Saturday, September 17, 2011

The Computer Based Interlocking Architecture

From the desk of
 Sandeep Patalay

The Computer Based Interlocking Architecture


The Solid state Interlocking systems for Railways should ensure the following:
  1. Fail safety
  2. Availability
  3. Reliability
  4. Maintainability
 Architecture and methodology

Generally following three types of redundancy techniques are used for achieving fail-safety in the design of signaling systems:

Hardware Redundancy – In this case, more than one hardware modules of identical design with common software are used to carry out the safety functions and their outputs are continuously compared. The hardware units operate in tightly syncronised mode with comparison of outputs in every clock cycle. Due to the tight syncronisation, it is not possible to use diverse hardware or software. In this method, although random failures are taken care of, it is difficult to ensure detection of systematic failures due to use of identical hardware and software.

Software Redundancy – This approach uses a single hardware unit with diverse software. The two software modules are developed independently and generally utilize inverted data structures to take care of common mode failures. However, rigorous self check procedures are required to be adopted to compensate for use of a single Hardware unit.

Hybrid Model - The hardware units have been loosely syncronised where the units operate in alternate cycle and the outputs are compared after full operation of the two modules. Therefore, it is no more required to use identical hardware and software. Although the systems installed in the field utilize identical hardware and software, the architecture permits use of diverse hardware and software. Moreover, operation of the two units in alternate cycles permits use of common system bus and interface circuitry.

To ensure the above said points hardware and software is designed accordingly. There are various techniques to meet the above said requirements as discussed below:



Table 1: Existing Failsafe Methods employed in Design of Computer Based Interlocking Systems

Method Name
Method of Implementation

Type of Errors Detected
Practical Problems with the Method
Time Redundancy


The same software is executed on the same hardware during two different time intervals
(Refer: Figure 5: Time Redundancy)



Errors Caused by transients. They are avoided by reading at two different time Intervals



Single hardware Fault leads to Shut down of the System. This method is not used since software faults are not completely found in validation. And the Self diagnostics employed for checking of hardware faults is not complete.





Hardware Redundancy


The same software is executed on two
identical hardware channels
(Refer: Figure 6: Hardware Redundancy)


Hardware faults are detected since outputs from both the channels are compared. And single hardware fault does not lead to shut down of the system

Software faults are not detected since the same software is running on two identical hardware channels. Software Faults at design stage are still not detected.



Diverse Hardware

Identical Software is Executed on Different hardware Versions
(Refer: Figure 7: Hardware Diversity)

Hardware Design faults at the Initial stage are Detected

Software Faults at the design stage are still not detected







Diverse software

The different software versions are
executed on the same hardware during
two different time intervals
(Refer: Figure 8: Software Diversity)


Software Faults at design stage are detected

Even though the software is diverse, they are executed on the single hardware channel; single hardware fault leads to Shut down of the system.


Diverse software on
redundant hardware




The different software versions are
executed on two identical hardware
channels
(Refer: Figure 9: Diverse software on redundant hardware)




Software Faults at design stage are detected and single hardware faults does not lead to system shut down


Hardware faults at the design stage are not detected.







Diverse software on
diverse hardware



The different software versions are
executed on two different hardware
channels
(Refer: Figure 10: Diverse software on Diverse Hardware)


Software Faults and Hardware Faults are detected at the design stage.





This methods is rarely used, Since design complexity involved is high





4 comments:

  1. Metronet Credit Solution is the best in terms of credit repair. I had many negative items on my credit report that were holding me back. I had late payments, inquiries collections, charge-offs and card debts which I had totally paid off and I am not sure of the others but they were all 3 years old. I filed bankruptcy a year ago and settled all of these. I was so surprised when I found out they were still on my reports. I recently got referred by my sister to hire Metronet via (METRONETCREDITSOLUTION at GMAIL dot COM) which I did and he helped me fix my credit report. I’m recommending him for anyone in need of credit repair. He is just the best.

    ReplyDelete
  2. I had built my credit score painstakingly over the past 3years with intention of getting a home loan by January 2023. My credit went down drastically when I went through a repossession which dropped my credit by almost 97pts; different negative items began to creep in. With my dream shattered I sought for help everywhere, and came across METRONET CREDIT SOLUTION. I contacted them after a careful review about their jobs. They helped boost my credit score across board to an excellent credit fix. I have been pre-approved for a home loan. You can reach them via customer service on WHATSAPP: +16265140620 or EMAIL: METRONETCREDITSOLUTION at GMAIL dot COM.

    ReplyDelete
  3. Lexington is a rip-off; I worked with them for almost 2 years with the aim of qualifying for a mortgage at the end of last year. But my target was not met and with last year gone without any tangible achievement, I lost my confidence in them. I tried every hack in the book but nothing changed. A realtor introduced me to (metronet credit solution) an expert who had been helping his clients. They helped me raise my score to 740; I was able to qualify for a home loan. I’m recommending their services on every forum as promised. You can contact them via mail: metronetcreditsolution@gmail.com or whatsapp: +16265140620

    ReplyDelete
  4. JerryLink Credit Group has made a huge impact in my life. When I was in college, I unfortunately had to rely on my credit cards to help me with my expenses. By the time I finished college, I was unable to pay back a lot of those debts. As a result of that I ended up with a very low credit score and quite a few collections, so I sought out JerryLink Credit Group. They were able to make a huge impact on my credit score. It went from the 500’s to the 700’s in a couple weeks. I couldn’t believe it. With that help, I was able to do so much more, lower the interest rate on financing my car, and to get approved for a great travel credit card which has really helped us while planning our wedding now. It’s been absolutely wonderful and I’m appreciative because I didn’t believe this process. Recommending as promised, you can reach out to them via: JERRYLINKGROUP@GMAIL.COM or text 626 514 0620.

    ReplyDelete