The safety people are torn and disagrees internally.
Formal methods for signalling interlockings Signalling words September Signalling and Telecoms Back in the day of mechanical signalling, it was comparatively simple to prove that signalling interlockings did what they were supposed to do.
There were drawings to study and a finished mechanical system that could be tested. The interlockings themselves were fairly limited in their application, perhaps covering one junction or, at most, a series of junctions such as at a station throat, but it was all fairly comprehensible.
Then along came computer systems. Suddenly, the problem was immeasurably more complex. Every line of code could alter how the system worked and interlockings grew to control larger areas, introducing possibilities of more interactions.
So how to check it? With teams of computer experts who were also signalling engineers, or signalling engineers who were also computer programmers, laboriously going through the program line by line? A sensible and standardised approach was needed. They seemed like obvious candidates, but would have to be modified to work on safety-critical signalling systems.
Clearly progress has been made with the standards. In the early s, few formal methods existed, with VDM Vienna Development Method and the use of Z-notation being two of the options available at the time. Not only were they the preserve of academia, but also they were certainly not sufficiently mature to allow industry adoption.
Most such methods lacked support for automatic formal proof. It has been a long journey to bring them to the level of sophistication that is available today. Written requirements, as we understand and use them today, just did not exist at the time. This was a major first hurdle.
The combination of a lack of understanding of the necessity for precise requirements and the toolset itself caused considerable challenges. The approach was not considered viable for commercial use at the time. Second foray Several years later, as a result Signalling words a technology change with the Trackguard Westrace Mk I interlocking configured by means of ladder logic being replaced by the Mk II interlocking that is in use today, the capacity for configuration data increased ten-fold.
With the higher potential for error as a result of increased data capacity, and the opportunity to apply the new technology onto Network Rail infrastructure through modular signalling, both templated design methods and formal methods approaches were investigated.
By this time, the understanding of the necessity for precise requirements had matured and, compared to the early days of the project, tool support had evolved considerably and established a long track record in railway signalling application. Clearly, using a formal approach such as this requires a definition of both the safety strategy and the approach, to gain acceptance with the customer and, internally, with the design and test community.
By this time, the process had evolved from just using formal proof to also including the generation of the data, test of the data and sign-off verification — in other words, complete automation of the process from a data configuration perspective. The architecture of the system comprises a suite of generic specifications, including the generic rules design, test and safetywhich have a 1: Once completed, the next step in the process is the specification of the specific installation, in other words the Scheme Plan.
This is either entered in XML extensible markup language format, electronically via SDEF standard data exchange format as specified by Network Rail, or using other electronic formats, for instance RailML European open data exchange format. Results of the second foray Data for the Shrewsbury to Crewe SYC Modular Signalling project for Network Rail was prototyped as a single interlocking today there are three interlockings.
This demonstrated not only the viability of the toolset, but also that any future changes to the layout or signalling principles could be easily changed and rerun in minutes. Whilst the process of generation and test had been proven by the prototype SYC installation, clearly the safety argument for the tool use was significantly more of a challenge.
The basic premise used for the auto-generation and auto-test of the configured data is for a suite of SIL0 applications which, once complete, are presented to the existing Westrace Graphical Configuration System GCS toolset in the same manner as would be used with templates.
The application data is then subject to consistency checking, compilation, de-compilation and reverse checking, prior to input to Prover Certifier for sign-off verification.
This process was subject to independent assessment by Professor John McDermid of the University of York, who concluded: Formal methods do offer benefits, but they are not a panacea and the approach adopted by Siemens seems to be balanced and to have due attention to the need to demonstrate the integrity of tools on which the process relies, and also to acknowledge the important role of humans in the process.
In essence, this encompassed the implementation of the sign-off verification certification process only, with the configuration data having already been generated and tested using conventional means.
This required a slightly different approach, whilst the definition and review of the safety requirements remained one of the major activities. The input of the geographical representation of the railway was a minor challenge this project entered the geographical representation manually.
After the third iteration of conventional testing, the forth iteration was tested solely with the toolset.
Lessons learned The biggest problems faced on any project are imprecise, ambiguous or conflicting requirements. During development of the generic application, requirements need to be presented in natural language so they can easily be translated into the toolset code.
This removes ambiguity and forces conflicting requirements to be expanded to a more explicit form so the conflict can be removed. The safety requirements, themselves based upon two previously and successfully implemented projects, still contained some requirements that were imprecise, ambiguous or conflicting.Mar 01, · On both sides of "the pond" we seem balance out our use of Ls (and bytes) in both directions!
AE/BE: signaling/signalling counseling/counselling. For clients to exchange metadata to coordinate communication: this is called signaling. To cope with network address translators (NATs) and firewalls.
In this article we show you how to build a signaling service, and how to deal with the quirks of real-world connectivity by using STUN and TURN. 1 [EvergreenEnergy – International Interdisciplinary Journal, New York, March ] Signalling Power of Dividend on Firms’ Future Profits.
The term ‘virtue signalling’ may sound as if it comes from the field of social sciences, but the Oxford New Words Corpus* reveals that it is used mainly by journalists writing opinion pieces in blogs or newspapers, especially those with right-wing sympathies such as Fox News or attheheels.com Signal Words 7.
Cause, Condition, or Result Signals (Condition or modification is coming up.) because if of for from so while then but that until since as whether in order that. Sep 09, · The signals of preference for the leadership of an adversary over that of the president came despite criticism from Democrats and discomfort among Republicans.