OSI Model Layer 1 - The Physical Layer

Welcome to Heavy Metal Cloud. My name is Matt. Today I want to discuss Ethernet wiring. And by the end of this video, I want to answer three questions. Number one, how are bits of information transmitted down the wire? Number two, why are the wires inside the cable organized in pairs? And number three, why are those pairs twisted? To help answer the first question, I'll be using a small oscilloscope. And for the other two questions, I'll be using a device called an electromagnetic radiation detector. But before we uh dive into the hardware, let's start with a little bit of theory. Let's say you're on your computer and you want to bring up a web page. First, you'll make a request to the web server. And what you'll get back is HTML that looks something like this. Okay, that seems simple enough, but how does this communication actually work? What you'll find in uh computer science is that most complicated problems are broken up into layers. The abstraction layers for networking is use uh something called the OSI model. There are seven layers and the flow of communication looks something like this. In this video, we're going to focus only on layer one, the physical layer. And the physical layer is ultimately responsible for sending low-level data or bits from one machine to another. At the bottom here, you can see a series of zeros and ones that follow a pattern. Let's dive a little deeper to see what's going on. If you go to Wikipedia and you type in Ethernet frame, you'll land on a page that looks something like this. Further down the page, you'll see a section that outlines a pattern of ones and zeros that actually start the communication between computers. See if we can find it. Here it is. So each one and zero here is called a bit. And if you group together eight ones and zeros, you get a bite. The transmission starts with a pattern of seven identical bytes. And you may wonder what is the significance of this pattern? And the answer is it has to do with timing. So if the computer on the left is sending information at one bit per second, but the server on the right is expecting bits every two seconds, you're going to be missing a lot of data, a lot of information. By the time the data is processed by the receiving computer, it'll think it's processing all ones instead of the alternating pattern. So to establish the timing, the computer on the receiving end will listen for each bit coming in and it'll adjust its clock speed to match that pattern. Again, in this simple example, every bit is coming in every second. Now that we have our timing established from the first seven bytes, we're ready to let the web server know that we can begin transmitting data. If you look close, you'll notice that the eighth bite, called the start frame delimiter, or SFD, is a little bit different. The last bit is a one instead of a zero. This lets the receiving computer know that everything is set up and we're ready to go. Next, we'll get into the fun part where I'll demonstrate how to send a signal down an Ethernet cable. Okay, so what I'm going to do is cut open an Ethernet cable. And to keep things really simple, I'll focus on just a single pair of wires, the orange wires. The reason I'm using this particular pair of wires, which are located at pins one and two, is they're typically transmit wires used by Ethernet. Okay, diving a little deeper, the striped wire is the signal wire. You use this wire to send a positive or negative voltage. If the voltage is positive, then the signal going down the wire is going to be a one. If the voltage is negative, it's going to be a zero. The solid wire is called the ring or the return wire. And for this simple example, you can think of it as a ground. Although, I'll get into the details of how it works a little bit later. Once I have the two orange wires separate, I'll connect them to a battery. This is connected to an oscilloscope on the other side. And um by the way, if you don't know what an oscilloscope is, it's a device that graphs voltage over time. Again, to create a one, the voltage running down the signal wire has to be positive. For a zero, the voltage has to be negative. So, uh to accomplish this in my demo, I'll just flip a battery back and forth. Now, flipping a battery is a little bit awkward and errorprone, as you'll find out pretty soon here. So, to make my life easier, I built a simple circuit called an Hbridge circuit. And um the circuit isn't important. It will just allow me to press a few buttons instead of having to flip the battery to send ones and zeros down the wire. My goal with this demo is to try and replicate the first bite of an Ethernet packet. So, uh let me give it a try and see how it goes. Okay, before we get started, let me give you a quick tour of the oscilloscope I'll be using. Each line on the x axis is equal to 1 second and each line on the y ais is equal to 1 volt. The line in the middle is uh zero. So anything above that line is going to be positive. Anything below that line is going to be negative. So uh let's start with a low tech example using just a battery. Okay. So what I've done here is I've u cut both ends of an Ethernet cable off and I've exposed all the wires. Um again for this demonstration we're going to be using only the orange wire pair and I already have one side wired up to my little