OSI Layer 2 - the Data-Link Layer

1. Introduction

We are going to carry on working our way down the layers of the OSI reference model. In this section and in the previous section, we covered layer three, the network layer. In this section, it's layer two, the data link layer. And we're going to focus specifically on Ethernet, which is the layer two medium that is used on local area networks. We're going to talk about wide area network layer two media. Later on in the course, we're going to talk about the format of the Ethernet header, and you'll also learn about the makeup of the Mac address.

2. Local Area Network Layer 2 – Ethernet

In this lecture you'll learn about layer two of the OSI reference model, which is the data link layer. Layer two frames are encoded and decoded into bits, ready to be put on the physical layer on the physical wire. Error detection and correction for the physical layer can be provided here depending on the protocol that we're using. And Ethernet is the layer two medium that is used on local area networks. Ethernet is pretty much ubiquitous on the land, and that's the layer of media that we're going to focus on in this lecture. In later lectures, when we get to the wide area network section, we'll talk about the two protocols that are used there.

But before we start getting into Ethernet, let’s actually have a look at some of the different layer two protocols. So you can see it in my slide here I've got a link going to a page on Wikipedia where there's a list of network protocols. Let's open that up now. So here I am, and up at the top it lists some of the common layer one protocols, like ISDM DSL. There are a lot of legacy protocols still listed here as well. Like ISDN isn't used so much these days. Then you can see the list of layer two protocols as well. So Ethernet is in here, the most commonly used on the land. There's a legacy protocol in here, a frame relay, and other kinds of protocols. We've got in here Fid and the point-to-point protocol that we'll be talking about later when we get to the one section that's where you can see the different protocols that operate at the different layers.

Let's go back to the slides. And the next thing I wanted to do was just clear up any misunderstanding you may have about the terminology, because this has been mentioned a few times as we've been going through the OSI lectures. But I just want to really make it clear here. So you've seen this slide several times before. We're going to look at the terminology now. When a packet is composed, obviously it's composed by the sender and it's going to put it on the wire and send it to the receiver. And as we go down through the OSI model, the sender will start off at the top layer, the application layer, and it will compose that part of the PDU. Then after it's done, layer seven will get encapsulated in the layer six header. Layer seven At that point, it's called the data. We then encapsulate it in the layer five header and then in the layer four header. When we put the layer four header on there, at that point it's called a segment. Then the layer three header goes on.

At that point, it's called a packet. And then finally, the layer to the data link layer will go on, and at that point, it's called a frame. But when the sender is sending traffic to the receiver It's not like it sends segments, packets, and frames separately. We're all part of the same PDU. It's really just terminology that we're talking about here. So we send that one PDU. When we're looking at it. From the point of view of layerfour, we call it the segment. From the point of view of layer three, it's a packet, and from the point of view of layer two, it's a frame. Moving on, let's look at that layer two Ethernet header, if it is Ethernet that we're using as our layer two protocol.

So at the start of the header, we’ve got the preamble that's used to help the sender and receiver synchronize. We then have the layer two destination and source address. That's the Mac address. When we're using Ethernet, we then have the Ethernet type, which is used to specify what is encapsulated inside the Ethernet header. So that will typically be IP. Version four, we then have the data and the FCS. The FCS is a frame check sequence. That's a cyclical redundancy check, which is used to check for the integrity of the frame to ensure that it has not been corrupted during transit. So lastly, let's look at that layer two Ethernet address, which is the Mac address. Mac stands for Media Access Control. The Mac address is a 40-bit hexadecimal address. If we look back at a slide, six bytes, each bite is eight bits. So six times eight, that's where we get the 48 bits from. And as concerns the Mac address, it is split into two different halves as concerns.

What makes up the Mac address? The first half of the first 24 bits is the "out." That's the organizationally unique identifier, and that's the manufacturer of the Ethernet port. So if you've got a Cisco router or switch and it's got an Ethernet port on there, it will have a Mac address. And the first half of the Mac address is Cisco's identifier. If you had a network card, and that network card came from IBM, for example, the first half of the Mac address is going to be IBM's identifier, and the second half of the address is going to be So the last 24 bits that are assigned by the manufacturer and the burned and Mac address are globally unique. The actual number of potential addresses is two to the power of 48. Or if you have a look at the slide, it was absolutely huge. big number, 281, quadrillion, or whatever you want to call it. There are a lot of possible Mac addresses. So that makes it possible for every Ethernet port in the world to have a unique Mac address. An important point that I want to make here is that there is no logical address with your Mac addresses.

It's just one big flat address space. We could have a PC with a network card from IBM, so it's going to have an IBM Mac address on there that could be in New York. We could have another one also from IBM in Beijing and another PC from IBM in London, so they're not grouped together There's no kind of a logical order with your Mac addresses, just one big flat address space and that's why this section is a lot shorter than the section where we were talking about IP addresses with IPad dresses. There is a logical order there and that's how we as administrators are going to control our networking The last thing to do in this lecture is to show you how to get information about the Mac address. I'm on my laptop here. So let's see how you get the Mac address on a Windows machine, so I'm going to open up a command prompt here. Type in CMD and then the command is Ipconfig space and then slash all to get the Mac address I've got a whole heap of output here because I'm running VMware on my laptop, so I've got a lot of virtual adapters.

