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Brian Randell forwarded me the Jon Jacky's report from COMPASS'88, and asked to comment it. These comments can be summarized by: FRIGHTENED! I am frightened to see attendants to a congress devoted to safety issues debating on topics (the 10**-9 figure and the like) when ignoring, or wanting to ignore, their real origin and their meaning: no significant contribution to the mortality rate in industrialized countries (see the statistics from OMS, the world wealth organization). I am frightened to see that they are confusing evaluation of a product and evaluation of a process: as a direct consequence of the former, such reliability goals cannot, by essence, be statistically assessed for a given product. I am frightened to see specialists of safety who do not seem aware of one of the major criticisms of the Inquiry Commission after the Challenger accident, i.e. that NASA had neglected quantitative evaluations. Jean-Claude Laprie, LAAS-CNRS, Toulouse, France.
I shall let Bev have the last word in this argument, especially since I
cannot figure out what we are arguing about. As far as I can tell, we
are in violent agreement. But he does issue a challenge to which I feel
I should respond, i.e.,
>Maybe Nancy could tell us how she would reduce catastrophic failure rates to
>acceptable levels and demonstrate the achievement of such levels?
I do not believe that we should be talking about catastrophic failures at
all. Catastrophic failures or accidents usually involve aspects of the
environment that are not under the control of the designer of the system.
Computers do not usually have catastrophic failures (unless one considers
the problems of electrical shock or fire, which are usually minimal). I prefer
the word used by Safety Engineers, i.e., hazards (states of the system being
designed that could lead to accidents or catastrophic failures given certain
environmental conditions). As a component of a larger, potentially hazardous
system, computer software can certainly contribute to the system hazards
and thus has some hazardous (but not necessarily catastrophic) failure modes.
Considering only catastrophic failures limits the problem too much.
So how can software-related hazards be eliminated or reduced? The practice
of System Safety Engineering provides some direction. Details will be in
my book (coming out next year from Addison-Wesley) and my survey article also
describes this (in less detail) but briefly: The first step is to identify
the hazards of the system being designed. System safety engineers call this a
Preliminary Hazard Analysis. This part of the process often involves
knowledge of previous systems and accidents and requires more creativity
for one-of-a-kind or first-time systems. However, even these can be
analyzed using information about general types of hazards such as
electrical shock, chemical effects, or radioactivity exposure. Analysis
techniques, e.g., Failure Modes and Effects Analysis (although this is
usually used more often for reliability analysis than for safety analysis)
and Fault Tree Analysis, are used to determine plausible events that
could create hazards. Design procedures are then used to try
to eliminate or minimize these hazards by either preventing the
precipitating events from occurring (i.e., designing them out), minimizing
the probability of their occurrence, or minimizing their chance of leading
to the hazard. Bev should note, here, that probability is involved and
thus measurement or estimation. But the key is that measurement alone
leads us to a fatalistic yes/no choice whereas measurement combined with
design allows us some further options in attempting to find designs that
have acceptable risk.
My books and reference materials are currently somewhere over the state
of Kansas, so system safety engineers may need to correct some of the
following. But the general goal of system safety design is to eliminate
single events that can lead to hazards and to minimize the probability
of multiple events or sequences of events leading to a hazard. Thus the
desire of the engineer with whom I spoke to get a number for the
event of a particular type of software behavior. Since I did not know
of any way that he could obtain this number with the amount of certainly
he and I felt were necessary when a nuclear plant meltdown was involved,
I thought it best for him to use 1.0 for this probability and design
around this event (i.e., design the system so that the event would not
cause a hazard).
Engineers have various ways of eliminating hazardous events (what I referred
to sloppily in my previous message as "catastrophic failure modes) from
systems or minimizing their probability of occurrence. For example, the
hazard of electrical shock can be eliminated by designing a purely mechanical
system. This is a rather extreme solution, however, and often unacceptable.
Another less extreme solution is to use interlocks to prevent certain events
or to ensure proper sequencing of events. For example, a door over a high
voltage area is often used to protect against accidents. The door can be
designed so that when it is open (and the high voltage equipment exposed),
the circuit is broken.
Note that the use of interlocks are often simply a matter of designing
the system so that multiple independent failures are required for hazards
to arise: The interlocks themselves may fail. Assuming that common point
failure modes (which engineers have techniques for identifying) are eliminated
or minimized, then risk will be reduced. Probabilistic Risk Assessment (PRA)
can be used to determine if the risk is acceptable, but even for
non-computerized systems PRA is controversial and criticized as inexact. The
most accepted use of PRA is for comparison of alternative designs. PRA has
been primarily used in the assessment of the safety of nuclear power plants,
and there is at least one interesting critique of this use that has been
published by the Union of Concerned Scientists.
Several accidents I have heard about have occurred when computer subsystems
replaced electro/mechanical subsystems without replacing the interlocks that
maintained adequate levels of risk. The same mechanical interlocks or back-up
systems can be included to protect against computer errors (probably the safest
since we can assess the risk involved fairly accurately using historical
information) and/or various kinds of interlocks can be included in the
software. In this case, software reliability assessment is involved since
the reliability of the software interlocks will be crucial. Since these
software interlocks are often quite simple and limited in required
functionality, I believe (and I realize that others might not agree with me),
high reliability MAY be achievable by using sophisticated software engineering
techniques including, perhaps, formal methods. I might be willing to accept
subjective assessments of risk here if the software is simple enough whereas
Bev might not. If subjective types of assessment (including the use of formal
methods) is not acceptable, then we may be forced to rely on mechanical
back-ups, probably in addition to the software interlocks (again, the goal is
to require as many independent failures as possible for a hazard to occur).
