN and ADSL few could perform the trivial calculation of transmission time for 1Mbyte of image data.

Question 7 Also a popular question examining appreciation of the ISO-OSI and its application. a) Whilst many responses could quote the advantages of open system standards few seem to understand that the ISO-OSI model is simply one of many possible methods of classifying the functional requirements of a general communication system in levels of increasing abstraction in order to simplify system understanding, flexibility and management; to open the design of associated hardware or software and to provide standardisation of system performance parameters by allocating performance criteria at each level. b) The large majority of responses correctly described the 7 layers of the ISO-OSI model but many had only a hazy notion of how the communication between layers and the virtual communication between peers are accomplished. c) Most candidates described, mostly in commendable detail, the Physical Layer, Data Link Layer and Network Layer functions of the 3 lowest layers in the ISO-OSI model. Fewer could give an example of level 3-to-level 3 communications such as the X25 packet structure. Question 8 A moderately popular question covering the range of syllabus topics relating to Radio systems. a) Several candidates correctly calculated the link parameters for this microwave link: Antenna effective area 1.237 m2; Antenna gain 34.4dB; Signal Strength 1.24 x 10-7 w/m2; Received signal 1.54 x 10-7 W b) Very few candidates appear to understand the basic problems of radio wave propagation. Most understand the concept of fading; some realise that short-term fading is mainly due to multipath propagation and longer-term fading to shadowing but no-one quoted the PDF of the former as Rayleigh and of the latter as log-normal. Question 9 A question covering the syllabus content on general modulation with specific reference to digital amplitude and angle modulation but which received relatively little response from candidates. a) IF modulation is used as a method of multiplexing several channels that allows r.f. processes at lower frequencies which generally is easier and cheaper than at transmission frequencies. It also enables frequency-tunable receivers without having to tune downstream circuitry. b) Most responses were able to sketch or otherwise describe the basic digital amplitude and angle modulation schemes covered in the opening chapters of most texts on Digital Transmission: OOK, BFSK and BPSK and some to outline simple block diagrams as to how they might be constructed in practice. Few were able to sketch PSD diagrams however. c) Responses indicated very poor understanding of constellation diagrams particularly the very widely used combination of amplitude and angle modulation known as Quadrature Amplitude Modulation.

Unit 224 Mathematics

General comments The results were generally very disappointing and there was a large number with low marks. As in previous years it is clear that many of the candidates are inadequately prepared for this exam. Many clearly did not have the knowledge or skills needed to answer some of the questions, but they are students of higher ability than the marks would suggest. They do not seem to have been very well prepared and it is particularly disturbing to observe that if they had prepared with just a little more emphasis on understanding, rather than learning mechanical procedures, many of them could easily achieve substantially higher marks. In answers to Question 6 for example they show evidence of having learnt procedures for finding eigenvalues but clearly do not really understand what an eigenvalue actually is. Comments on individual questions Question 1 - 2D optimisation using partial differentiation This was a popular question but not very well answered. The most surprising thing was that a majority of candidates left out the conditions fx = fy in part(a). Most candidates were unable correctly to work out the second derivatives in part(b) due to poor skills at algebra and bad technique. Question 2 - Vector calculus Part (a) was reasonably well answered but many students’ answers were over long and clumsily expressed, especially in part (ii). In part (b) (i) only some candidates correctly used the result from part (a)(ii). In part (b) (ii) very few progressed beyond writing down the Divergence Theorem. A lack of knowledge of spherical polar coordinates was very evident. Question 3 - Green’s theorem and complex analysis Part (a). A very common fault was to assume x2 + y2 = 1 over the whole domain. The use of plane polar coordinates seemed largely unknown. In part(c) the complex integration was not well done. Although many knew about residues very few could do the evaluation. Question 4 - Z transform and Laplace transform Part (a). There were few good attempts at this. The Z transformation did not seem well known and many candidates showed utter confusion. Many were unable to transform the differential equation in part (b). In past years questions like this used to be much better answered. Question 5 - Fourier series Part (a) was probably better done than many of the questions on the paper, possibly because the procedure was routine. Part (b) was fairly well done but often uncompleted because of an inability to apply boundary conditions correctly and/or (and especially) the initial condition. Question - 6 Discretisation of ordinary differential equation an eigenvalues Most showed a very poor understanding of eigenvalues and eigenvectors. Except for parts (a) and (d) the rest of this question was often poorly answered if at all.

Question 7 - Finite difference solution to Poisson’s equation Part (a) Very few attempted this question. It is sad and disturbing that this question is ignored. It is such an important topic to all engineers and scientists and involves such simple mathematics it ought to be one of the most popular questions. Question 8 - Applied statistics This was quite well done on the whole although many did not distinguish between Hypothesis Testing and Confidence Intervals. Question 9 - Steady state difference equation and Gauss-Seidel iteration There were very few correct answers to part (a). Part (b) was much better done although some confused the Gauss-Seidel and Jacobi methods while others tried their own methods.

Unit 225 Dynamics of mechanical systems

General comments Again this year, the candidates showed a relatively poor awareness of how to apportion time to gaining marks. Short explanations of what they would do (given enough time) could have gained significantly more marks in most cases even if there was not time to complete the computations or if these had gone wrong. The average marks per attempt at each question were good for four questions but very poor for three others and two of the questions were not attempted by anyone. The examiners impression is that the standard of understanding was generally sufficient to obtain a pass mark but candidates have not managed their exam time well and/or they have not got themselves to such a level of proficiency that they can address the questions in suitably short order. The comments on individual questions reports specific comments on the way in which the questions were addressed. Specifically, it enumerates difficulties in interpretation, common errors made and other retrospective remarks on the goodness of the questions based on the attempts made by the candidates sitting the examination. Comments on individual questions Question 1 This question was about the vibration of single degree of freedom systems and 2 degree of freedom systems. Broadly similar questions arose in 2005 and 2004 and a near-identical question occurred in the 2003 paper. The response to this question was generally fairly acceptable. Part (a) should have been easy for anyone sitting this exam and indeed most candidates gained most of the marks for this part. The attempts at part (b) were also generally fair. None of the candidates demonstrated any useful knowledge of the principle of a vibration absorber. Question 2 This question combined a small number of basic principles: conservation of angular and linear momentum, conservation of energy and s = ut+½at2. It was not similar to any recent question but perfectly within the syllabus. Part (a) should have been found easy. Candidates made a reasonable attempt at this part. However none of them had a good understanding of the assumptions. For part (b), the logic for whether the base of the block would slip was not clearly set out in either attempt. In this question, candidates would have done well to ask themselves “what would happen if the arrow struck the block exactly at the centre” and “what would happen if the arrow struck the block at the very top”. In the first case, the foot of the block would slide backwards. In the second case, it would slide forwards. Question 3 This question was a very standard “Rayleigh Quotient” question. Moderately similar questions occurred in 2005 and 2003. Nobody attempted this question.

