Power over Ethernet Interoperability by Sanjaya Maniktala.
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1 The Evolution of Power over Ethernet
Part 1 An Overview of Ethernet Introduction
Part 2 The Historical Evolution of PoE Introduction
2 Overview of PoE Implementations
5 Inrush and Power-Up
7 Maintain Power and Disconnect
8 PoE State-Machine Diagrams
10 Isolation, PCB Design, and Safety
11 Surge Testing and Protection
12 Lab Skills, Thermal Management, and Decoupling
13 N-Pair Power Delivery Systems
14 Auxiliary Power and Flyback Design
Preface: December 2007, San Jose, California: It seems a long time ago. I walked into a big networking company to head their small Power over Ethernet (PoE) applications team. Surprisingly, I hardly knew anything about PoE prior to that day, having been a switching-power conversion engineer almost all my life. But it seemed a great opportunity to widen my horizons. As you can see, one notable outcome of that seemingly illogical career choice five years ago is the book you hold in your hands today. I hope this small body of work goes on to prove worthy of your expectations and also of all the effort that went into it. Because, behind the scenes, there is a rather interesting story to relate—about its backdrop, intertwined with a small slice of modern PoE history, punctuated by a rather restive search for our roots and our true heroes, one that takes us back almost two centuries.
PoE seemed exciting from the outside, certainly enough for me to take the plunge. Intuitively, it represented the union of two huge, hitherto parallel worlds of modern development—power and networking. It seemed obvious that the scion too would one day mature into a fine young adult. Soon after taking up the mantle, and the gauntlet, I somehow managed to resuscitate my skeletal systems team. Despite that near-heroic effort, even on a good day, we remained at best a freshly greased jalopy, lurching dangerously from side to side under the uneven weight of a large heirloom we discovered in our backseat—one which they insisted was a qualified principal hardware engineer reporting to me, and I of course begged to differ. On occasion, I had silently prayed that one day this artifact would materialize in our PoE lab, and even be spotted moving slowly but surely toward that dusty ol’ Le Croy oscilloscope! But that was never meant to be, I was only dreaming. My job was not destined to get any easier. Finally in 2010, defying all odds (even a sneaky curator along the way), we somehow managed to get our rickety vehicle, not only to the finish line, but first—in the entire industry. We had received the first-ever American safety certification given to a PoE chip, from Underwriters Laboratories (UL) under their UL 2367 category for 30-watt PoE applications. Yes, it was my tiny team, all pumped up with steroids, which did that.
What exactly did it bring to us anyway? Bragging rights for sure. But beyond that, with a UL-certified PoE chip under our belts, we could enable our customers to drop all their port fuses, a savings of at least 5 cents on every port on almost all their Ethernet switches and hubs. That was huge from a commercial viewpoint. Not to be caught resting on our laurels, barely a few months later, in 2011, we acquired another industry-first safety certification—the very first PoE chip certified under the UL 2367 category, but now using four-pair construction, for enabling 60-W PoE. Keep in mind that, to date, a 60-W IEEE standard does not exist. This certification was initiated under a very specific customer request, to support a radical market-driven standard being pushed by a major original equipment manufacturer (OEM). That standard, alternatively called UPoE or UPOE, stands for Universal PoE.1 It seems that just before this OEM reached out to us in distress, the very launch of UPOE from their side had been put on indefinite hold on account of UL refusing to certify product safety based on their existing PoE solution. I was asked to explain, to a rather sleepless customer late at night, why we stood a far better chance than the competition of getting them through the UL safety barrier, and also how much time that would likely take, considering my previous experiences related to safety testing and of achieving UL certification. The happy ending was straight out of Bollywood: the OEM did finally release its new UPOE products, but now completely based on our PoE chips, the ones we managed to get certified for up to 60 W barely a few weeks after that telephonic lullaby. It was delightfully obvious that at least for now, UL certification was no longer just a case of saving money on fuses—it had turned out that no one could even legally sell a 60-W PoE solution in the marketplace without our UL-certified chip, the only one out there at the time. And so, for a brief moment, our rundown vehicle had dared to pretend it was a Ferrari.