[Note that some hazards are unavoidable, and the design goal then becomes
minimizing the amount of time in the hazardous state.] Probably neither Bev
nor I would agree to the use of another computer as the backup, even if the
code is written by a separate group of people because of the problem of
eliminating common failure modes, i.e., non-independent failures
(e.g., the same software requirements specification). The point is that this
does not mean that the reliability of the entire software system (which may
be quite large and complex) needs to be ultra-high, only the smaller
safety-critical parts. If the software is designed, as is common with
software engineers who do not know enough about safety, so that it is
ALL potentially safety-critical, then the problem becomes quite overwhelming,
and Bev and I (and others) find ourselves with planes or other devices
that we do not feel safe using.
Hopefully, I have not made too many mistakes in this brief summary of System
Safety Engineering. If I have, I am sure that the other Risks readers will
correct me :-).
Nancy
The telephone feature of delivering the calling number to the terminating
line is part of a group of features called
'CLASS', although there are other ways it could be done in certain
special cases. There are a number of Bellcore publications that
describe it in some detail. Among these are TR-TSY-000031 on the
basic feature, (TA) 000030 on the signalling between office and
customer terminal, 000391 on the feature to block delivery of the
calling number, 000218 on selective call reject, and (TA) 000220,
also related to selective call reject. TAs are an early version
of TRs. If you don't find one in a reference,look for the other.
There are several other TRs that relate to these features, but this
list should sate most of us.
Calling number delivery, selective call reject, and calling number
delivery blocking are all involved with the 'Signalling System 7' which is just
beginning to be deployed amongst local exchanges, although some of the long
distance carriers are much farther along. Among other advantages, SS7 enables
the transfer of much more information between network nodes than was previously
generally available. This should allow the introduction of many new network
services in the near future. On the other hand, CLASS and calling number
delivery in particular will not likely become common until large areas are cut
over to SS7, since otherwise they would not work much of the time. (Only
within the local switching office, or among those that had already implemented
SS7)
It looks to me like a subscriber to calling number delivery gets telemetry
intended to allow display of the number calling concurrently with ringing. I
suppose proper customer premise equipment could pick this off and feed it into
a computer or use it to determine what to do with the call, eg. route to an
answering machine only if not long distance. If the number isn't available, as
would be the case if the originating and terminating offices were not linked by
SS7, the telemetry sends ten 0s. If the number is available but the originator
is blocking delivery, it sends ten 1s.
Calling number delivery blocking is itself a CLASS feature that can be set
on by a service order or, depending upon the tariffed offering, turned on or
off on a per call basis. How it is offered, if at all, is up to the local
telco and PUC. The TR makes it look to me like it is not available to party
line subscribers. I think there is a technical reason for this.
Selective call reject allows the subscriber to set up a list of up to N
directory numbers (N might be on the order of 6 to 24) that would be sent to
'treatment' instead of ringing the subscriber's phone. A caller using blocking
could be put on this list after one call by using a control that says, in
effect, add the last caller to my list, but that number could not be read from
the list by the subscriber. It doesn't look to me like the blocking code
itself can be put on the list; maybe somebody else knows a way or has tried it.
Call reject can be turned on or off also, and can be maintained from either a
DTMF or dial phone.
There might be something here for everybody. If I can block delivery of
my number and Mr. Townson can send me to treatment we would be almost as well
off as with Internet addressing from Bitnet to Portal.
The foregoing opinions and interpretations are mine, not my employer's.
My interpretations of the referenced documents are based on a cursory reading.
They probably contain some errors.
John McHarry McHarry%BNR.CA.Bitnet@wiscvm.wisc.edu
A co-worker of mine called the Police last year to report a burglar alarm in his neighborhood which was going off. (He lives in Baltimore County, Maryland.) The dispatcher received the phone number, his name and an address automatically. The 911 dispatcher read back the address that was displayed. It was where they had lived two(2!) years previously. When they moved, they kept the old phone number and gave the phone company his the address. Unfortunately, the change of address was not passed on to 911. Although it would be nice to have 911 come if you were in trouble and and could only lift the phone, I would like them to arrive at the Current address. (I know the people who live at my old address do not know my current address, although I assume they have a current phone phone book. Since I am listed, They could direct the police to my home.) Quite a waste of time, esp. in an emergency.
The Alameda County phone book has a privacy notice right below the 911 number which warns callers about ANI and advises them to use the regular 7-digit number if they don't want their number displayed on the dispatcher's console. -Al Stangenberger
Here's another scam for ANI. Set up a free phone service:
time and weather, point spread predictions, sports score line,
Dow Jones business news brief. It's just a taped message someone
can call into. Now set up a PC to capture the ANI information on
people who call. Take the diskette of phone numbers to a service
that offers CNA (customer name and address) and presto! You have
yet another profiled mailing list ready to be sold to hungry marketers
of sports equipment, business journals, etc. Where'd they get MY
name? you ask. You'll never know.
ANI is going to be big business. Just north of Atlanta is one of the new
AT&T regional billing centers. Their goal is to fully integrate ANI with their
customer inquiry department. So when you call 1-800 whatever, the AT&T rep
will answer "Good morning Mr. Jones, how's the weather in Macon? I'll bet
you're calling about that collect call to Bogota." They'll have your name,
address, and billing info on the screen in front of them as they answer your
call.
Hmmm... try forwarding your calls to AT&T. What will happen?
Brent Laminack (gatech!itm!brent)
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