Question 4 This question had appeared before in almost identical form in 1999. Related questions appeared in two of the last three years but not in a very similar form. Q1 of 2004 had much in common with it. In essence, this question required only Newtons law of acceleration applied once in the frame of reference of the car (for part (a)) and once in a stationary reference frame (for part (b)). Candidates were also expected to be able to verify that a hypothesized solution provided for an ordinary differential equation did actually satisfy that equation. This required them to know that d/dx(ln(x)) = (1/x). Part (b) required mainly that candidates would insert the answer from part (a) into the Newton equation of acceleration for the vehicle in the stationary frame and k. It was very disappointing that nobody appreciated this. Question 5 This question about dynamic balancing of a rigid rotor was comparatively well attempted. A similar question appeared as Q1 of the 2003 paper this question was a close replica of a question posed in 2000. Candidates generally made a reasonable fist of drawing the force and moment polygons. Several missed the important point that when the rotor is dynamically balanced, the bearing reaction forces are zero – and so they can be discounted. This was a relatively hard question to correct because the writing on the diagrams was difficult to make out in several cases and the benefit of doubt was given where it was required. Question 6 This question was all about moments of inertia of a body and about gyroscopic couples. A question on gyroscopic couples has appeared in some form in every exam paper since 1998 – and perhaps before then. The present question was very similar to Q5 of the 2003 paper. Part (a) required only the recognition that about the z axis, only the inertia of the outside would be felt whereas about the other two axes, the inertia of the rotor would be added to that of the stator. Part (b) was most easily solved by first principles – using the rate of change of angular momentum. It was surprising that parts (a) and (b) were not more successfully attempted. For part (c) one had to know how to apply the Euler / gyroscopic equations and it was not very surprising that this part was not seriously attempted. Part (d) was looking only for a comment about the assumptions. Question 7 Questions which were similar (at least to the extent that the same broad approach is needed) appeared as Q3 of 2005, Q5 of 2004 and Q3 of 2003. The structure of the question in 5 parts actually guided the candidates through the correct approach – calculate velocities first and then work on accelerations. The existence of a rigid link in every case informs us of a constraint on both the relative velocities of two points and on their relative accelerations. This was a question where marks could easily have been obtained simply by observing that point velocities had to be in particular directions and where the relative accelerations between two points A and B on the same rigid link had to be –�2r in the direction of the vector between those points. Gross errors were made even at the stage of working out the geometry of the mechanism at the instants shown. Question 8 This question was about eigenvalues (natural frequencies) and corresponding natural mode shapes of multi-degree-of-freedom systems. A 3-degree of freedom system was given. The question is similar to Q4 of 2005, Q6 of 2004 and Q6 of 2003. Nobody attempted this question.

Question 9 This question comprised mainly a standard balancing-of-reciprocating-engines component (worth 70% of its marks). Questions of this genre appear regularly in the Dynamics of Mechanical systems paper – for example Q2 of 2004, Q8 of 2002 and Q9 of 2000. The first part was addressed relatively competently by most of the candidates who attempted it. However, none of them were able to shed any light on part (b) which required the reasoning that the vertical component of conn-rod force accounted for the acceleration of the piston. Marks were available for even the indication of how it might be approached.

Unit 227 Control systems engineering

General comments • There is a need for students to place more emphasis on basic digital measurement techniques

and particularly those applicable to modern machine systems. • The use of the root locus method applied to the design of controller compensators needs more

emphasis. • Practical frequency response methods for controller design, eg Bode and Nichols, need better

coverage. System performance interpretation of frequency plot plots is also an area for improved understanding.

The results for 2006 indicate that typically students do not have broad coverage of the syllabus and that certain topics are either omitted or given scant consideration. The following comments are offered in relation to the questions of the 2006 examination. Comments on individual questions Question 1 This question deals with one of the most widely applied digital position measurement techniques based around the incremental encoder and it is clear that this topic receives little coverage. The level of detail required by the question is not great. There is a clear need for effort in this topic and in the associated basic digital logic methods used in modern instrumentation. The encoder forms the basis of velocity and position measurement and such a widely used method requires attention. Question 2 The state space method is generally understood and applied correctly. Question 3 The topic of A/D and D/A conversion is essential to modern measurement techniques and to the application of any computer based system used in machine processes. Very few students attempt questions on this topic and correct solutions are rare. It is clear that that this part of the syllabus receives little attention. Question 4 & Question 9 In general a large number of candidates successfully apply the basic root locus method to problems that are set purely in a mathematical form with transfer functions provided. However, questions in which the technique is used as a design tool, such as Question 9, result in few successes. This indicates a clear need for tuition in using the technique to the design of basic controller compensator functions. Question 5 The question deals with the use of z transforms and did not in attract many attempts, although those candidates who did so provided a successful solution to parts (a) and (b). No one attempted part (c) which involved writing a difference equation.

Question 6 Part (a) is a basic block diagram manipulation question which was attempted by most candidates, although only approximately 30% achieved a correct solution. Part (b) required the use of a given formula relating to transient response. Again there were many unsuccessful attempts indicating a need to understand the basics. Question 7 There is clear indication frequency response methods are not given sufficient coverage and that many students are not able to plot and manipulate frequency response characteristics. More preparation is needed in applying the techniques to practical problems. Part (b) of the question involved the use of given standard transient response equations and again yielded few successful solutions. Question 8 The questions deals with the application of PID control to a given system with the first 2 parts dealing with mathematical manipulation. These were in general successfully handled. The final part called for a practical interpretation of the system frequency response and did not yield any successes. Question 10 The manipulation of open loop data to generate closed loop information using Nichols or Bode was in general lacking. In general candidates are poorly prepared in being able to interpret an experimental data set which is a basic requirement of evaluation of experimental test results. Students are poorly prepared to handle any problems that require the design of simple series compensator control actions.

Unit 228 Information systems engineering

Comments on individual questions Question 1 This question covered Data modelling using the Entity Relationship approach. Specifically this involves design and mapping an ER model to form a relational data set (schema). There was no requirement to code up of the schema in SQL but an extension into applying entity and referential integrity constraints was required. The extension to conventional ER modelling was asked in the last part of the question. This type of question has been asked frequently in the past usually with reference to a case study/discourse. The discourse allows candidates to show their skill and technique in analysing and interpreting a database application. In attempting this type of question candidates can only arrive at an answer by following a technique and showing their working out. Also it is important to state any assumptions that are valid (ie do not contradict the discourse). In the answer the examiner was looking for: An outline of the Entity Types is given below from which referential integrity constraints can be derived ENTITY TYPES: Products ProductsOnOffer (suggest a sub-type of Products) Customers Members - (another sub type this time of Customers) CustomerOrders - a central entity Cart - another word for a Customer orderline Suppliers SupplierOrders SupplierOrderLine Warehouse Candidates may have unresolved Many to Many Relationships and have them resolved later into I to Many Relationships. Relationship Types are shown as links between the Entity Types above. These are: Products to Cart (single) (1:M) Products to SupplierOrderLine (1:M) ProductsOnOffer to Products (1:1) Mandatory/NonMandatory Cart to Orders Warehouse to SupplierOrders SupplierOrders to CustomerOrders (partial)

b) For example Entity Type CustomerOrder is assigned attributes as follows: cust_orderno#, custid, orderdate (PK marked #) similarly customer , supplierorder and supplierorderline respectively thus: Customer (custid#,ismember,custname,address) Supplierorder (sorderno#,supplierrid,orderdate) supplierorderline (sorderno#,product#, deliverydate, qtyonorder) interdependencies that should be explained include how to associate a product that originates from a customer order with the product ordered from a supplier to meet the original order. This is achieved by creating a FK (foreign key) CustomerOrderID to SupplierOrderID (because a product supplied to a customer must have originated from Products dispatched from a supplierorder - but not necessarily vice versa. c) The OOM aspect of the question could result in a range of answers depending upon whether candidates have a strong OO programming background or not. In particular answers that cover two approaches are possible. One approach is to use the notation of a Class as a direct mapping from an Entity Type. The second approach is a code-based approach used in Java that is a Class which has an explicit inheritance (for example) expressed directly in code at run time. Expect candidates to use Java or other OO language (eg VB.NET/c#) Private Class ProductsonOffer Private Class Products inherits ProductsOnOffer Private dsNW As DataSet dsNW = New DataSet("Northwind") 'step 2. create a data table Dim dtCustomers As DataTable dtCustomers = New DataTable("Customers") 'step 3. add data table to dataset collection dsNW.Tables.Add(dtCustomers) There are a couple of texts that students should reference and these are listed in the reading list: Data Analysis for Database Design (David Howe) Database, Design & Management David Stamper & Wilson Question 2 This question covered Methods for modelling information systems including UML (an explicit reference is made in the syllabus) and is well represented in previous papers. There are many new books and web references to UML. One particular recommended text which has only recently come to press is: Object-Oriented Systems Analysis and Design with UML by Robert V. Stumpf Publisher: Prentice Hall ISBN-10: 0-13-143406-3 This was a more discussion based question with a range of answers expected. Candidates with real design experience following a typical software products life cycle would be able to draw upon that experience and reflect on the techniques used.

