Okay, so let me propose a scenario to you. You've got your IoT deployment, maybe you have a bunch of devices out there, a bunch of base stations connected over wires, and one of your base stations is out there. Maybe it's like a mile away, you got a wire going out there, and you can't communicate with it anymore, and you're wondering what's going on. Did something go wrong with the cable? Maybe a cable got damaged or cut or something like that. What would you do? How would you figure out what happened? So what you can do is you can drive out, look at the base station, it looks fine, look at your connection it looks fine. Something wrong with the cable. Well, what are you going do? Are you going to figure out, are you going to walk along every inch of that cable for the whole mile and look and see where it's broken? That would take forever. When you need to troubleshoot a problem in the wide area, whether it's wireless or wired, you're faced with this problem of you can't really physically inspect all your cables, so this is a big challenge if you need to troubleshoot something. Well, it turns out there's this really neat trick you can do that's used by professional network engineers and is used to locate anomalies using something like a radar, and it is used for wireless deployments, it's used for wired deployments. It's like a radar that you can shoot down wires and figure out if anything's messing with your signal on that wire. This technique is called Time-Domain Reflectometry or TDR. I'm going to talk about this because this is a very useful technique for when you're deploying in realistic environments. So the idea behind TDR is it is a tool that can detect and localize variations in a cable or in a wireless deployment. It can detect things like deformations and cuts, splice taps if someone's tapping into your cable. It can detect crushed cable, termination points, sloppy installations and so on, anything that changes the impedance of the wire. What's really neat is it can actually tell you the difference between these things. It can tell you if your cable got deformed or if it got cut and so on. So the main idea behind TDR is we're going to send a pulse down the wire and measure reflections, and we're going to measure things like the delay of the reflection because the delay of the reflection localizes the location of the anomaly. But we're also going to look at the structure of the reflection, how the reflection comes back because that gives us information about the type of anomaly. So to motivate this, I'm going to show you a video. So what we did here is I went online and I bought a big slinky, a really big one, and what we did is we took the slinky outside and we had one person be at one end of the sidewalk and the other person be at the other end of the sidewalk. Then what one person is doing is they took the slinky and they shook it and sends the pulse down the slinky. They send a wave down that slinky, and you can see that when they do that with their hand, the pulse goes down that slinky. It goes down and goes down but something very interesting happens when that pulse hits the other end. When the pulse hits the other end, it gets reflected back and it doesn't just get reflected back, it gets inverted. So someone sends a pulse on the right side and then the reflection comes back on the left side. So why does that happen? Why does the pulse come back reflected? You'll also notice that the shape of the pulse changes, it gets more square. You also notice that the amplitude of the pulse decreases. There's some reasons for this. There's actually a lot of information about what's going on on the other side of the slinky from this pulse. The pulse is going out and it's hitting something and we can tell a lot about how the other person is holding that slinky by how it gets reflected back. In particular, the person on the other side is holding the slinky fixed. If they had their hand loose and the slinky moves, the slinky pulse would not get inverted. The pulse would come back and it'll be non-inverted. So this is something that's really interesting and it applies to slinkies, but the really weird thing is this applies to wires as well. If you send an electrical pulse down a wire, you can send a pulse down and if it hits a cut or if it hits moisture, if it hits anything it'll get reflected back and the way it gets reflected back tells you a lot about what it's hitting there and where it's hitting that, and this is such a useful technique to diagnose what's going on. It works for electrical wires. It also happens to work for optical cable. It also happens for wireless signals as well. It's amazing how well this works. So let's study this in a little bit more detail. If you have a slinky or you have some pulse on a string and you send the pulse down the string, the way the pulse gets reflected back tells you about the termination on the other side of the string. I'm just going to tell you, and you can think about this. If you think about this is intuitive and it makes sense that if you have a string or a slinky or something like that and you send a pulse down it and no one's holding the other side, it'll be just like this animation here. The pulse will go down and it'll just disappear. Nothing will get reflected back. So if you have no termination, your wave pulse will not be reflected at all. Now, if you have a termination on the other side and that termination is loose, it's soft. Someone's holding the slinky and it's loose or if something is able to move up and down a pole or things like that, you will get a reflection but a reflection from a soft boundary is not inverted. You'll get the same pulse back. On the other hand if you have a reflection from a hard boundary, your boundary is fixed and it's hard, your reflection will go down the wire, the string and it'll get inverted when it comes back. On the other hand suppose you have a rope or a string where you have a thin string and it's connected to a dense string. What will happen there is some of your pulse will go through it and some of it will get reflected. If you go from something where you're going from high speed to low speed or a thin rope to a thick rope, your pulse reflected back will be inverted and it'll be reduced in size. If on the other hand you go from a thick string to a thin string, your pulse will get reflected back and it won't invert. So what you can do is tonight right before you go asleep, pull up this slide and reflect on this a little bit and think about it. Imagine yourself taking a string and doing this and all these things will make sense because we know how strings work and things like that. If you think about all these things are true. So the weird thing is these things are true for electricity too. If we take electricity and we send it down a wire or you take light and send down optical cable or wireless and we send it through the air, we can learn about impedance changes and changes in propagation and things like that. For example if you have a cable and the cable has melted, what happens is the cable's impedance changes. Not as much electricity can get through that chunk of the cable. So this is similar to a hard boundary. You're hitting something that's like a hard boundary. Electricity is trying to get in and not as much can get in and some of it gets reflected back and it's going to be inverted because when you hit a hard boundary it's inverted. So what I have over here on the right is the output from an oscilloscope measurement of a real melted cable. So what we did here is we send a pulse down the wire. That's the initial pulse shown on the left and what gets reflected back is shown on the right, and you can see the pulse gets sent down and some pulse gets reflected back and when it gets reflected back is smaller and it's inverted. So this is a big hint that if you see this in a cable, if you're sending these pulses down, you're going to get reflections back that you have some melted cable. On the other hand, if you have a cut cable, things look totally different. Here we have a cut cable, you send a pulse down and your reflection comes back, it's reduced in size but it's not inverted because here we have an open boundary. There's nothing holding down the electricity. Electricity hits this cut cable and it gets reflected back. So it's an open boundary that can move. Another example is moisture in the cable. Moisture is a little bit different because moisture gets around the cable in different locations. It does different things to the cable. It improves the ability for electricity to go through the cable. In some locations it reduces it and so you see that in the TDR map which I'm showing you down here. We have some water-soaked cable and you see the initial pulse goes down and some of it goes through and some of it gets reflected back and you see like a little bit more of a mess here. This is the signature of cable moisture. If you're doing TDR and you see this signature, you probably have water in your cable. If you have a faulty amplifier. So you have a wire deployed, something amplifying your signal that's getting faulty, you're going to see a signature like this, where you're going to see some of your reflection come back positive, some of it negative. If you have a wiretap, it's going to look like this. You're going to see an impedance change, just a little bit of an impedance change as it hits that tap. So this is really amazing if you think about it, that all of these different situations, the signatures just show up so well. TDR is a really powerful technique for you to figure out what's going on in your cable when you're doing real deployments. You can't go out and inspect everything. So in the future, when you do real deployments you can use TDR yourself to figure out what's going on.
