Layer 1 Basics

Where layer 1 fits

Going back to our OSI model, let's see how layer one fits into the rest of the pieces. Layer one is this very bottom one, the physical layer. This defines our media and how we're going to transfer those bits. We'll take a look at those different components, but just from a data type perspective, we call that a bit. These are the ones and zeros that are going across the line.

Just a refresher, though: the TCP/IP model combines those first two layers, the data link layer and the physical layer of the OSI model, into just one layer, the network access layer. The reason why is because a lot of the technologies that are out there nowadays that we use, and the technologies we're talking about in this class, have some gray areas. It's not as defined as it was with the OSI model. Just realize that this physical side that we're going to talk about actually creeps up into the data link area as well.

Let me take a little deeper dive into what I mean by that. We see here layer three, the network layer. We have the data link layer, and then the physical layer. If we look at the different standards that are out here, here are some of the standards for Ethernet, here are some of the standards for wireless LAN, and here are some of the standards for a wireless personal area network. There are different standards here, and some of these standards define some of the aspects that get up into the data link layer. Traditionally, when we saw the different protocols, it would separate that physical layer out and that data link layer out into two separate pieces, but what we see with a lot of the technologies now is it's all kind of grouped up to a large degree. So we see some gray areas with this, and that's why the TCP/IP model combines layer 1 and layer 2 into a single layer.

The three components of layer 1

The three components that go into layer 1 are the physical side of it, the encoding, and the signaling.

What the encoding is: there's a lot of data that's coming in that needs to be transferred across. These are the ones and zeros. Encoding is lining up those ones and zeros into an order that's going to be understood by the other side. We have to define what order those ones and zeros are going to go into.

Once we've got to the encoding side, now those ones and zeros need to be changed to something physical. It needs to manifest itself into pulses of light, or electronic signaling that's going across, electrons that are being sent across a wire, or it's going into waves that are going through the air. Somehow we need to convert those ones and zeros into some sort of physical manifestation. That is the signaling side of this.

And then finally the physical aspect, the actual copper lines, the connectors on each side. So those are the three components: the physical side, the encoding, and the signaling.

The physical side

When you stop and think about the physical side, we have NICs on both sides. So these two computers, let's say they're communicating, we have NIC cards on each side. Then we have some sort of media that's going between them, whether it's fiber that's going between them or copper, or perhaps we're going through the air with this. We've got that physical media that's being sent across there. And then we've got connectors on each end that connect an Ethernet cable into those NIC cards. That's the physical side of this.

Even the cable that goes across here, there's a lot of defining of what that cable looks like, and how do we protect this data, whether there's some sort of sheathing around it. There are a lot of aspects to the physical side that get defined, and I'm always amazed at this, because you've got these really tight tolerances. What I mean by that is you've got these connectors on each side of these cables that are made by many different companies. And then you've got the NIC card that goes into these computers that are probably different companies that make those NIC cards. Those connectors have to fit in so perfectly, with very tight tolerances. It can't be too big, otherwise it doesn't fit in, or it can't be too small, otherwise it will just fall back out. When it fits into there, then it needs to click into place, and then somehow you need to be able to unclick it and pull it out. So there are some really tight tolerances with this physical side, and it gets really granular on where the spacings of those conductors are going to need to be, and how do you make a connector between the two conductors. It gets really granular on what's defined with this physical side of connecting these devices.

Encoding

Encoding is lining up the bits that are going to be interpreted on the other side. Obviously, both devices need to know what order those bits are going to be in so they can determine what data is within those bits.

Let me give you an example here. I've got a graph. When the graph is low, that's going to represent a zero. When it's high, it represents a one. The middle eight bits are going to represent the data that's being sent across. So the data, this is going to be a 1 0 0 1 1 0 that's being sent off to the other side. In this example right here, I've got a starting bit of zero that's going to flag the other side that I'm starting to communicate, and then an ending bit of one that says I have sent you the data now.

They could look many different ways depending on the encoding standards that are there. Maybe it's several bits that are flagged on each side; this could look many different ways. But essentially here I've got a starting bit and an ending bit, and possibly somewhere in here I want to fit some parity bits in there, so that way the other side can do a check to make sure that the information it's getting is accurate. So there's going to be some sort of encoding, the order in which the bits are put in, so the other side can understand what is being sent to it.

Signaling

Signaling is actually taking those bits and then turning them into some sort of signal that's going to be sent to the other side. For instance, maybe we're going across a copper line. You can see that we're sending volts, or we're not sending volts, and that could be the way that we're signaling across this cable.

If it's a light, same thing. We could be off, on, off, off, off, on, on, on, on. So that's the light that we're turning on and off, and then there's some sort of sensor on the other side that's picking that up. Or let's say if it's going through the air, then we can somehow change the waves. We'll talk more about waves later, but we can change waves that can be interpreted on the other side, whether it's a zero or a one. That's the signaling.

The difference between encoding and signaling

There can be some confusion between encoding and signaling, so let me go over the difference in this example of Morse code. Morse code is a standard that's out there for encoding. We have dots and dashes that represent letters and characters. That's Morse code, and that's encoding.

But it can go across many different media, whether we're tapping it out on a wireless CB unit, or maybe it's going across some sort of copper lines, or we're using a flashlight. It can go over different media. A dot and a dash is going to be represented by an on or off of the light, or it could be represented by pulses going down a wire. That's signaling. And then encoding is that cheat sheet that you use; that's Morse code, that defines what lettering those dots and dashes represent.

Standards

There are a lot of different standards out there. For this course, the ones that we're most concerned with revolve around Ethernet and wireless, but here's a list of some of the Ethernet standards that are out there. You don't need to know all of the different nuances with these. Just know that these different standards exist. If you're taking the Network+, you may need to be a little more familiar with the ones that are listed here. The ones on the left are the ones that revolve around copper lines. The ones on the right are the Ethernet standards that revolve around fiber lines. We can see that through the years these have been developed. 10BASE-T used to be the older connections that were at 10 megabits per second, and then we worked up to 100 megabits per second.

I'm just going to show you another resource for this, because this is constantly being developed and we're constantly coming out with new standards. I'm going to bring up a browser, and in this browser I'm just going to type in "Ethernet physical layer" — oops, if I can spell that right. This is just Google, so I don't know that spelling matters all that much. We'll click on here; it's a Wikipedia article. Then we'll be able to see some of the different physical layers that are out there in this wiki article. As we scroll through it, we can find some of the different standards. Right at the top is a list of the contents for this specific page, and we can see all of the different speeds. Obviously, we've got faster and faster connections as we've gone along. Some of these — I mean, we have a 1.6 terabits per second. So some of these are really fast connections, and are constantly being developed.

I'm just going to jump to one of the older technologies, fast Ethernet. Some of our switches and stuff that we're going to be working with, a lot of it's going to be fast Ethernet. Let's just take a glance at these. We can see here the first part of this section defines the twisted pairs, so our Ethernet cables that we have. The second part is the fiber optic cable, so Ethernet over fiber optics. As we look at here, it's got the standards that it has on the end here of the fiber optics. We see ST and SC. We see LC that's in here. What these are doing is they're defining the encoding and the signaling and the physical layer of what that looks like. So we can see how these standards define this physical layer, and have the encoding, signaling and physical aspects all defined.

So we've talked about the layer 1 components — physical, encoding and signaling — and then we talked about some of the standards that are out there. Once again, you don't need to know all the nuances of the different standards that are out there. Just know that these exist, and it gets pretty complex as we dig down into what is defined, how the dimensions of the physical layer work, and how the encoding and the signaling happen. It's pretty amazing. I really love getting down into the nuts and bolts of this.