In the answer the examiner was looking for: a) i) The choices depends on scale, robustness, emphasis (data driven vs process driven) platform, system automation tools , development language. ii) SWIFT would use a 'evolutionary' life cycle approach with key phases rolled out in a particular order as each phase depends on the previous phase. Each roll out would develop a prototype that reveals increasing functionality around the key phases: 1. user analysis phase (capturing requirements and building data model/process models) 2. stand alone design phase of database and some functionality(user testing) 3. web/client server phase (scalability testing) 4. web services (service oriented phase - full multi-user testing) 5. functionality-wise incremental development - evaluate/test then start from 1 again Candidates may suggest flavours of the above as a variety of the waterfall approach. Some justification of other design methods/life cycles would be expected b) UML has usefully combined a set of robust design techniques that fit well into Visual CASE tool automation and system management and allows users the flexibility of choosing their own methodology. Basically designers cherry pick the design technique most useful to communicate ideas to design groups and perhaps to users as well. UML also is not restricted to Information Systems (IS) but transgresses ground in the area of Real Time (RT) and other areas meaning that systems that have facets of both RT and IS can be designed using the same technique. Although mainly an OO approach Class diagrams in UML are easily adapted to express ER models with corresponding support from CASE tools that can automate the production of schema code. Question 3 The question covered: Effective implementation, evaluation and testing of an IS. An introduction to the feature of a multiuser relational database product including its data Management. Data integrity and quality control. Transaction processing. There are two traditional text books on the reading list: Fundamentals of Database Systems Elmasri, Navathe Introduction to Database Systems Date, CJ Addison-Wesley This is a general type of question that covers many general areas on the syllabus. It mainly covers database management presented in previous papers as a practical problem raised through a discourse. Knowledge of SQL as a data manipulation and constraint language is also represented in this question and this is covered throughout the syllabus. This was a more practical based question with a range of answers expected. Candidates with real database management experience would appreciate the problems raised in the question and have a routine or procedure to follow and would be familiar with administrative tools to support recovery/security. In the answer the examiner was looking for: a) Trigger - a specific database driven routine that is invoked by a database event such as INSERT/UPDATE DELETE and carries out processing on a specified table. Maybe to check validity of an INSERT statement.

Check constraint is a resident constraint built into a table definition when it is created. Check constraints are more limited than Triggers because the constraint code is limited to single line conditional expressions and by the SQL definitions allowed in a particular SQL version. Check constraints are also only affected by SQL INSERT and UPDATES clauses on columns in Tables and have no exception handling code that deals with exceptions. b) The answer depends on candidates knowledge of trigger functionality on the server they are familiar with. Basically the decisions are often quite general such as: Is a Business Rule more easily programmed/handled client or server side? e.g. javascript. Is a Business Rule more easily programmed/handled by another technique that has for example structured exception handling that runs on the application server middleware? e.g. Java , C# , ASP.NET Need to consider whether a Business Rule is better handled through the use of Views or stored procedures that have some deterministic characteristics, for example, calling a Business Rule whilst the interface is running (instead of automatically)? Also consider whether a Business Rule is better handled by a scheduled job or alert send by the DBMS either as a message or written into the database log when a Business Rule is violated? Taking all the above into account the best candidates would be BR1 and BR4 BR2 needs an alert to indicate failure but there is nothing a trigger can do to force the issue (ie nothing to write to a database) BR3 is best implemented as a View. c) The most obvious check would be to see whether the database has proper referential integrity controls in place i.e. are the customers unique so that Mrs Smith (for example) is uniquely identified even if they could be the same person with a different account. Also check that address details are correct could be that a former customer lived at the address that the complainant lives. The next part of the procedure is to rerun the SQL/stored proc that caused the error and check this in the database log files. Sometimes a DBA may have set the isolation level on transactions incorrectly, transactions could be interleaving (i.e. not serialisable) with each other and a ‘write-ahead’ operation occurred twice to an order with a random customer being picked up. Many database servers adopt an optimistic approach to concurrency control. Ultimately a decision has to be made whether the database has become corrupted and whether a back up is drawn from the day prior to the incident. Question 4 The question covered World wide web (WWW) based information systems and the choice of tools for web enabled information processing this implies security and access control. The main traditional text book on the reading list is: Fundamentals of Database Systems Elmasri, Navathe 2005 edition covers the latest web based development approaches. This is a general type of architecture type of question. In the past the web based IS architecture has been used synonymously with client server or 3 tier clients server architecture. Candidates should draw upon any practical experience they have on developing web based IS architecture similar to SWIFT. As a IS developer / strategist they should be familiar with the security and access control issues of the WWW that the question raises. Candidates are advised (particularly for part c) to use SQL as descriptors of security control mechanisms.

In the answer the examiner was looking for: a) The diagram must include the major software, netware components of a distributed IS. This contains standard components which candidates could see in any system along with some specific components relevant to SWIFT Include standard 3 tier architecture DATA tier : Database server / Profiling tools/ DBA connections to Middle Tier via TCP/IP and TDS (tabular data streams) connections to other remote database using replicator agents MIDDLE tier: Application server plus web server. connections to other web servers using SOAP/XML CLIENTS connect to a network through UDDI and authenticate to membership database. Some clients run HTML others smart clients with components executed on clients like Java applications or .NET versions b) SSL Secure Sockets Layer is a protocol for transmitting private documents via the Internet. SSL uses a cryptographic system that uses two keys to encrypt data - a public key known to everyone and a private or secret key known only to the recipient of the message. SWIFT would use the protocol to obtain confidential user information, such as credit card numbers. By convention, URLs that require an SSL connection start with https: instead of http: so customers would know their transactions are secure and guaranteed to be delivered and not hi-jacked in situ. c) i) CREATE VIEW VWCURRENTORDERS AS SELECT * FROM ORDERS o,CART c WHERE o.orderid=c.orderid and Orderstat = 'Pending' AND CUSTID IN (MEMBERSlist) GRANT SELECT INSERT ON VWCURRENTORDERS TO PUBLIC ii) CREATE VIEW VW BGorders as SELECT * FROM ORDERS o,SUPPLIER s WHERE s.orderid=s.orderid AND Supplierid = 'B&G' CREATE USER 'BROWN' , Password = 'fqhiq5*peq8w2y' ADD USER TO DATABASE SWIFT GRANT SELECT ON VWCURRENTORDERS TO 'Brown' Question 5 The question covered SQL. Relational calculus and algebra. SQL standards and Structure. Operators available in single and multiple (Join) table queries. The main traditional text book on the reading list is: A Guide to SQL Philip J. Pratt Boyd & Fraser (on the reading list)

Questions on SQL are very common and covered in different parts of the syllabus. SQL is the de-facto database language and thus an important part of the syllabus SQL is a very expressive language and there are many ways to formulate an SQL query. Syntax errors is not a major issue but accuracy and succinctness of the code is important in this question In the answer the examiner was looking for: a) Query 1 is a subquery that uses the inner select to compute a data set that is then passed to the outer part of the query to select customername. In effect those customers that have ordered different products on any date apart from today Result is Andrews and Hutton b) Inner Join is a standard JOIN type i.e. both sides of the joined table must have matching rows. NO it must have a subquery to compute the first self join. c) and d) Evaluation strategy: subquery evaluated once to produce set of orders not = today Each row (from tbl_customers as C) tested against this set. Relational Algebra Operators are (expressed as an inverted tree with progressive filtering: PROJECT RS1 as Tuple variables T and C (only local to subquery) SELECT Tuple variables matching values for T=C CORRELATE (sort merge) : subquery generate resultset where T=C The value of d2.OrderDate parameterises an evaluation of the subquery Subquery must (at least) be re-evaluated for each distinct value of D1.CustId=D2,CustID Correlated queries are expensive to compute but the expression tree represented above would be the more efficient if the filter columns were indexed. Question 6 The question covered SQL. Introduction to embedded SQL. WWW dynamic web development. The use of Forms (HTML encoded) Mapping database tables to a HTML Form. The main traditional text book on the reading list is: Fundamentals of Database Systems Elmasri, Navathe (latest edition from the reading list) It is common to find SQL across the paper this time showing its versatility combined with Java in an application programming language environment. It has been common in previous papers to see some reference to implementation/coding the user interface of an IS. It is expected candidates have prior knowledge of Java or C++ and again were aware of the main programming techniques of implementing dynamic web pages. This means web content changes depending on user requests and updates made to the content held in a database. In the answer the examiner was looking for: a) The examiner was looking for an understanding of dynamic SQL, a programming technique that allows SQL code to be generated automatically (in part or fully) during the execution of an application program. Dynamic sql can accomplish tasks such as adding “where” clauses to a search based on text fields that are filled out on a form; or to create tables with varying names depending on execution path. Static SQL is the usual way that SQL is executed and is very efficient because the execution plan is fixed

