1 00:00:00,050 --> 00:00:01,290 In this section of the course, 2 00:00:01,290 --> 00:00:03,810 we're going to learn all about the OSI model. 3 00:00:03,810 --> 00:00:06,570 Now, you may be wondering what is the OSI model? 4 00:00:06,570 --> 00:00:09,630 Well, OSI stands for the open systems interconnection. 5 00:00:09,630 --> 00:00:11,220 And when we talk about the OSI model, 6 00:00:11,220 --> 00:00:13,800 this was developed all the way back in 1977 7 00:00:13,800 --> 00:00:16,620 by the International Organization for Standardization. 8 00:00:16,620 --> 00:00:18,300 Now, this organization is responsible 9 00:00:18,300 --> 00:00:21,180 for creating different standards, which we refer to as ISO 10 00:00:21,180 --> 00:00:22,770 and then some number behind it. 11 00:00:22,770 --> 00:00:26,610 For example, if you see ISO 7498, that is the standard 12 00:00:26,610 --> 00:00:28,470 that refers to the OSI model. 13 00:00:28,470 --> 00:00:30,030 Now for the exam, you do not need 14 00:00:30,030 --> 00:00:32,580 to worry about memorizing any kinds of ISO numbers, 15 00:00:32,580 --> 00:00:34,440 but I do want to introduce you to this concept 16 00:00:34,440 --> 00:00:36,420 because everything we cover in computing is going to be 17 00:00:36,420 --> 00:00:38,490 associated with some kind of standard with it. 18 00:00:38,490 --> 00:00:41,310 Now, in the case of the open systems interconnection model, 19 00:00:41,310 --> 00:00:43,740 most people simply call this the OSI Model, 20 00:00:43,740 --> 00:00:44,760 but you'll also hear it referred to 21 00:00:44,760 --> 00:00:46,770 as the OSI Stack sometimes. 22 00:00:46,770 --> 00:00:49,710 Either way, we're really talking about the exact same thing. 23 00:00:49,710 --> 00:00:52,050 Now, the OSI model is an extremely important thing in 24 00:00:52,050 --> 00:00:53,940 Network Plus because it's a fundamental thing 25 00:00:53,940 --> 00:00:55,380 that we're going to use to discuss all the pieces 26 00:00:55,380 --> 00:00:56,730 and parts of our networks. 27 00:00:56,730 --> 00:00:58,170 In fact, it's so important 28 00:00:58,170 --> 00:01:00,240 that you're going to see some questions on the exam about this 29 00:01:00,240 --> 00:01:03,270 concept and how it relates to other things in our networks. 30 00:01:03,270 --> 00:01:05,637 The OSI model is going to be made up of seven different layers, 31 00:01:05,637 --> 00:01:07,860 and we're going to talk about each of these seven layers in the 32 00:01:07,860 --> 00:01:10,320 next seven videos covering one layer per video 33 00:01:10,320 --> 00:01:12,060 because this model is just that important 34 00:01:12,060 --> 00:01:14,430 to our understanding of our computer networks. 35 00:01:14,430 --> 00:01:16,230 Now, as we work through this section of the course, 36 00:01:16,230 --> 00:01:18,150 we're going to be focused mainly on domain one, 37 00:01:18,150 --> 00:01:19,380 networking concepts, 38 00:01:19,380 --> 00:01:22,620 and exclusively we'll be covering all of objective 1.1, 39 00:01:22,620 --> 00:01:23,700 which states that we must be able 40 00:01:23,700 --> 00:01:25,230 to explain concepts related 41 00:01:25,230 --> 00:01:28,920 to the open systems interconnection or OSI reference model. 42 00:01:28,920 --> 00:01:31,470 Now, the seven layers that make up the OSI model are useful 43 00:01:31,470 --> 00:01:32,910 when we're trying to troubleshoot a network, 44 00:01:32,910 --> 00:01:33,990 because if we have a problem 45 00:01:33,990 --> 00:01:35,850 and we think about it from all the different layers in the 46 00:01:35,850 --> 00:01:37,200 seven layers, we're going to be able 47 00:01:37,200 --> 00:01:39,480 to start identifying exactly where the problem is 48 00:01:39,480 --> 00:01:42,000 and then we can troubleshoot it more accurately. 