If I scroll back and then find my wireless adapter, which is going to be in this list somewhere, let me just keep scrolling through until I find it OK. Here it is to my wireless network adapter because I'm on wireless now. Both wireless and wired Ethernet use a Mac address and the entry here. Physical address That is the Mac address, so my Mac address on my laptop is six eight OK. So that's how you find it on Windows. Let's have a look at Linux next. So I'm going to open up party and I'm going to SSH into a Linux box that I've got running, so I'm going to secure shell in here. I'll get the command prompt in Linux, so on Windows it was Ipconfig and you have to use slash all to get the Mac address in Linux. It's F confit for interface configuration and I'll scroll up a little bit here you can see Here's my Ethernet interface that I'm using here. The Mac address is the hardware address, so that's zero zero three29 is the oui portion and then the vendor assigned portion c four E eight. Seven is okay. So that's how we finally find the Mac address on Windows and Linux.

Let's have a look at a Cisco router or switch, so I'm going to open up another party session and this time I'm going to SSH onto my router. I need to enable to get to the enable prompt and then the commands This is going to give me a heap of output about all my different interfaces I could also just enter one interface to more target the output and I can see on fast Ethernet the address is 187-3748 D five six in brackets here. It tells me the bias that's the burned in address and it's exactly the same value here. It is possible in software to change the Mac address on the internet, but normally we won't do that. Normally, we'll just leave it at the address provided by the manufacturer. OK, so that was everything I needed to tell you about Mac addresses. See you in the next lecture.

OSI Layer 1 - the Physical Layer

1. Introduction

Our last section on the OSI reference model is because we've reached layer one, the physical layer. Just like in the last section when we covered the data link layer, we're going to be focusing on the local area network and Ethernet here. So you'll learn about the different connection types. For Ethernet, we'll discuss the specifications of copper cable and of fiber optic cable as well.

2. Ethernet Connection Media

In this lecture, you're going to learn about the final layer in the OSI model, which is layer one, the physical layer. OSI layer one conveys the bit stream, meaning it puts the actual bits on to the wire. That could be an electrical impulse over copper cables or light over fiber optic cables or radio signals over wireless. So it does that and it takes control of the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data, including defining the cables that will be used, the interface cards, the Ethernet ports or the Wan port types, and the physical aspects of the network. In this lecture, just like in the last one, we’re going to focus on the local area network. So we're going to be focusing on Ethernet again.

We're going to cover wide area networking in a later section. Our Ethernet LAN connections can be carried over coaxial cable, which was the original implementation, but that's not used anymore. What we have nowadays is twisted copper pair cable, fiber optic cable, or wireless. Starting off with our copper cables, we use UTP cables for this unshielded twisted pair. They are commonly used to connect desktop computers to switches. The connector type is RG 45, and the maximum length of the cable is 100 meters. And if you have a look at the picture on the slide here, I'm sure you've seen a standard network cable before. These are the copper cables we're talking about here. A standard network cable has an RG45 connector at the end. Let's have a look at the link I've got in this slide as well to get some more information on UTP cables. So the link is on Wikipedia and you can see with our standard UT cables we've gone through several different iterations. Cat Five was pretty popular not that long ago. We're up to cat seven now, and cat eight is in development. The difference between the cats is the categories. The difference between the categories? Well, newer categories have support for higher bandwidth connections. Back when category five first came out, we didn't have ten gigabits, 40 gigabit, or 100 gigabit Ethernet back then. So category five was fine for carrying the maximum bandwidth connections that were available then.

And you will still find some offices that have got Cat Five cable in there. If you want to carry ten gigabit Ethernet, then the minimum is Cat Six cable. So cat five and cat five e can go up to gigabit Ethernet. Cat Six supports 10GB Ethernet for the newer standards, which are 40 gigabits and 100 gigabyte Ethernet. Now, I save our standards because they're standardized, but we're not really all that common in implementation yet. They haven't been out for all that long. You'll find in a lot of enterprises now that they go up to ten gigabit Ethernet. But 40GB Ethernet and 100 gigabyte Ethernet are going to see more common deployment as time goes on. Okay, so those are the different categories of cables, and the next thing to tell you about is that the proper cables can be either straightthrough or crossover UTP cables. The receiving transmitter wires can be wired to the RG 45 connector as either straight through or crossover. If I go back a slide, you see the end of the cable there but an RG 45 connector and there's individual receive and transmit wires in there.

