Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or neither?

From: Mike Ellims < >
Date: Sun, 24 Apr 2016 13:23:12 +0100

> as software does not "fatigue" or randomly "break" the way hardware does

Reliability engineering is much more than just fatigue or randomly breaks, it encompasses everything about the control of variation and error in a system, most commonly using statistical methods.

BS 4478 defines reliability as "The ability of an item to perform a required function under stated conditions for a stated period of time".

O'Conner states that "reliability is usually concerned with failures in the time domain. This distinctions marks the difference between traditional quality control and reliability engineering".

He goes on to list a number of reasons why a failure may occur as follows (abridged)

Henley and Kumamoto give a potted history of the development of reliability engineering and track its roots to work done by Lusser on the V2 (V1?) missile which was spectacularly unreliable at first. But obviously not a wear out issue...

This is of course separate from whether in any context reliability is useful concept e.g. software. You can obviously measure the reliability of Windows98 is terms of mean time between crashes (failures in time) likewise you can measure the reliability of Linux in the same manner. Whether that has any deep meaning aside from the fact that it shows Windows98 to be pants compared to Linux for some distribution of uses/input/outputs a different question.


-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces_at_xxxxxx Coq, Thierry
Sent: 24 April 2016 07:24
To: The System Safety List
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or neither?

I find this list hugely informative. In particular, I find PBL's posts factual, useful, interesting and intriguing. As well as the many other debaters who agree with him or challenge him. As it should be. I wish to express my gratitude to all debaters.

However, this last exchange seems to me a debate on authority. On our left, we have DO-178. B now C.
On our right, we have IEC, IEEE, Musa, etc.

To go further, it is plain fact that the aeronautics industry has demonstrated it doesn't need "software reliability" to deliver highly reliable automated systems, or systems of systems. It seems evident with the knowledge we have of the aeronautics success that in order to use "software reliability" in other industries, or in aeronautics, there needs to be a clear use case where the value of "software reliability" is demonstrated, compared to other methods or techniques, in order to apply "software reliability". The analogy of software and hardware does not seem valid, as software does not "fatigue" or randomly "break" the way hardware does, which is the basis for all probabilistic reliability theories for hardware. The analogy that does seem valid between hardware and software is the presence of systematic faults, in design, manufacturing, installation, testing, misuse, etc. Which in hardware also does not have a probability number. Formal methods can be used to identify such systematic faults in software. If one can be found, then a test environment can be devised in which the software will fail 100% of the time. Random hardware faults do not behave like t hat.

Best regards,
Thierry Coq
The opinions reflected here are my own and are not necessarily those of my employer

-----Original Message-----
From: systemsafety
[mailto:systemsafety-bounces_at_xxxxxx Peter Bernard Ladkin
Sent: dimanche 24 avril 2016 03:27
To: The System Safety List
Subject: Re: [SystemSafety] Does "reliable" mean "safe" and or "secure" or neither?

On 2016-04-23 19:43 , Nick Tudor wrote:
> DO-178C

In the absence of a complete sentence, let me suggest one.

It's fine for some industry consortium to find it has no use for a specific concept. RTCA likely has no use for the notion of a cup of tea, either (BS6008). But that doesn't mean it makes any sense to argue that there isn't any such thing as a cup of tea.

PBL Prof. Peter Bernard Ladkin, Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany Je suis Charlie Tel+msg +49 (0)521 880 7319

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Received on Sun Apr 24 2016 - 14:29:10 CEST

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