when the SQL code runs. The downside is this type of SQL is less flexible. Similarly HTML can be statically defined prior to user interaction but can be programmed to change appearance depending on the operation of a web page. b) The examiner was looking for answers that tested the candidates understanding of how a database can be viewed as a web form either as multi-dimensional (or master-detail) for example an order header (single row) and an orderline (multi-row) form. The form must preserve the link to the database through the content and through the integrity constraints (eg referential integrity). All of these characteristics are implied in the question. Specifically in relation to SWIFT: Design of SWIFT’s dynamic web forms would need interaction with a data source (connection to a database) that dynamically populates the HTML text boxes when the user selects or enters some value. A submit will invoke a write to a database when validation rules are OK. These are the possible mappings that should be implied in the screen design/layout. Web Form ->Form -> Orders and Cart TextBoxes -> Fields -> OrderID (auto) CustomerName -> CustomerID (autolookup) OrderDate (auto) OrderLine (cart) -> ListBox (productID Qty UnitPrice, Extprice) drop down controls used to enter pre-calculated data. Button -> Submit (commits the order) possibly with a confirmation window. c) The code uses Embedded SQL in Java the breakdown of the code was expected i) Mention the namespace : Uses Imports.data.sql namespace ii) A object of class string is instantiated with a SQL string which is dynamically built using data from a text field on a data entry form. ii) It returns all customers that have CITY equals to entered city value. e.g. London iii) Explain the JDBC library as a component that reconciles (interprets/compiles) SQL code within a programming framework supported by Java. Although detailed knowledge of the JDBC API is not required beyond what is supplied in the code, other than an understanding of its significant role to support cross-DBMS connectivity to a wide range of servers and SQL databases. The JDBC API as the code suggests exposes Java to any tabular data sources, such as spreadsheets or flat files and combines this with SQL or Java code itself. With a JDBC technology-enabled driver SWIFT would connect their customers table to a web client and application server running J2EE. d) JDBC allows you to call a database stored procedure from an application written in the Java programming language. The first step is to create a CallableStatement object. As with Statement and PreparedStatement objects, this is done with an open Connection object. A CallableStatement object contains a call to a stored procedure; it does not contain the stored procedure itself. The code below gives an example of what candidates will do to creates a call to the stored procedure SHOW_CUSTOMER_CITIES using the connection object called 'con' . The part that is enclosed in curly braces is the escape syntax for stored procedures. When the driver encounters "{call SHOW_CUSTOMER_CITIES}" , it will translate this escape syntax into the native SQL used by the database to call the stored procedure named SHOW_CUSTOMER_CITIES.

CallableStatement cs = con.prepareCall("{call SHOW_CUSTOMER_CITIES}"); ResultSet rs = cs.executeQuery(); Question 7 The question covered Simple ideas of data management, data mining and the concept of a data warehouse. These are the two traditional text books on the reading list that cover the concepts of a data warehouse (DW) also known as OLAP (on line analytical processing). Later editions are best and up to date as this is a fast moving technology. Fundamentals of Database Systems Elmasri, Navathe Introduction to Database Systems Date, CJ Addison-Wesley This is a specific question that had a lot of bearing on the SWIFT case study. It is hard and unusual to see a stand-alone data warehouse question as candidates would generally regurgitate notes. Therefore the opportunity to apply knowledge of data warehouse concepts to a case study is a good opportunity to test analysis and problem solving skills. This was a more practical based question with a range of answers expected. Candidates with real data warehouse experience are quite rare as this technology is only really effective in large organisations and developed by specialised staff. Therefore applying knowledge and making some sound justification for data warehouse techniques was sought in candidates’ answers. For this type of question candidates are advised to avoid repeating or overlapping answers across different parts of the answer and produce direct answers. In the answer the examiner was looking for: a) Main differentiation are in processing types DBMS mainly OLTP and some fast OLAP queries. Tuned for throughput and I/O performance DW solely OLAP and decision support queries that computes a lot of new data. DW use more specialised query language such as MDX (multi-dimensional query language). b) DW models data as Facts , Cubes and Dimensions. This means rows and columns and large pre-computed data sets. These data sets are in 2NF and perhaps 1NF. They are OK as updating a DW is rare, rather the data is generally appended or re-built c) Variety of answers depending on interpretation of the case study and perhaps candidates experience of using a DW/OLAP. The examiner expected some discussion/understanding of hierachical dimensions. An indication of where these are relevant to SWIFT such as Time and drill down expansion to Year, Quarter, Month for example. Region and Location and expansions after drilling down (e.g. Country, Region, County, City, Post Code) for example. Product Category and expansions after drilling down as above. Part ii) deals with the creation and population of a DW. Clearly a separate and more complex process than would be used in a DBMS. There are problems caused by the volatility of the data (transactions) that causes data to temporarily lose its value. Therefore expect some discussion on techniques such as DTS (data transformation services) and other transformation techniques that can be automated and scheduled.

Question 8 The question covered Engineering issues involving the distribution of data (including multimedia data formats) and processing over networks (including the world wide web). The later chapters in these text books on the reading list cover this material in their later editions:- Fundamentals of Database Systems Elmasri, Navathe Introduction to Database Systems Date, CJ Addison-Wesley Distributed databases have been asked before and this is standard text book material. However with the emergence of XML and simple message based protocols such as SOAP a new technology called web services has arisen driven by the major vendors (IBM websphere; Microsoft – dotNET and Sun J2EE). This technology is quite new so candidates should always read up the latest professional computing journals rather than rely on text books that can soon get out of date on web development technologies. There are two contrasting parts, both parts represent solutions to handling the distributed nature of SWIFT’s business. Therefore candidates should carefully read the case study to first of all understand what the needs are of a distributed system for SWIFT. If this is not done the context of the question is lost and the answers would be quite aimless. In the answer the examiner was looking for: a) One of the primary advantages of the XML Web services architecture for SWIFT is that it would allow programs written in different languages on different platforms to communicate with each other in a standards-based way. A simple XML based protocol called SOAP brings down the barriers to entry for a web based ecommerce companies like SWIFT. The other significant advantage is that it would be easy to move from client server to Web services as they could reuse existing infrastructure and use standard Web protocols, XML, HTTP and TCP/IP. An XML Web service can be defined as a software service exposed on the Web through SOAP, described with a WSDL file and registered in UDDI. Only a working (not detailed) knowledge is expected of this technology. Candidates should also answer this in relation to SWIFT "What I do with XML Web services?" Exposing existing applications as XML Web services will allow users to build new, more powerful applications that use XML Web services as building blocks. For example SWIFT could develop a purchasing application to automatically obtain price information from a variety of vendors, allow the user to select a vendor, submit the order and then track the shipment until it is received. The vendor application, in addition to exposing its services on the Web, might in turn use XML Web services to check the customer's credit, charge the customer's account and set up the shipment with a shipping company. b) There are three different replication scenarios relevant for SWIFT - simple read-only replication - replication to and from a mobile client - multiple updates. They provide:

• data distribution to a network of servers, including those that are mobile or occasionally connected (illustrated in scenarios 1 and 2)

• data consolidation to a central server (scenarios 1 and 2)

• process separation onto more than one server (scenario 1)

• information flow from one server to another (scenario 3)

• data sharing across multiple sites (scenario 3) SWIFT would hold copies or replicas of a fragmented database partitioned on organisational, historical or structural lines. A replication agent keeps copies synchronised in real time if necessary. Therefore replication is another distributed technology but instead distributes data between physically remote connected systems rather than messages (as in XML web services). Thus it is not necessary for a web based /web oriented architecture to be built on top of replication technology. Also replication is vendor specific and thus little standardisation has occurred despite being much older technology.