They can be cabled. Either straight through is more common or the wire can be reversed, which makes it a crossover cable. Straight through cables are used to connect an end device (a PC or a router) to a switch. So they're more commonly used. Crossover cables are used to connect devices together directly. They are most often used to connect two devices of the same type. For example, two computers to each other, or two switches to each other. Cabling two computers to each other is not a normal thing to do. It's not a very big network if you've only got two computers there. So if you're in a pinch and you need to copy some files from one computer to another and you have no other way of doing it, you could actually connect them back to back with a crossover cable and you could transfer the files that way. But we've got things like USB sticks nowadays, and in a normal office we'll have an access switch there that we could connect them through.

So it's very rare that you would ever consider connecting computers back to back with each other like that. What is common, though, is connecting your switches to each other. So we've got our building at work; we've got the bottom floor and everybody on the bottom floor is connected to our bottom floor switch, and then we've got the first floor and everybody on the first floor is going to be connected to our first floor switch. Well, we want our staff on both floors to be able to communicate with each other on their computers. So we're going to need to connect the switches together as well. The standard way to connect two switches together is by using a crossover cable. However, modern switches support auto MDX, whereby received and transmitted signals are reconfigured automatically to get the expected result. So with modern switches, you don't have to use an crossover cable, you can use a straight over or a straight through cable as well and that will still work. So the normal way to do it is a crossover. But with really modern switches, a straight through will work just fine as well. But if you are going to use a straight through, do check that it does support MDX.

Okay, so that was our copper cables moving on to fiber. Fiber supports longer distances and higher bandwidth than is possible with copper. The kind of places where you use fiber cable would be if you're connecting two separate buildings within the same campus. So again, we're still talking about the local area network here. So all of our PCs in the local area network are going to be in the same building or within the same campus at most. When we are in a campus kind of environment, like maybe a university, we want to table our buildings together. Typically, the connections between switches and different buildings will use fiber optic cables. Another reason for using fiber is forward will switch to switch connections inside the building because we'll typically require higher bandwidth between direct connections between switches in the building and copper might not be able to connect with fiber. We've got two different types: single mode and multi-mode fiber. Single mode supports higher bandwidth and longer distances than multi mode, but it's more expensive.

Let's have a look at the link here now. I've got this on my second tab. So this is the Wikipedia page for multimode fiber. And you can see, just like we had different categories released over time for copper cable, there are different standards for our fiber cable as well. This is on multi mode.So you can see that for ten gigabit Ethernet, depending on the type of cable we're using, it can go up to 400 meters. 140gb of Ethernet is supported on fiber as well. They're not supported on copper yet, and that can go up to 150 meters. So looking at multi-mode fiber, you're looking at a maximum length of a few hundred meters. Single mode can go much further; it can go up to several hundred kilometers again, depending on the cable type and the other physical hardware you're using. So our last slide here is a picture of some of the different fiber connectors that are available. There are many different types of fiber connectors depending on the type of cable you're using, whether it's multi-mode or single-mode, and the distance it's travelling over. Another difference between fiber and copper is that the copper RG 45 connector will plug directly into the switch.

But fiber connectors, they'll normally go into an transceiver, which then goes into the switch. So you've got a switch with an open port in it, a small transceiver goes into the port on the switch, you choose the correct type of transceiver that matches the connector and the type of cabling that you're using. There is one last thing that I want to tell you about here, and that is Poepower over Ethernet, which you can use as a convenient way to get power down to your Poe capable devices such as IP phones and wireless access points. So you can see in the slide here we've got an office and they've got IP phones on their desks, and we're also using wireless access points as well. And the traditional way to connect them to the network is to connect them all to a standard switch. And if you're using a standard switch, which means it does not support Power over Ethernet, then you're going to need to have power supplies plugged into all of your devices as well.

So if there are 100 IP phones on the desk there, every single one of those IP phones is going to have a network cable plugged into the switch for the network connectivity, and it's also going to have a power supply plugged into a wall socket. But there is a more convenient way that we can do this, and that is by using a PoE Power Over Ethernet switch. They look the same as standard switches, but they have the added POE functionality. When you use a PoE switch, the power is sent to the device over the standard network cable. So that saves you from having to use a separate power supply for all of those different devices. So in our example in the office with 100 IPhone, then that is 100 phones now that have a power supply plugged into the wallpaper. So there's less cabling, it's just a whole lot more convenient for your users as well. So when you do have an office that has got IP phones deployed in there, it is often used that you're going to have a PE switch there rather than standard switches.

Another thing that you'll sometimes see with Poe, but not as commonly, is if you're not using IP phones, but you do have just wireless access points there. Well, if you've got a wireless access point which is in a location where it's hard to get power to, for example, maybe it's on the roof of a warehouse and there's no power near there, then what you can do is use a power injector. So rather than putting in an entire Power Over Ethernet switch, if you've already got a standard switch in there, what you can do is buy a power injector. The power injector gets plugged into power, and then it supplies Power Over Ethernet, over the Ethernet cable, towards the wireless access point. That Ethernet cable can go up to 100 meters. So it's a way that you can get power to a wireless access point in a location where it would be normally difficult to get power to. Okay, that's everything I needed to tell you here. And that was the last lecture on the OSI model. We'll see you in an extra.

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