After this industry-leading UPOE chip certification,2 my company’s fortunes in this particular business unit turned north, though I doubt anyone around us really understood the technical reasons for their unexpected windfall, who delivered it to them, or under what circumstances. PoE was still considered a niche area (a hobby post, in effect). To make matters worse, PoE is largely analog electronics at work, whereas a good part of our modern world is fashionably digital. In a way, we were therefore low-tech, at least to most people at the helm. The PoE systems team was, I suspect, not really considered smart enough to even pass judgment on any one of the hundreds of “revolutionary” ideas emanating from just one person firmly ensconced in the hallowed office of the chief technology officer. To make matters worse, the systems team was now making incredibly feeble attempts at “incremental ideas” such as the industry’s first commercially successful single-pair PoE (now appearing in the emerging automotive PoE market too3 —see Fig 13.16). So, finally, inundated with tons of congratulatory messages that flowed in (not), shortly thereafter in early 2012 I walked out of what I consider an inconsequential meeting with someone in urgent need of a backbone, to join another company down the road, and to continue my PoE explorations without being forced to make professional compromises. It seemed a natural career move for me, because this new company had recently acquired an innovative Israeli company, which was one of the original pioneers of PoE, as detailed in Chap. 1 of this book. It would help me “close the loop,” technically speaking. I could return to the roots of it all and enhance my perspective too. I could also practice both my power-management and PoE skills under one roof and under one business unit. And last but not least, it also allowed me to complete this book relatively unfettered, rather than having to watch over my shoulder for beady eyes in suits.
In Chap. 1 of this book, I have tried, first and foremost, to give credit where due, perhaps because I have felt cheated too at times. But I have also tried to quantify exactly what we, in PoE, owe our past lives. I am a firm believer that people must declare their inheritance before showcasing their wealth. We need to recognize and reward the truly deserving persons, even if they are long gone. Posthumous recognition does have tremendous value. Many have fought just for a lasting legacy all their lives, and died for it too. To that end, I have carefully retraced the history of PoE, and its underlying innovations, deep into the 19th century. My research points to the startling revelation that PoE started not at some high-stakes IEEE forum in the 21st century (you knew that of course), but almost 200 years ago. The real heroes of our times, largely unsung and unknown today, resided in that era. On closer examination, a lot of our modern achievements pale in comparison to theirs, especially when we learn that they had almost no available resources at their disposal, except sheer resourcefulness and dedication. Once we become aware of our rich legacy, we realize it is our lack of historical perspective which often makes us rather self-indulgent about our own achievements, and often puts us in the embarrassing position of crowning the wrong heroes of our times—perhaps the first ones who just happened to walk through the door, or the ones who spoke the loudest, or perhaps the ones who were just adept at using modern media or communications to stay in our faces all the time, or worse, the postman (the marketing or sales guy).
As an example, in Chap. 1 I point out that modern PoE is built on a singular principle of data and power sharing the same lines (or medium). This principle, often called phantom power today, is much like two persons sitting on the same seat of a train, oblivious of each other (with due apologies to J. K. Rowling). I go on to show how the well-known Wheatstone bridge can be mentally morphed to describe this fascinating phantom circuit principle, a fact recognized very early by a man called John Joseph Carty.4 Carty, a former engineer at Bell Telephone Company, went a step further and replaced the familiar resistors of the traditional Wheatstone bridge with inductors. Voilà! In doing so, he had laid the foundations of phantom power feeding, without which PoE as we know it today would not have existed. This sequence of events and the birth of the phantom circuit is recounted in a scanned image of The New York Times dated July 9, 1911, available at The New York Times archives (it has no direct mention of PoE, though, which is understandable back in 1911).5
All of Carty’s patents can still be found, though as scanned images only.6,7 We conclude that 1886 is the exact year the phantom power principle of modern PoE emerged. The National Academy of Sciences started giving out the John J. Carty Award for the Advancement of Science, the first recipient being Carty himself, just after 1932, the year Carty died.
