Monthly Archives: August 2013

Removing Oscillations From The Rising and Falling Edges

Improperly terminated digital transmission lines will result in ringing

A Digital Pulse With Ringing

The test engineer was puzzled. He thought he had provided the correct commands to the programmable counter, but the results he was getting back from it were all wrong. To be within the test specifications, he should have been measuring a pulse-width around 500ms. Instead, he was getting a pulse-width of less than 10 nanoseconds. His first thought was that there must be a bug in the test script that was sending the commands to the counter. So, he manually programmed the counter from the front panel of the device. This didn’t help. He was still getting those short pulse-widths. Next he tried manually measuring the pulse-width using a digital storage scope. “That’s strange” he thought. It looks like the pulse width is 500 ms. He adjusted the time scale on the storage scope to 10 nanoseconds per division, and observed the rising edge of the pulse. There it was. The pulse was ringing on the rising edge. Ringing is the process where a signal that is transitioning from a low to a high state or from a high to a low state oscillates back and forth before settling on the final value. When viewed with an oscilloscope, this signal looks like the step response of a filter that causes oscillations until the oscillations are damped out. The recently graduated engineer didn’t know what was causing the ringing, since he had not included inductors, capacitors or resistors in the circuit that connected the device under test to the programmable counter. Still, he figured he could get rid of those oscillations by including a low pass filter before the input to counter. Another solution that he found easier was to program the counter to ignore any falling edges that occurred within the first 20 nanoseconds of the pulse. Years later, with a bit more experience, he realized what he had not realized then. He had failed to include a terminating resistor matching the impedance of his 50 ohm coaxial transmission line. If he had done this, the reflections that occurred at the point where the impedances were mismatched would have been made insignificant, and programmable counter would have measured 500ms from the beginning. Many times later when facing problems, the test engineer would think of this and muse “Perhaps the problem is I don’t know what I don’t know. Now, how do I solve that problem?”

By adding a terminating resistor to the transmission line, the oscillations can be removed.

A Digital Pulse Without Ringing

This schematic shows a terminating resistor at the receiving end of the connection.

Transmission Line Including a Terminating Impedance

Here’s a nice technical discussion on terminating digital lines…

http://www.ni.com/white-paper/3854/en/

Here’s a nice technical discussion on calculating the impedance of a transmission lines…

http://www.allaboutcircuits.com/vol_2/chpt_14/3.html

How long will the Internet last?

Will the Internet last as long as the Pyramids?

When one looks at a timeline of the 7 wonders of the ancient world…

http://en.wikipedia.org/wiki/File:A_timeline_of_the_Seven_Wonders_of_the_ancient_world.png

it is the striking to note that the first wonder built, the Great Pyramid of Giza, is the only wonder still standing. It is also striking to consider that, while the Great Pyramid of Giza stood for over 4500 years, the period of time when all seven of the ancient wonders stood simultaneously lasted only 21 years.

Today, different groups of people have assembled different lists of the seven wonders of the modern world. Most of these lists are of civil engineering wonders, but some lists include wonders from other branches of engineering.

http://listverse.com/2007/09/07/top-7-wonders-of-the-technological-world/

If you were to make your own list, perhaps you would include the Internet. The Internet is made up of other modern technological wonders, including the computer, the microcomputer, operating systems, and telecommunications systems. It is powered by a global energy distribution system, and has developed a mutually dependent relationships with many energy distribution systems.

Will the Internet last as long at the Great Pyramid of Giza? Like the Great Pyramid of Giza, the Internet was designed to last. In part, its strength is due to its distributed design. It has become so large, so self-healing, so redundant and so distributed that is never entirely down. Of course, there always some parts of it that are not working quite right.

Back in 1998, a hacker named Peiter Zatko, aka Mudge, claimed before the United States Congress that it was possible to take down the entire Internet.

http://www.youtube.com/watch?v=VVJldn_MmMY

Whether one believes that something like this was possible then (or might still be possible today), the idea that large parts of the power grid or the Internet could encounter long duration outages should be considered. This is because the operation of either currently depends the operation of the other, and many lives now depend on both.