49 00:01:42,000 --> 00:01:45,510 Now, the OSI model is also called the OSI Reference model, 50 00:01:45,510 --> 00:01:47,100 and you may be wondering what do you mean 51 00:01:47,100 --> 00:01:48,540 by a reference model? 52 00:01:48,540 --> 00:01:50,910 Well, a reference model is simply something that we use 53 00:01:50,910 --> 00:01:52,140 to categorize the functions 54 00:01:52,140 --> 00:01:54,120 of a network in a particular layer, 55 00:01:54,120 --> 00:01:56,190 and that's what we're going to do when we use the OSI model 56 00:01:56,190 --> 00:01:57,930 during our troubleshooting efforts. 57 00:01:57,930 --> 00:02:00,090 Now, when you start to look at the OSI model in depth, 58 00:02:00,090 --> 00:02:01,140 you're going to start to notice 59 00:02:01,140 --> 00:02:04,020 that the OSI model doesn't actually line up cleanly 60 00:02:04,020 --> 00:02:06,240 or easily or accurately with the way 61 00:02:06,240 --> 00:02:08,370 that our modern networks are going to operate. 62 00:02:08,370 --> 00:02:10,949 For example, some things are going to operate at multiple 63 00:02:10,949 --> 00:02:13,380 layers of the OSI model in our modern networks, 64 00:02:13,380 --> 00:02:16,080 especially when we're discussing the upper three layers. 65 00:02:16,080 --> 00:02:18,390 Now, there's a good reason for why the OSI model isn't 66 00:02:18,390 --> 00:02:20,640 perfect the way it is the way our networks are operating 67 00:02:20,640 --> 00:02:21,750 today, and that's 68 00:02:21,750 --> 00:02:24,450 because our networks today operate under a model known 69 00:02:24,450 --> 00:02:26,550 as the TCP/IP model. 70 00:02:26,550 --> 00:02:29,010 Now, to keep things simple for right now, just remember 71 00:02:29,010 --> 00:02:31,170 that the OSI model is a reference model, 72 00:02:31,170 --> 00:02:33,060 and we're going to use it in our network operations 73 00:02:33,060 --> 00:02:34,680 and troubleshooting because again, 74 00:02:34,680 --> 00:02:36,240 it's very generic in nature 75 00:02:36,240 --> 00:02:37,320 and it's going to allow us to work with 76 00:02:37,320 --> 00:02:38,610 any and all network types 77 00:02:38,610 --> 00:02:41,160 we come across, not just the most commonly used network 78 00:02:41,160 --> 00:02:44,160 that we use today, which is actually TCP/IP. 79 00:02:44,160 --> 00:02:46,590 Now, one of the benefits of using a reference model like the 80 00:02:46,590 --> 00:02:49,590 OSI model is that it can be used equivalently across lots 81 00:02:49,590 --> 00:02:52,530 of different technologies and devices and manufacturers. 82 00:02:52,530 --> 00:02:55,080 So if I'm going to look at a particular wireless network card 83 00:02:55,080 --> 00:02:57,450 for my computer, I can compare how it operates 84 00:02:57,450 --> 00:02:58,860 through each of the seven layers of the 85 00:02:58,860 --> 00:03:01,170 OSI model and then compare it equivalently 86 00:03:01,170 --> 00:03:04,020 to a different manufacturer's wireless networking card. 87 00:03:04,020 --> 00:03:05,640 When you can understand the functions at each 88 00:03:05,640 --> 00:03:07,890 and every layer that's being performed, this helps you 89 00:03:07,890 --> 00:03:10,170 to better understand the flow of data in your network 90 00:03:10,170 --> 00:03:11,850 through the card, how it communicates, 91 00:03:11,850 --> 00:03:13,770 and how you can troubleshoot it too. 92 00:03:13,770 --> 00:03:14,790 All right, now that I've mentioned 93 00:03:14,790 --> 00:03:15,969 there are seven layers of the OSI model 94 00:03:15,969 --> 00:03:17,700 several times in this video, 95 00:03:17,700 --> 00:03:18,840 I still haven't really showed you 96 00:03:18,840 --> 00:03:20,100 what those seven layers are. 97 00:03:20,100 --> 00:03:23,490 So let's take a look at the seven layers of the OSI model. 