Incidentally, the vaunted twisted-pair cable, which we use freely today in Ethernet, came from Alexander Graham Bell (hark: the telephone guy was the cable guy too!)—in 1881.11 It is almost embarrassing to sense that we may have been patting ourselves on our backs all along for their achievements, or patting an unknown person with 100 dubiously acquired and questionable patents. Carty and Bell would surely turn in their graves.
We will notice that through time immemorial, genuine contributors to society always do it for love, not for money. Money is incidental in their minds. And that is how some of them brought us to the point at which modern telecommunications really started to take off. The ancient patents on which our modern networking world is based.
are probably not too many, their value is not proportional to their numerical count, but those key patents are indeed rock-solid as it turns out today. They have not only stood the test of time, they have changed our times completely—almost 200 years later and still counting. At the end of the day, any innovation or idea, however it was acquired or pitched to the general public, must pass a litmus test: can it hold up to (impartial) technical scrutiny, not just within the organization or close-knit community it was supposedly created and peerreviewed in, but by the larger scientific and engineering community? And not just today, but a hundred years from now? That is the test which ultimately distinguishes the real Alexander Graham Bells and J. J. Cartys of yesteryears from some unprincipled modern wannabes. We remember the sad case of Milli Vanilli in the music world.12 We also had the sad case of Janet Cooke in journalism.13 More recently, we had the extremely sad case of Lance Armstrong in sports.14 Extremely sad for us that is, not for them—because we let them do it to us for that long. There are also perhaps eight impostors in modern science listed on the Web.15 That is ignominy, not fame. So we ask: are there any such waiting-to-happen scandals in our brave new networking world? That could be terribly embarrassing to all, especially if it turns out that we gave them a stage to strut around on, padded them with generous financial incentives, and then created an ecosystem around them based on completely inadequate checks and balances, one which even protected them ferociously.
I did a fairly comprehensive Web survey while writing this book and came across a bunch of recently filed PoE patents at the United States Patent Office Web site.16 I learned that a staggering number of patents listed there in PoE came from just one person. I counted over 350 U.S. patent applications pending, and another 138 U.S. patents already granted by the office. A rate of one patent per week or so it seems, judging by the dates. Is he the new Edison? The new Carty? The new Bell? How about the father of energy-efficient Ethernet? Time will tell of course. But I did start to wonder if innovation had become a numbers game now. Are 100 patents better than one? Couldn’t that one patent happen to be Alexander Graham Bell’s famous patent, number 174,465, issued on March 7, 1876—the one that brought the telephone into our houses?17 I wondered if these 100 modern patents will change the world, or just the lifestyle of their inventor (and perhaps the inventor’s mentors and carefully chosen co-inventors too). With these closing thoughts, let us now turn our attention to more recent PoE patents, to see how they are perhaps shaping our world and how they might contribute to the future growth of technology. Here is a tiny sampling. All are available at www.uspto.gov or Google Patents at www.google.com/patents. I do caution: please read the original filings carefully for yourself; judge for yourself eventually. Assume I am making off-the-cuff and ignorant remarks. Because the truth is PoE is still evolving, and so are we.
1. U.S. patent number 5,065,133 on November 12, 1991. “Method and apparatus converting digital signals to analog signals and simultaneous transmission of AC power and signals over wire conductors.”18 This patent is perhaps the earliest modern reference to injecting (AC) power on to the center taps of data transformers, similar to what we do in PoE today.
2. U.S. patent number 5,994,998, on November 30, 1999. “Power transfer apparatus for concurrently transmitting data and power over data wires.”19 This injects DC power on the center-taps of data transformers, as in modern PoE.
3. U.S. patent number 6,115,468, on September 5, 2000. “Power feed for Ethernet telephones via Ethernet link.”20 This injects DC on the center-taps of Ethernet data transformers, as in PoE today. An extension of the previous ideas.