Question 9 The question covered Effective implementation, evaluation and testing of an IS. An introduction to the feature of a multiuser relational database product including its data Management. Again the two traditional text books on the reading list: Fundamentals of Database Systems Elmasri, Navathe Introduction to Database Systems Date, CJ Addison-Wesley This is a general type of question that covers many general areas on the syllabus. It mainly covers database management with a special emphasis on performance. The style of question is more factual but qualified by practices that would apply to SWIFT. The style of question is new although the report type answer is not Again some leeway for a range of answers based on knowledge gained in testing for performance in a multi-user environment. The depth of answers was not expected to be too high rather an awareness of the issues and some reasoned solutions. Use of software tools will depend on the candidates background and whether they use open source (fewer tools generally) or commercially licensed DBMS software. This question is looking at presentation of ideas in a structured fashion with a few topics as the focus. Slides should be general and not DBMS specific but use common techniques. General principles would be mentioned such as balancing between what is good for performance but potentially bad for data integrity (for example). Here are some indicators How to measure and optimise performance (for example indexes and denormalisation) How to monitor using system tools (e.g. showplan, profiler, statistics) Technique of data caching - will include slides on

-caching query execution plans -use of buffer memory and write ahead logs -caching clustered index structures

Technique of Connection Pooling - mechanisms to alleviate the need to close connections - threading of connections in a OLTP application - timeouts from long duration transactions.

Unit 230 Software for embedded systems

Comments on individual questions Question 1 This question covered the syllabus topic Hardware/Software interface and to a lesser extent the syllabus topic - Partitioning the division of functionality between hardware/software/firmware. Recommended text books: Embedded Microprocessor Systems, by Ball, S. Publisher: Newnes. Embedded system design by Berger, A. Publisher: McGraw Hill. This is an important topic area and a common theme for this subject and questions have appeared frequently in previous years. This question refers to the Case Study and it was possible to gain marks from general answers. High marks could be gained by candidates who qualified their answers with analysis of the Case Study. The last part of the question tested candidates who have read recent literature on the development of embedded processors and the hardware/software interface. Again high marks would be gained by applying knowledge to the application in the Case Study. In the answer the examiner was looking for: PCMCIA has developed a standard for small, credit card-sized devices, called PC Cards. Originally designed for adding memory to portable computers, the PCMCIA standard has been expanded several times and is now suitable for Solid State Memory Cards. The PDA could also use this as an expansion port and work similar to USB in that cards can be inserted and withdrawn without rebooting. Universal Serial Bus is a new type of external bus contending to replace serial and parallel ports. Many devices can be connected to USB ports, which support Plug-and-Play and hot-swapping. USB transfers data at speeds of 12 Mbps (megabits per second). USB is more versatile than PCMCIA as a dedicated slot is not needed. Bluetooth is a technical industry standard that facilitates communication between wireless devices such as mobile phones, PDAs (personal digital assistants) and handheld computers, and wireless enabled laptop or desktop computers and peripherals. A single Bluetooth-enabled wireless device is capable of making phone calls, synchronising data with desktop computers, sending and receiving faxes, and printing documents. An essential function for the PDA would for digitising locations (a location tracking system) and for direct communication to a server computer. Bluetooth is limited in range to about 30m between computers and other GPS PDA's without the use of wires. SCSI (Small Computer System Interface) A type of parallel interface that the PDA could use for interfacing a mass storage devices. SCSI can transfer data at rates of 4 MB/sec.; in addition, there are several varieties of SCSI that support higher speeds: Fast SCSI (10 MB/sec.), Ultra SCSI and Fast Wide SCSI (20 MB/sec.), as well as Ultra Wide SCSI (40 MB/sec)

b) Discussion on Systems on Chip (SOC) should deal with reasoning/judging the pros and cons. Pros - 1) higher performance, since all the circuits will be on a single chip; 2) smaller space requirements; 3) lower memory requirements; 4) higher system reliability; and 5) lower consumer costs. SOC may be advantageous for high-volume production of not-too-complex systems, but not for low-volume production of complex systems that require different technologies. The challenges posed by SOC technology include: 1) higher design and prototyping costs; 2) longer design and prototyping cycle time; 3) more complex for testing/debugging; 4) integration of intellectual property from multiple (and possibly independent) sources. Due to the complexities and high costs of developing viable SOC technologies, even large semiconductor companies have opted to co-develop SOC-based products with partner companies instead of going about it on their own on a wide variety of products - from set-top boxes to MP3 players to networking equipment. SOC development is a complicated activity, building a SOC from the design library of your own company is one thing, building one from several design libraries from different suppliers is another. Making the various blocks from these different design libraries work together, even if they're designed to be compatible with each other in the first place, is indeed a big challenge by itself. A high level of design reuse among design groups is needed to attain high productivity rates in SOC design. Unfortunately, source reuse is not a very effective system in many cases, since it still involves understanding and redesigning of IP (intellectual property) blocks on the part of the SOC designer to make them useable in a new product c) Variety of answers looking for design pragmatics of PDA's The most natural way to do this is to enable the Bluetooth or Infra red devices in readers and beaming a Bluetooth enabled PDA. There are many pointers referenced in the discourse. The discourse states that direct access requires fixed sensors on the entry points to buildings or similar. Location awareness using a GPS/PDA would use a direct system of identification with GPS data captured locally on a PDA, but GPS only works outside buildings so candidates would need to discuss the configuration of both fixed location identification and mobile location identification between buildings. Answers should be practical/well thought through so that any advantages in using a PDA in this way are convincing and imaginative. Question 2 This question covered Fault tolerant system (part of Partitioning and Real Time systems) The main text book is Burns and Wellings (B&W). Also reference from the reading list; Real Time system design and Analysis by Laplante. Questions on fault tolerance are frequently represented in previous papers. Questions on the practical design of fault tolerant real time systems have been qualified in previous years by the use of a Case Study. The Case Study usually sets the criticality of the real time control either for safety or operational performance. This year safety is not a major issue, rather it’s concerned with accessibility, reliability and performance. In previous years there have been related questions covering Redundancy and N-version programming. Again candidates should draw examples from the Case Study which forms a major part to this question. The last part implies applying an appropriate design technique and this should reflect candidates understanding of the nature of Case Study application. In the answer the examiner was looking for: a) A transient fault occurs once and then disappears. If the operation is repeated then the system will behave normally. An intermittent fault arises, then goes away, arises again and so on. A common cause of an intermittent fault is a loose contact on a connector. These faults are very annoying and hard to fix

because of their sporadic nature. Lastly there are permanent faults caused by faulty components. The system will not work until the component is replaced. Burnt out chips, software bugs and processor failure (explored in Processor Faults) are all examples of permanent faults. Transient faults are usually associated with the temporal state of a system and can be due to resource depletion such as memory capacity exceeded and as such these faults occur when a portion of the system is unable to obtain the resources required to perform its task. Permanent faults occur when adequate resources are available, but the system does not behave according to specification. Permanent faults may be the result of improper design or implementation. Permanent faults may occur in hardware or software. (eg Floating point error) b) Principles are: - duality or resilience (standby software/hardware that can take over in case of failure) - recovery and isolation (one fault doesn’t bring all system down) - communication (the systematic communication of errors - the user needs to know about for example a failure of service and what has caused it) c) This is fairly open-ended with a range of answers, but essentially the candidate should demonstrate an understanding of testing, redundancy in design and structured approach as this fault is the most likely common type of fault concerned with errors in design requirements being imprecise (for example the device works outside its specification). This is a case in point in the Case Study and suggests designers have not expected some behaviour such as what happens on progressive zoom-ins. This failure is likely to be because the hardware graphics card does not support raster or perhaps the layers of maps are not available. There are other possible causes so candidates will gain marks if they have appreciated the difficulty of designing across different technologies and the knowledge of hardware/software interface. The second part requires mention of failure mode classification schemes as referenced in the Burns and Wellings (B&W) text book. This method is used to trace a fault through various stages and makes it possible to gather diagnostic information about how a system might fail - these are in effect similar to more formal testing techniques such as HAZOP and CCS. There are other answers that candidates could offer such as fault prevention methods as suggested in B&W. The recovery from failure may indicate whether the system has any protection and whether the system can crash altogether or effect performance. Question 3 This question covered System level software - programming languages for control/real time system software. The main text is Burns and Wellings. B&W – Real Time systems and Programming languages Burns, Wellings. Addison Wesley. Not many questions have been asked specifically about real time programming languages as only recently has Java real time features been covered in text books. Questions in previous years have been more general and not been associated with a Case Study. RT Java or Java extensions for real time is covered in B&W but candidates should not simply regurgitate notes but think a bit about the lead in questions earlier. For developing Java for real time control candidates should avoid simplistic answers like ‘we simply extend the class libraries’ – NO candidates need to consider extending the language.