98 00:03:23,490 --> 00:03:25,110 First, we start at the bottom layer, 99 00:03:25,110 --> 00:03:27,570 and we work our way from layer one all the way to the top, 100 00:03:27,570 --> 00:03:29,370 which is known as layer seven. 101 00:03:29,370 --> 00:03:32,250 These layers are the physical layer, the data link layer, 102 00:03:32,250 --> 00:03:35,070 the network layer, the transport layer, the session layer, 103 00:03:35,070 --> 00:03:37,740 the presentation layer, and the application layer. 104 00:03:37,740 --> 00:03:40,620 Now for the exam, you need to remember these seven layers 105 00:03:40,620 --> 00:03:42,810 and know them in the proper order. 106 00:03:42,810 --> 00:03:44,520 You need to know them going from bottom to top 107 00:03:44,520 --> 00:03:46,200 and from top to bottom. 108 00:03:46,200 --> 00:03:47,610 Now, to help you with that, I like 109 00:03:47,610 --> 00:03:50,070 to use a memory aid known as a pneumonic. 110 00:03:50,070 --> 00:03:52,050 Now in the case of the OSI model, I like 111 00:03:52,050 --> 00:03:53,220 to think about my favorite kind 112 00:03:53,220 --> 00:03:55,170 of pizza when I'm listing the things from the 113 00:03:55,170 --> 00:03:56,760 bottom to the top layer. 114 00:03:56,760 --> 00:03:59,490 Now, my favorite kind of pizza is sausage pizza. 115 00:03:59,490 --> 00:04:02,310 So if you want to remember the seven layers of the OSI model, 116 00:04:02,310 --> 00:04:05,070 just remember what I always say about sausage pizza. 117 00:04:05,070 --> 00:04:07,230 Please do not throw sausage pizza away. 118 00:04:07,230 --> 00:04:10,470 After all, I think sausage pizza is pretty darn tasty, and 119 00:04:10,470 --> 00:04:11,910 therefore we shouldn't be wasting it 120 00:04:11,910 --> 00:04:13,470 by throwing it in the garbage. 121 00:04:13,470 --> 00:04:15,330 Now, by using this sentence, you'll be able 122 00:04:15,330 --> 00:04:17,850 to remember the seven layers from bottom to top. 123 00:04:17,850 --> 00:04:19,680 Just take the letters from the sentence, 124 00:04:19,680 --> 00:04:24,060 please do not throw sausage pizza away or P-D-N-T-S-P-A 125 00:04:24,060 --> 00:04:27,360 and replace it with the words physical, data link, network, 126 00:04:27,360 --> 00:04:30,450 transport, session, presentation, and application. 127 00:04:30,450 --> 00:04:32,190 And there you have it, the seven layers 128 00:04:32,190 --> 00:04:35,100 of the OSI model from bottom to top, at least that's 129 00:04:35,100 --> 00:04:36,840 how I like to remember it, but you can use 130 00:04:36,840 --> 00:04:38,310 whatever mnemonic you want. 131 00:04:38,310 --> 00:04:40,020 I've heard many interesting ones over the 132 00:04:40,020 --> 00:04:41,520 years from my different students. 133 00:04:41,520 --> 00:04:43,830 Once I was teaching a class to a bunch of navy sailors 134 00:04:43,830 --> 00:04:45,450 and they told me their saying was, 135 00:04:45,450 --> 00:04:47,880 please do not tell shore patrol anything. 136 00:04:47,880 --> 00:04:50,040 For those of you who don't know, shore patrols basically 137 00:04:50,040 --> 00:04:52,260 like the military police officers in the Navy. 138 00:04:52,260 --> 00:04:53,670 So I'm guessing they didn't want to get in 139 00:04:53,670 --> 00:04:55,020 trouble with their ship's captain. 