4. U.S. patent number 8,026,635 B2, on September 27, 2011. “Power over Ethernet power sourcing equipment architecture for variable maximum power delivery.”21 This says that if a PoE chip inside the power sourcing equipment (PSE) has integrated pass-FETs and thus gets too hot, the PSE chip can be designed to include a control for driving an external FET which can be switched in parallel to the main (integrated) FET. The port current will thus get split, a fraction of it going through the main (internal) FET, so it will not get too hot— problem solved. We will learn in Chap. 6 of this book that there are limits on port current in standards-compliant PoE. By switching in an external paralleled FET, we actually lose information about the net port current, because only the current in the internal FET is being monitored by the PSE. This idea could work if the PSE can somehow accurately sense the current in the external FET too, but that aspect is not addressed.
5. U.S. patent number US 7,956,616, on June 7, 2011. “System and method for measuring a cable resistance in a power over Ethernet application.”22 This invention uses the available time between end of classification and power-on to measure the cable resistance (see Chaps. 4 and 5 of this book). It also thinks there is an available time slot between detection and classification, though, after detection, if you raise the port voltage, any normal PD will assume the PSE is doing classification. Such minor details aside, as the PSE raises the port voltage VPORT above about 22 V (see same chapters), a short-circuit module (SCM) inside the Powered Device (PD) suddenly conducts and applies a 22-V zener across the port, causing a certain port current to flow. The inventor bypasses the problem of infinite currents resulting from short cables (under an applied voltage source as explicitly stated) and proceeds to eliminate the zener drop (diode offset) by using Rcable = (V2 – V1 )/(I 2 – I 1 ), essentially subtracting two potentially infinite numbers to produce a finite number always. But we also need a differential current sensing concept spread across the time dimension now, so that infinite numbers don’t need to be processed or stored at all by the chip. It is an inarguably awesome display of Ohm’s law being used to clobber conventional number theory.23 We can almost feel the infectious eagerness with which the inventor’s mentors must have pursued this idea relentlessly through their brilliant patent review committee and highly paid law firm. And even I admit, in such cases, the typical five-figure incentive stock option award per patent seems rather incongruous. Taking the idea to the bench, a typical PSE cannot distinguish between a shortcircuit inside a PD versus a short-circuit on the cable. So it will jump in to protect from both, by applying current limiting, eventually turning the port off, as is also evident from Chap. 8 of this book. The PSE will try again and again to power up with this new-fangled PD placed on the other side. Interoperability is expected to be the first casualty. This may work with a proprietary PSE, but it is not backward compatible.
6. U.S. patent number US 8,217,527 B2, on July 10, 2012. “Midspan powering in a Power over Ethernet system.”24 The idea of this is that you can inject power into the data pairs by inserting a center-tapped transformer en route to the PD, rather than injecting PoE at the starting point (the switch) itself. Midspan manufacturers may have used this idea for years.25 Though this inventor omits citing the other patent for whatever reason, he or she says that this innovation involves “insertion of inductance at the Midspan to overcome killer patterns that can cause baseline wander.” Baseline wander is described in Fig. 9.2 of this book. It is not clear what exact inductance the inventor is proposing at the Midspan level, except to suggest it is above 350 mH, same as in any Endspan. In Chap. 9 of this book we will learn that all practical data transformers (up to 100 Mbps) have an inductance of over 350 mH, because that happens to be the minimum specified value. In Fig. 2.9 of this book I have given out another such method of PoE injection on the data pairs using a Midspan. Hopefully no one will ask you to pay for that. It is common sense. This inventor proposes the usual known inductance value of data transformers, but since it appears that somewhere in the Ethernet standards it was not explicitly spelled out that a data transformer could be found inside a Midspan, or alternatively, someone did not explicitly write that a Midspan could use a transformer (to inject PoE), the inventor has claimed rights to it. Though, one option, perhaps, was to simply bring it to the notice of the IEEE committee to plug, as most others did. But perhaps this inventor was not present in those IEEE meetings. That would explain it. So, the final remaining question is: Is this an innovation which will change the course of technology?