Network Outage Finger-Pointing

What to do then your providers blame each other for a network outage

Network Outage Finger-Pointing

The Chief Information Officer had run into this sort of problem before. His Network Manager was telling him that the leased line provider had an outage on Line AB. The leased line provider was telling him that there appeared to be something wrong with Port 1 on Router A. What he hoped would be a productive information-gathering meeting was turning into an exercise in finger-pointing with both sides indicating that they checked and rechecked their work. So, quite seriously, the two sides were squared off, both convinced the other side had messed something up.

Sometimes it's hard to know where a problem originates.

There was a loss of connectivity between two routers

Because the Chief Information Officer had run into this problem before, he knew exactly what to do. He proposed an experiment. The idea was to connect Leased Line AB to Port 2 and Leased Line AC to Port 1. If the problem remained on Lease Line AB and not on Lease Line AC, then the problem was with Leased Line AB. If the problem moved from Leased Line AB to Leased Line AC, then the problem was with Port 1 of Router A. Both sides eagerly agreed to experiment. This was just the sort of evidence they needed to show that they had done their jobs correctly. Of course, Router A’s configuration would need to be temporarily modified to maintain consistency with the IP addressing scheme in place. Fortunately, this was a simple modification, and the Network Manager had the changes ready within 15 minutes. All that was needed then was to swap the cables and reboot Router A.

To avoid finger-pointing, it is sometimes necessary to gather more information.

An experiment was proposed to isolate the problem

The result was that the problem remained with Leased Line AB. Fortunately, the Leased Line Provider was a reasonable guy, and he was quick to accept what this new evidence meant. He reviewed the provisioning of Lease Line AB for the third time, comparing each parameter with the parameters of Leased Line AC. These two lines were supposed to be provisioned identically. When he found one parameter that was not identically configured, he knew he had found the problem. This was quickly corrected and full connectivity was restored to the wide area network.

The problem was with the provisioning of the leased line.

The experiment indicated the source of the problem

If the Leased Line Provider’s ego was bruised, he didn’t show it. In any case, everyone was relieved that the problem had been solved.

The Hacker-Proof Automobile

The Information Security Analyst sat quietly in the audience. He had driven for hours to hear this presentation, and he could barely believe what he was hearing. The speaker, the head of a government organization, an organization responsible for protecting his country’s information systems, was downplaying the importance of automotive cyber security, comparing those worried about the situation to “Chicken Little,” running around and complaining that the sky was falling. “Wow” he thought. “Does this guy just not understand the situation, or is he pretending that it isn’t a problem for some reason?” The analyst knew full well there was a problem, because he had read two important papers on the topic.

The first was titled “Comprehensive Experimental Analyses of Automotive Attack Surfaces.” The second was titled “Experimental Security Analysis of a Modern Automobile.” These two papers, both written by a team of researchers from the University of California, San Diego and the University of Washington painted a very different picture of automotive cyber security. Not only did the papers point out that there were vulnerabilities. The researchers demonstrated exploits against the vulnerabilities. Three experiments were most notable. First, they demonstrated that it was possible to hack a vehicle through a music file, which would play fine on a computer or a stereo system, but would deliver software updates to onboard computers called Electronic Control Units (ECUs) when played on a vehicle stereo system. Next, they demonstrated that it was possible hack a car while the car was in motion, disabling the brakes at 40 miles per hour. Finally, they demonstrated that multiple cars could be hacked and then commanded to respond to remotely issued commands in unison. This was done while the cars were geographically separated by a large distance.

The authors left it to the reader to speculate what sort of major cyber-attack might be possible should some gifted hacker, terrorist group or some nation state decide to get very nasty. The idea of millions of cars simultaneously losing the brakes while driving over 55 mph came to the analyst’s mind. “Guess that means I’m chicken little” he thought. “Well, at least I’m not running around claiming the sky is falling.” Of course, he would do something about it. He was planning to get another car. This car would be cyber hardened because it would contain no ECUs. This car would be a 1966 Corvette.

The Hacker-Proof 1966 Corvette Stingray

Two important papers on automotive cyber security…

http://www.autosec.org/pubs/cars-oakland2010.pdf

http://www.autosec.org/pubs/cars-usenixsec2011.pdf

Smart Device Design

Words and thought from NetChime Research LLC

Monthly Archives: August 2013

Removing Oscillations From The Rising and Falling Edges

How long will the Internet last?

Network Outage Finger-Pointing

The Hacker-Proof Automobile