This was a more practical real time system programming type of question that references Java and the JVM framework. Therefore it would be advantageous if candidates could blend their Java programming experience with real time control development experience. The Case Study is only used to give context to answers and to draw out examples. Again reading recent literature will help with answering part c) – the last part of the EITHER OR question In the answer the examiner was looking for: a) A JVM is a piece of software that is responsible for running Java programs. A new JVM is started whenever you type in a program name on the command line. It is called a virtual machine since it is software that emulates a physical computer. b) PDAs can run Java programs on the JVM, allowing them to port programs on any real machine that also has a JVM. For example a Java Virtual Machine running on the Veito PDA, supports all features of a normal JVM except for the graphics part of the runtime libraries. It is targeted at embedded systems (smartcards, handhelds, mobile phones etc.) and can run on 8-bit-systems c) Candidates have a choice to transpose their specialist knowledge of JVM-based architecture across to the .NET framework. The text B&W supports Java as the main RT embedded software, therefore candidates are expected to reinforce their reading with recent journals and web sites. For example RT Java: Candidates should state the real time elements such as real time clock, timeouts, deadline specification and scheduling. A real time program needs a lot more than simply being:- logically correct, compilable and executable. RT Java must also adhere to a real time model of asynchronous processing and these are largely ad-hoc. In fact a real time programming language should resemble a constraint language or a formal specification of an abstract machine. There have been dedicated real time languages but these have always been quelled because power and versatility have outweighed expressiveness. For example B&W mentions the importance of the IMPORT class This allows programmers to override existing namespaces for example. In the real time model this would normally result in the need to build a model appropriate for a particular OS real time executive. Java therefore provides an abstract model of real time threads and processes. For example Java supports operations such as the notification of asynchronous threads. This model has not been produced piecemeal but has become a standardised specification that can be adapted to deal with different OS. Question 4 This question covered Human Computer interface (HCI) The main reference text for STD is Burns and Wellings. B&W – Real Time systems and Programming languages Burns, Wellings. Addison Wesley. For HCI design principles the best text is: Designing the user interface B Schneiderman McGraw Hill. There has always been a question on HCI and related topics in previous years. This question is split into 4 parts, the first three parts are discussion based with a range of possible answers dependent on interpreting the Case Study, the last part involves ‘doing’ some modelling using STD notation. Candidates will find frequent references to STD notation in past papers as candidates invariably need to demonstrate an understanding of applying a design technique for any interactive system. HCI design is no different in this regard.

Again candidates would benefit from drawing upon experience of designing user interfaces and apply this to the application in the Case Study. There is no real difference in the mobile PDA application from any other user interface once the design parameters are understood. The Case study specifies the design parameters and the ‘look and feel’ of the user interface pretty well but not the dynamics of interaction – hence the need to understand the symbolic metaphors (e.g. a map), a story board and an STD. In the answer the examiner was looking for: a) Answer not as obvious as it sounds but key points are :- comparative visual metaphors. Mention the symbolisers such as legend, ‘northstar’ , scales and icons that direct the user to interact with a map. Contrast the two approaches; for example, the need for pointing devices and the need for scroll bars as the screen is limited. Mention the opportunity to get properties and more information in context to an operation (panning, browsing, drilling down layers) b) The answer is quite open ended but should get the main points across. Expect discussion on adding value Quality of Service (QoS) and ease of use for example. Candidates should relate to the means of interchanging information across HCI boundaries – user controls; the cognitive evaluations of using these controls; the interpretation of user actions by the system. The examiner is therefore looking to see if candidates have a basic understanding of HCI concepts within the context of the Case Study particularly the use of maps as metaphors and walking through the issues using fundamental principles. c) STD is a more formal notation and includes precise characteristics that cannot be modelled using a story board However a Story Board illustrates cognitive aspects of the user interface revealing the users domain of knowledge. Story Boarding allows more informed feedback (from the user’s perspective) on the HCI design. The role of the PDA is important because it provides the means by which informative feedback would be presented to the user. Candidates should reveal awareness of the various trade-offs encountered in terms of the structure and layout of the HCI. How they would overcome the constraints that you would work within and how these affect the usability of the system d) This part may require assumptions as the STD is an accurate representation of states of interaction. Therefore some margin for error is allowed but any assumptions should be stated. Jim has these basic behaviours: Planning a route ; Persisting a route table/lookup Walking a route Communicating with coincident objects. Recording waypoints /markers Setting status of stages between stages and duration

These are the important states that the model needs to define: S1 Jim starts in stationary position with GPS signalling enabled S2 Jim waits for a positioning event to occur. In this case that means GPS has fixed Jim's location, identity and time. S3 Prepare to walk. This state is triggered by the direction sensor to prepare the device for a state ‘start walking’, a change of state is picked on the receiver and fed back to Jim. S4 Walk from stage 1 to stage 2. The map appears and the route is program is running. S5 Off the route is a state that is detected when Jim leaves the route (error between say 10-20 ms ) S6 New route - computed to get Jim back on track to route / or the next node S7 Stage of the walk completed within time constraint. S8 Stage completed with exception. S9 Locate - a state that displays the relative location of other objects S10 Walk completed. States S4-S9 are essentially repeated then this state is true. Question 5 The question covered System level software. Operating system features such as threads stacks and software interrupts. Burns and Wellings devote a chapter to this mainly biased to real time system development but focusing on Java implementations in particular This year more effort has been given to the supply of code samples that are written in Java for students to inspect and disassemble. In previous years more general questions have been asked but mainly for unix based operating systems. Again programming knowledge of Java is not essential as the code supplied is descriptive enough to allow candidates to identify systems concepts within the Java code. Java is a very expressive high level language so it should be very easy to pick out the main concepts of exception handling; the program control of a stack and so on. Candidates should try to draw upon examples possibly derived from the code sample supplied whenever they can. This also applies to part a) where an illustrative piece of code saves a lot of writing. In the answer the examiner was looking for: a) Software interrupt handlers are internal mechanisms to protect the integrity of the Operating System (OS) when hardware is interfaced to a computer system. The handler will detect hardware faults and protect the memory address space. Integer arithmetic overflow or underflow; Floating Point arithmetic anomaly - are all examples. A software interrupt handler can reload the page from disk (virtual memory) and restart the instruction which generated the exception; e.g. Page fault; Memory protection violation Exception handlers are programmer controlled and include routines that might capture software interrupts before they happen and crash the existing program that is running. Exceptions belong to a special type of software interrupts. They are generated by the processor itself whenever some programmed event occurs. A thread is an OS term describing a light weight process this means the code for a thread is re-entrant and has no process boundaries. A process has more strict access in multi-user conditions with no opportunity for access to the same code when the process is running. Processes strictly obey ACID acronym. Thus threaded code is relatively fast and unreliable (in some cases) whereas process code is relatively slow and reliable.

b) i) Push exception when stack too full. Exceeds capacity. Pull exception when stack is empty. There should be at least one element on the stack. The code that manipulates a stack could easily be applied to a system level stack. The main principles unique to OO languages (C# included) is the ability to encapsulate objects and thus we see a exception object with its own instance within a structured error handler using ‘catch try .. finally’ operators. The code presented is based on an example in the B&W primary course textbook. ii) Code Solution: first import namespaces including Java stack primitives --------------------------------------------------------------------------------- import Stack; import FullStackException; //note renamed to distinguish import EmptyStackException; //ditto // define the class to manipulate the stack and declare a method (main) public class UseStack { public static void main(...) { // simply use (via try) pop and push and catch the exceptions Stack S = new Stack(100); try {S.push(SomeObject); SomeObject = S.pop(); } catch (fullstackexception F); catch(emptystackexception E); } } ------------------------------------------------------------------------------------- c) C-shell is the preferred unix shell script but other types of shell script exist meaning there is little or no standardisation of syntax. Also C-Shell is interpreted and therefore not type checked so this can cause major logical errors Java is a better programming language for exception handling but does not perform well on low level operations. Also it depends on the library of routines that Java supports on a particular operating system. Though technically these are standardised they can behave differently as the intrinsic low level features OS features are hardware dependent. Question 6 The question covered System concepts; basic definitions. Concurrency control synchronisation. Overlaps with another part of the syllabus - close coupled and distributed systems. Burns and Wellings devote a chapter to this topic area. It also has a traffic signal scenario which though different shows some similarities with the problem definition supplied in the question. There have not been many questions on this topic before as question have focused more on high level inter process communication using CORBA for example.