7. U.S. patent number US 8,082,453 B2, on December 10, 2011. “Power sharing between Midspan and Endspan for higher power PoE.”26 Though filed as a separate idea, and much earlier in time, it actually seems like an extension of the previous idea, described in number 6 above. Clearly, these two patents (numbers 6 and 7) have been culled from one. This part-patent is remarkable. The inventor implies that instead of just using a simple Midspan to inject power, we can send power from the switch over the spare pairs to the Midspan. The Midspan will pass through the power coming over the spare pairs, but will inject power on to the data pairs as described in the previous patent (number 6). In effect we now have power on all four pairs going into the PD, which is strictly not standards-compliant (is that what the patent implies?). Note that “N-pair” delivery systems are discussed in detail in Chap. 13 of this book. One statement in the patent is noteworthy (see the link in this entry):
For example, one scenario that could occur when no power coordination between Endspan and Midspan PSE is used includes both Endspan and Midspan, each independently powering the PD, thereby providing the PD more than two times its required power. However, with Endspan having the ability to configure the first and second output powers, Endspan can have either of the two PSEs, but not both, power the PD, thereby reducing by a half the overpowering inefficiency. In Chap. 3 of this book we will learn that the basic idea behind back off is simply to never allow the Midspan and Endspan to power up the cable simultaneously. But here, both the PSEs (Midspan and Endspan) have somehow powered up. So the new problem seems to be that the two PSEs are not just making available, but providing, twice the power needed by the PD. This is akin to food being forced into someone’s mouth against his or her wishes, or grain being jammed into a harvesting thresher, never mind the actual load across the PD. This spooky overstuffing of the PD results in a catastrophic situation, which the inventor labels “overpowering inefficiency.”
I am the first to admit the inventor has coined a novel term here, which may in fact highlight certain process efficiencies and/or deficiencies of our times, and may lead to a far more energy-efficient Ethernet.
8. U.S. patent number US 8,217,529 B2, on July 10, 2012. “System and method for enabling power applications over a single communication pair.”27 This concerns power delivery over a single pair. The problem with one-pair PoE is, as explained in Chap. 13 of this book, that with one pair we no longer have two available center-tapped nodes for connecting the PoE forward and return wires. Some PoE engineers have therefore copied the phantom circuit approach used in audio microphones and in landline telephones as shown in Fig. 13.16 of this book. In this particular patent, a separate winding is placed around the data transformer to counterbalance the flux produced by the PoE current, thus ensuring we do not need to oversize the data transformers. However, how do you derive power to drive this PD flux-cancellation circuitry (and successfully power up), without having already powered up? Usually, we cannot afford to bombard magnetic cores with 350 mA rather than the 8 mA they are probably designed for. And that is what this patent set out to accomplish. Did it do that? Does it work? When will it get built and tested?
legacy we inherited. There is still lingering ambiguity in the IEEE 802.3at standard. There is thus a strong need for a book like this one. I hope you will use it to innovate in a way that makes future generations look back at us proudly, not askance. Patents are consulted forever. They form our combined legacy, and are also a fogless mirror of our times. Therefore, the underlying process behind creation and innovation is very much worth defending every single day. I do feel the number-of-patents situation, in particular, stoked by generous incentive schemes which reward quantity not quality, needs to be scrutinized. Or we will end up with (more) inventions which aren’t and inventors who didn’t.
This book does happen to be the very first book on the subject. I apologize in advance for any unintentional mistakes or misinterpretations. There are no references other than the IEEE standard to consult really. The dust has still not fully settled. Please double-check and validate everything for yourself. I hope this book will help you in that process and that it will also be fun and enlightening.
Power over Ethernet Interoperability by Sanjaya Maniktala pdf.
⏩Author: Sanjaya Maniktala
⏩Publisher: The McGraw-Hill Companies, Inc
⏩Puplication Date: 2013
⏩Size: 21.1 MB
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