oscilloscope here. On the other side, I'm going to be using this simple AAA battery and I'll connect it to both the orange pairs. If I touch it on the oscilloscope pair, you can kind of see that it's sending a positive uh voltage. So, this would be equivalent to sending a one. If I flip the battery around, it's a little bit difficult. That's why I made that circuit. Okay. If I flip the battery around and then I touch it, you can kind of see I'm sending or creating a negative voltages. So these would be equivalent to a zero. So again, this is a little bit cumbersome trying to flip my AAA battery back and forth. So what I'm going to do next is I'm going to hook up my very simple Hbridge circuit. I'll press a few buttons and I'll try to replicate that first bite of data. Okay, so off camera I have my Hbridge circuit all wired up and yeah, let me try to send that uh first bite of data down the the wire here. I'll count out the bits as I go here. So, we got 1 2 3 4 5 6 7 and 8. Okay, I paused the oscilloscope so we can take a look at what happened. You can see here we've replicated the first bite. And now imagine the voltages are coming from a network interface card. It might look something a little like this. And instead of one bit per second, um the data is moving much much faster. What about the two remaining questions which are why are the wires organized in pairs and why are the wires twisted? To answer these questions, let's take another deep dive into the wiring. Okay, in my demo, I had the current flowing down the signal wire and uh returning to the battery. Um the circuit is effectively a loop. In electronics, this is called single-ended signaling. But is this how Ethernet actually works where a signal is returned back to its source? And uh the answer is no. No, that's it's not how it works at all. Um the way Ethernet actually works is something called differential signaling. With differential signaling, you have two signals that have equal voltage but opposite polarities. Since the sum of the voltages is approximately zero, there's no need for a ground or a neutral wire to um return the current back to the source. Also, if the striped wire changes from positive to negative, the ring wire, the solid wire also has to change polarity to match. Again, both the wires have to be the same voltage but with opposite polarities. So, why does Ethernet use this type of signaling? Let's take a look at that bite of data that was recorded on the oscilloscope. You can see that we have a relatively clean positive and negative voltages which correspond to ones and zeros. But what happens if our wire is next to other wires or electrical devices? This is an electromagnetic radiation detector. It detects magnetic fields that could potentially be dangerous or cause interference with other electrical devices. And this is a big power cable for my surge protector. Let's see what happens when I hold it near the detector there. See that? So, the challenge we face here is that uh this electromagnetic interference can alter the voltage being sent down the Ethernet cable and cause some problems. Next, let's take a look at how that works. So, here's that power cable uh we just seen that's altering the voltage in a very small way. If we hold it next to an Ethernet wire, you can see that the voltage is going to be a little bit higher or lower than 1.5 volts. And again, this is what a clean signal looks like using a single wire as a signal. The receiving computer is able to easily determine um if it's receiving a one or a zero based on the voltage. And this is a very exaggerated view of uh the voltage with electromagnetic interference. You can see the voltage is jumped around and the receiving side is having a hard time determining if uh the bits are a one or a zero. To solve this problem, we introduce a second signal wire that has an equal voltage but opposite polarity. Now the receiving side can reconcile the signal. The electromagnetic interference will alter the voltage by the same amount but in opposite directions. So now we know why the wires are in pairs. But why are they twisted? Okay, let's say we have wires laying side by side. In this scenario, one of the wires would be receiving more interference than the other one. This is bad because we need both the wires to be exposed roughly to the same amount of interference. Otherwise, the receiving computer won't be able to reconcile the difference, and it ultimately won't be able to figure out if the bit is a one or a zero. To solve this problem, we twist the wire pairs. This allows the EMF interference to be roughly distributed between the two wires. Okay, to wrap things up, we explain how a signal is sent down the wire and we explain why the wire is organized with twisted pairs. In the next video, we're going to move up the OSI layer to layer two, the data link layer. Thanks for stopping by and I'll see you in the next

In this video, I discuss how a signal is sent down an ethernet cable. I also explain why ethernet uses wire pairs and why those pairs are twisted. This video is the first in a series of Networking fundamentals. 00:00 Intro 00:32 Theory - The OSI Model 02:49 Explaining the demo 04:21 Demo Time! 06:47 Why Ethernet wires are organized in pairs 09:26 Why Ethernet wire pairs are twisted Links referenced in this video: https://en.wikipedia.org/wiki/Ethernet_frame