Part a) involves ‘process synchronisation and concurrency at the inter process level’, involving interleaving and mutual exclusion. Although candidates could give a text book answer here and simple talk about semaphore systems, this would not attract many marks as it is necessary to consider a much more robust approach than what is usually mentioned in the simplistic examples given in text books. Therefore high marks could be gained by qualifying answers that are relevant to the scenario. In the answer the examiner was looking for: a) Semaphores though have to deal with issues such deadlock, livelock and those mentioned by B&W pg244 that are pertinent to the scenario, for example critical regions ; protected objects (fault tolerance) as implemented in Java. b) Candidates need to assess the basic weaknesses of a centralised traffic control (it is real time system with a local controller that operates the lights). This is actually how traditional systems for traffic management work as they attempt to influence vehicle flow by controlling traffic signals or ramp meters. However, these approaches ignore an important component of traffic management: coordination of traffic by the flow and volume of vehicles themselves in REAL TIME. Distributed systems would be needed to provide a global view of traffic control and will require a network of independent processors that communicate by message passing or its variants, including supporting frameworks such as RMI/CORBA. The load on this system would then be shared and processing can operate at the point it is needed. Also this method of approach offers failure protection but this depends on having sufficient redundancy and equally important recovery schemes like 2 phase commit (2PC) built in. Networking would also support quite simple protocols and would allow various distributed traffic telematic systems to be configured. Therefore applications would communicate with each other over a network while maintaining the shared memory logical view of data. The network/communication harness would be based on TCP/IP-protocol and client/server architecture and would be independent of the physical network type. Question 7 This question covered Computer System Management Processes. Of the recommended texts only B&W cover the issue of networks but the recent development of security means it is best to refer to the numerous text books that have been published in the last 2 years on Computer Networks. The book list is to be updated to include a couple of these books. There has been frequent reference to networks mainly from an administrative point of view in previous years. This year the network question is more technical and applied to the Case Study. This is very much a discussion based question on a specialised topic, with candidate’s depth of knowledge and experience playing a major part in the answer. In the answer the examiner was looking for: a) Network servers logically sit on the boundary between an organisation and the outside world. Often the application servers providing services to the outside world are in the same physical location. Therefore they are the weakest link in the security of servers and need to be logically placed in a secure/safe location in a network infrastructure. The area in which they are placed is often called a demilitarized zone (DMZ). The firewall (discussed in part c ) directs traffic to the DMZ based on a set of addresses assigned to that area. Within the DMZ, you can ensure that servers do not have access to corporate resources. This way, if a security breach does occur on those servers, the perpetrators cannot then move on to other computers. Need to secure client computers and accounts to ensure that only authorised users can use them to access a network. If you cannot physically secure a client computer,

then make certain that the accounts used on it have few privileges and that you use file encryption, secured screen savers, and other local security strategies. Very relevant to mobile network enabled device such as a PDA!!! b) Wire-tapping occurs when parties on the public network look at data packets (information) for content that interests them. Once they identify packets of interest, they may intercept the packets and access any content that is not encrypted. This content may include information such as an IP address, login information and passwords. Once identified, an IP address can be used as a disguise to launch attacks against Web sites and other IP addresses. Another form of security threats is called impersonation. This can occur if a hacker learns an email configuration. Once known, the hacker can use this information to send messages using that email address. Violation of message confidentiality: It's easy for someone to hi-jack a message that has been sent and forwarded to other parties. Have to recognise that anyone, anywhere with an email account could get to see what has been sent. c) Firewall description (best described using metaphors – encouraged in candidates answers. In simple terms a firewall is a hardware or software device that acts like a fence around a system or network, it would be great just to put up a big fence and never let anyone in (some systems do this) but in real terms it would lose all communication with the outside world a bit like a siege. So what is needed is a walled garden with access to and from strictly controlled and limited to applications and data that can be trusted and that is exactly the job of a firewall. They range from free software only firewalls to full on corporate wide dedicated servers running firewalls as the interface to the outside world. Firewalls protect against wiretapping etc on networks by using a simple set of rules, this permission based scheme plugs the holes that operating systems are sadly renowned for. A simple rule set starts from the position of all ports (or doors) closed and then as you start each application you have to create a rule to let it get to the internet or network, most firewalls will come pre-configured to allow internet and pop3 email access from start-up but instant messaging systems will often need permission to work. Question 8 This question covered Inter-process communication. Scheduling of processes in real time/critical systems This topic is very much the territory of B&W textbook where 3 chapters have been written on this important topic. The concepts are very well explained in this book with numerous examples that the examiner has adapted in previous exam papers. There has always been a question on this topic in previous exam papers. This was a more text book based question in which an analytical part that required a particular result. Therefore working out is important as errors in calculation will be compensated so long as the correct procedure was followed. The key to getting high (and easy marks) is to choose the correct analysis technique and be able to check the plausible process states (of interleaved processes) using a graph or schedule chart. This is one of the few questions on the paper that did not require reference to the Case Study.

In the answer the examiner was looking for: a) Scheduling demands are different. Unix schedules processes and threads which have no deadline and have variable priorities for example can change priority. Real Time (RT) processes are scheduled according to deadlines being met; if not critical sections of functionality will fail. b) This is well covered in B&W, although B&W do not have any examples of Earliest Deadline First (EDF) scheduling in this context. B&W mainly focuses on Fixed Priority Scheduling (FPS) so candidates will have to apply EDF to a FPS process set. The relevance here is the concept that a process will be schedulable AND pass the utilisation test (given by a formula). Therefore candidates will use timeline analysis plus utilisation based tests relevant to each scheduling scheme. The process set would be scheduled as follows Process A priority 1 (lowest because of the period T), Process B has the highest, thus pre-emption will occur. A timeline graph/table would show this. The following scheme would be schedulable under EDF with these deadlines: process A (T=4 D=4 and C=1) process B T=12 C= 3 process C T= 16 C=8 c) Aperiodic processes are the biggest effect on the utility on any priority based system such as the EDF and FPS schemes. Another factor that has an impact is the frequency of blocking and priority inversion which results in poor performance of a real time system. Question 9 The main topic on the syllabus is called ‘Quality Process’ and this question more or less cover most of the sub topics as well. There are two text books on the reading list: Software Process Improvement , Haugh et al , Springer-Verlag publisher. Software Process Improvement concepts and practices, McGuire Idea group publishing. This is a specialised area of the syllabus that attracts questions but not on a regular basis. Candidates can gain high marks particularly if they have working experience of Quality Assurance and/or Quality Control. It was expected that candidates develop their answer along the lines of presenting the facts and background and then argue the merits of applying, for example TickIT, to a given problem; possibly for their own situation or their own organisation. In the answer the examiner was looking for: a) The primary objective of the Quality Process Assessment (QPA) is to assist the client in identifying the areas of opportunity for reducing time to deliver, reducing cost to deliver, and improving quality at delivery for their software initiatives. In engineering and software, quality control is a set of measures taken to ensure that defective products or services are not produced, and that the design meets performance requirements.

b) Process Capability is defined as the extent to which a stable process is able to meet specifications, monitor process stability using control limits; access process stability using specification limits and to compare relative capability to determine which process is more capable. Process Capability Assessment assesses process capability using numerical methods It uses Statistical inference such as standard distribution. When a process spread is just about equal to the specification spread, the process is capable of meeting specifications, but barely so. This suggests that if the process’s mean moves to the right or to the left just a little bit, a significant amount of the output will exceed one of the specification limits. The process must be watched closely to detect shifts from the mean. Control charts are tools that can be used to model this. c) Choosing Tickit. Some background could be mentioned such as : The TickIT project started in 1991 with the principal aim of stimulating software system developers to think about: - what quality is meant to achieve in the context of the processes of software development, - how quality may be achieved, and how quality management systems may be continuously improved. A further major objective is to provide industry with a practical framework for the management of software development quality by developing more effective quality management system certification procedures. These involve publishing guidance material to assist software organisations interpret the requirements of ISO 9001, training, selecting and registering auditors with IT experience and competence. Introducing rules for the accreditation of certification bodies practising in the software sector Discussion Pros might include The TickIT Guide is now in its fifth issue, this has been the mechanism for encapsulating good practice guidance, that is, the know-how and experience of a large number of quality professionals, and disseminating it throughout the world. TickIT guidance is available for any software organisation to use and benefit from, regardless of whether they intend to proceed with certification. Discussion Cons might include : Although certification is a contractual requirement for many organisation’s software, it is rarely applied and it could be argued that the overhead of applying TickIT credentials is too much of an overhead for small to medium size software companies.

