## Problems in RBD modelling.

## Problems in RBD modelling.

(OP)

Hi all,

I have some problems to modelling the following system by RBD scheme.

I have a sequence of N equal devices, N/2 belong to system A and N/2 belong to system B, which are installed in the following alternative order: device A, device B, device A, device B and so on. The fault happens only when two consecutive devices fail, so I think that I have to model a series of N-1 parallel of two devices.

Problem: all the N/2 devices of system A are connected between them and to another system "1", the same is for N/2 devices of system B which are connected to system "2". So the fault globally happens when:

1)two consecutive devices (obviously one of system A and one system B) fail

or

2)both two system "1" and "2" fail

or

3)only one devices of system A and "2" fail

or

4) only one devices of system B and "1" fail.

Now I can’t model the first series of N-1 devices parallels by adding in series the block parallel of “1” and “2”. How can I resolve? I need to calculate MTBF of global system.

Thanks to everybody

I have some problems to modelling the following system by RBD scheme.

I have a sequence of N equal devices, N/2 belong to system A and N/2 belong to system B, which are installed in the following alternative order: device A, device B, device A, device B and so on. The fault happens only when two consecutive devices fail, so I think that I have to model a series of N-1 parallel of two devices.

Problem: all the N/2 devices of system A are connected between them and to another system "1", the same is for N/2 devices of system B which are connected to system "2". So the fault globally happens when:

1)two consecutive devices (obviously one of system A and one system B) fail

or

2)both two system "1" and "2" fail

or

3)only one devices of system A and "2" fail

or

4) only one devices of system B and "1" fail.

Now I can’t model the first series of N-1 devices parallels by adding in series the block parallel of “1” and “2”. How can I resolve? I need to calculate MTBF of global system.

Thanks to everybody

## RE: Problems in RBD modelling.

TTFN

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## RE: Problems in RBD modelling.

I hope this drawing will be useful.

http://we.tl/Guan4Ek454

Tks

## RE: Problems in RBD modelling.

2> (1-P1)(1-P2)

3> (1-PA)(1-P2)

4> (1-PB)(1-P1)

Assuming I understood what you described. You seemed to imply that the failure of any single A results in total system failure.

TTFN

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## RE: Problems in RBD modelling.

Middle A is connected by a ring topology to system “1”. The same for middle B to system “2”

Thanks for your help IRstuff.

## RE: Problems in RBD modelling.

2> no change

3> (1-PA^3)(1-P2) 1 minus probability that all 3 PAs are working

4> (1-PB^3)(1-P1) 1 minus probability that all 3 PBs are working

As for 1> I would just brute force it; there are 64 possible states for the 6 A/B devices, i.e., 2^6. Create a table with those states and mark them as good or bad, and count up the bad ones.

TTFN

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## RE: Problems in RBD modelling.

I fully agree your approach. My problem is that I have a number of devices that is very bigger than the example in drawing. I don't have a specific line guide from the client, so I would ask to you if it is reasonable modelling an RBD only for the worst case. I mean: to calculate the availability in the point of global system that is the worst, in my example (see the new drawing) it will be the green service (because it is the service provided by the two devices depends ever also of their near devices of the same network to reach system "1" and 2" respectively).

http://we.tl/Cp05Ki8k4J

## RE: Problems in RBD modelling.

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## RE: Problems in RBD modelling.

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## RE: Problems in RBD modelling.

the fault of service in one area covered by two consecutive devices, does not cause fault of the services in the other places of system. For this reason I proposed to you the middle couple fault as worst case of analysis. Consider for example that A e B devices are access point of a wireless network or CCTV cameras: if we lost the service in one area we don't lost the service in the other one.

I agree with you that in my OP I was talking about that any A/B faults cause a global fault of the system but now I'm reworking the analysis.

In this condition, I again would ask to you if it is reasonable to make an analysis of the worst case to demonstrate availability of design.

Thank you

## RE: Problems in RBD modelling.

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## RE: Problems in RBD modelling.

Please note that drawing is only an example i should have a number very big of devices.

Sorry if I am repeating again but i want to be sure that your opinion is not based on a wrong english sentence about the question.

## RE: Problems in RBD modelling.

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