Unit 231 Computer systems engineering

Comments on individual questions Question 1 This was a very popular choice amongst candidates and scores averaged at 32%. This question considered aspects of RISC pipelines. Part a) involved identifying the operations involved at each stage for selected instruction types. Many candidates failed to identify the appropriate operations for each pipeline stage for these instruction types. Part b) involved calculations: speed-up factor is 3; CPI for pipelined is just over 1 and for non-pipelined is 3; max throughput for pipelined is approx 250 million instructions per sec and for non-pipelined is approx 83 million instructions per sec. Part c) included a request for a sequence of instructions where data conflict could occur and how to resolve this. It was generally answered well except for the explanation of the problem in some cases. Part d) focused on branch instructions and pipeline flow – this was often poorly addressed. Mano’s text is a good reference for this material. Question 2 This was a popular choice amongst candidates and scores averaged at 32%. Many candidates attempted only part b) which focused on JK flip-flops. The question as a whole involved designing a sequential circuit in the form of a sequence detector to detect the sequence 010 within a bit stream. Many who did attempt the whole question scored well. Part a) requested a state diagram and table for a Moore machine. Several candidates did not produce a completely correct diagram, in some cases it would not cope with overlapping occurrences of the sequence correctly. Some others produced a Mealy machine instead. Most candidates produced a table which corresponded to the state diagram. Follow through was used in marking part c) so that if a candidate erred in parts a) or b) the answer given to part c) should follow from their earlier solutions, thus avoiding further penalisation. In part c) some candidates failed to consider the output equation. Sample flip-flop input equations are: J1=A.Q0 ; K1=A’.Q0 + A.Q0’ ; J0=A’ ; K0=A where flip-flop 0 has inputs J0 and K0 and output Q0 and flip-flop 1 has inputs J1 and K1 and output Q1, with corresponding circuit output equation of Q0.Q1. This is not the only possible solution however. Errors occurred during working by many candidates in part c) but follow through in marking was then applied in order not to overly penalise candidates. Question 3 This was a very popular choice amongst candidates and scores averaged at 38%. It was on the topic of cache memories. Part a) involved some calculations but was not generally well answered. Numeric answers include: number of bits in a main memory address = 24; number of words within a cache block = 16; number of blocks within cache = 2048; with main memory address split into the following fields: tag (9 bits), block (11 bits), word within block (4 bits). Part b) focused on cache misses but descriptions of how a miss would be detected in a single-level directly mapped cache system was often poor. Part c) explored the hit ratio and effective memory access time – both 18.15ns (0.93 * 15ns + 0.07 * 60ns) and 19.2ns (0.93 * 15ns + 0.07 * (15ns +60ns)) were accepted as answers. Percentage improvement in access rate should follow through from this (access rate with cache)/(access rate without cache) * 100% e.g. (1/(18.15* 10-9s))/(1/(60* 10-9 s)) *100% = 331% approx, although subtracting 100 from this was also accepted. Part d) asked how set-associative mapping could improve the hit ratio – this was not generally answered well. Part d) on write back policies was often well answered.

Question 4 This was the most popular choice amongst candidates and scores averaged at 42%. This question explored adders. Part a) requested Boolean expressions, a corresponding circuit and delay calculations for a Full Adder. There are several possible correct answers. This was generally answered well but some candidates did not produce circuits which corresponded to the expressions they provided or else used gates which were not available in the given table. Some miscalculated delays by adding delays for all gates in the circuit. The diagram for a Ripple-Carry Adder requested in part b) was correctly produced by most candidates but the delay was miscalculated by many. The delay depended on the circuit given in the candidate’s solution to part a) and follow-through was applied in marking. For many circuits the delay for the final carry-out (for example) is not simply 4 times the delay for the carry-out for a single full-adder – it may be less if the carry-in input is not on the longest path for the delay of a single Full Adder. Part c) considered a Carry-Look-Ahead Adder. Some candidates tackled this part well except for the delay, where it was not always the longest path which was considered by the candidate. Several omitted this part of the question. Question 5 This was a popular choice amongst candidates and scored the highest average at 50%. Part a) mainly considered production of an instruction sequence for a given calculation using a stack-based, zero-addressed instruction format. A solution to part a)i), the expression in Reverse Polish is ab/cd*-c+ In part a)ii) some candidates used instructions which were not zero-addressed and hence did not answer the question asked. Part a)iii) was generally answered well by those who coped with a)ii). In part a)iv) the expected advantage was increased speed and expected disadvantage was increased cost. Part b) focused on different addressing modes, seeking general descriptions and specific examples with diagrams. Part c) was poorly addressed by most. A sample answer would be that directives are instructions to the assembler and are not translated to machine code, unlike other instructions in an assembly language program which are translated by the assembler in to machine code. Question 6 This was a popular choice amongst candidates and scores averaged at 35%. This question explored aspects of dealing with interrupts in a system utilising vectored interrupts for interrupt-initiated I/O. Part a) explored when and how the CPU checks for interrupts and the actions taken during the interrupt cycle. Many candidates omitted stages involved. A solution to part b), a problem with use of non-vectored interrupts, is that with many potential sources for the CPU to poll, the time required before polling a device could exceed the time available to service the I/O device. Part c) enquired about the daisy-chaining approach to dealing with priority interrupts from I/O devices. It was answered in a suitable amount of detail by many but not all candidates. Part d) sought comparison between this and an approach for system bus arbitration in some shared-memory multiprocessor systems. Question 7 This was attempted by very few candidates and scores averaged at a low 6%. It was on the topic of virtual memory and addressing which appears in the syllabus in the section headed ‘Memory Organisation’. Material can for example be found in the recommended text by Mano. In part b) answers to calculations are number of bits per page table entry = 3 + 22 = 25; size of page table = 226 *25 bits = 200 MB. Parts a) and c) asked for descriptions of the purpose of various bits and the process of translation from virtual to physical address. The high-order 26 bits of the virtual address form the index in to the page table. The frame number is 22 bits in length. The offset within page/frame is the low-order 14 bits of the virtual address. Part d) focused on two separate possible improvements: use of multi-level page tables and use of a translation-lookaside buffer.

Question 8 This was a very popular choice amongst candidates and scores averaged at 38%. Part a) considered mainly the strobe approach but also the handshaking approach to asynchronous communication between the CPU and an I/O device. Some candidates failed to label diagrams appropriately. Part b) considered a variety of multiprocessor systems, seeking categorisation and brief descriptions to distinguish between them. Some candidates dealt incorrectly with some systems or could have scored more by expanding on very brief answers. Question 9 This was attempted by a reasonable number of candidates and scores averaged at 33%. It explored noise margins of a logic family, propagation delays of gates and gate fan-out in parts a) to c). The fan-out in the example in c) is 10. Parts d) and e) explored behavioural, structural and physical representations in the design process of digital systems and cost and performance metrics in design evaluation. Parts c) and e) were reasonably addressed by those who attempted these parts – many did not address e). Other parts were generally poorly addressed with incorrect formulae given in answers to a) and b) by many. The topics are covered by the Design options and Design methodology sections of the syllabus. The text by Gajski is a relevant source for reference in this question.