140 00:04:55,020 --> 00:04:56,430 So they believed in the mantra of, 141 00:04:56,430 --> 00:04:58,530 please do not tell shore patrol anything. 142 00:04:58,530 --> 00:04:59,910 And this kind of thing is like saying 143 00:04:59,910 --> 00:05:02,640 what happens in Vegas stays in Vegas, right? 144 00:05:02,640 --> 00:05:03,630 Exactly. 145 00:05:03,630 --> 00:05:05,160 Now, another one of my students told me 146 00:05:05,160 --> 00:05:06,660 that their favorite mnemonic was, 147 00:05:06,660 --> 00:05:09,390 please do not teach students pointless abbreviations. 148 00:05:09,390 --> 00:05:10,860 Now, I thought this was kind of funny 149 00:05:10,860 --> 00:05:12,960 because the abbreviation is exactly what we're trying 150 00:05:12,960 --> 00:05:15,990 to do here to remember the seven layers of the OSI model. 151 00:05:15,990 --> 00:05:18,330 Now, whatever works for you is fine with me. 152 00:05:18,330 --> 00:05:20,700 Just remember that you need to memorize these seven layers 153 00:05:20,700 --> 00:05:22,140 before you take your exam 154 00:05:22,140 --> 00:05:24,390 because on the exam, you are going to get questions 155 00:05:24,390 --> 00:05:25,980 about these seven layers. 156 00:05:25,980 --> 00:05:27,690 Now you might see something easy like 157 00:05:27,690 --> 00:05:29,487 what is the seventh layer of the OSI model? 158 00:05:29,487 --> 00:05:31,140 And in that case, you would just pick 159 00:05:31,140 --> 00:05:32,970 application and be done with it. 160 00:05:32,970 --> 00:05:34,800 Or they might give you something a little bit harder 161 00:05:34,800 --> 00:05:36,930 and say, what layer is the session layer at? 162 00:05:36,930 --> 00:05:38,790 And you would say, layer five. 163 00:05:38,790 --> 00:05:41,190 Now again, they might go a little bit more complex 164 00:05:41,190 --> 00:05:42,630 and ask you things about a device 165 00:05:42,630 --> 00:05:45,150 and ask you what layer is that device operating in? 166 00:05:45,150 --> 00:05:47,400 For example, a router is going to operate at layer three 167 00:05:47,400 --> 00:05:49,200 or the network layer of the OSI model, 168 00:05:49,200 --> 00:05:51,600 but a hub only operates at layer one 169 00:05:51,600 --> 00:05:53,280 because it's considered a physical device and 170 00:05:53,280 --> 00:05:55,050 therefore it works at the physical layer 171 00:05:55,050 --> 00:05:56,430 of the OSI model. 172 00:05:56,430 --> 00:05:58,320 Now because of this, we're going to cover each 173 00:05:58,320 --> 00:06:00,300 and every layer of the OSI model in this section 174 00:06:00,300 --> 00:06:02,580 of the course, so you can learn what devices are used at 175 00:06:02,580 --> 00:06:04,380 what layer, as well as which protocols 176 00:06:04,380 --> 00:06:06,540 and other information about them too. 177 00:06:06,540 --> 00:06:08,790 Alright, before we finish up this lesson, we need 178 00:06:08,790 --> 00:06:11,190 to talk about one more thing, and that's data 179 00:06:11,190 --> 00:06:13,140 because our networks are designed for the purpose 180 00:06:13,140 --> 00:06:15,810 of making data flow across those networks. 181 00:06:15,810 --> 00:06:18,240 Now, data moves through the layers of the OSI model, 182 00:06:18,240 --> 00:06:21,030 and as it does, the name of it actually gets changed, 183 00:06:21,030 --> 00:06:22,770 and it's not called data anymore. 184 00:06:22,770 --> 00:06:24,660 So if you hear the word data, 185 00:06:24,660 --> 00:06:27,180 what we're really talking about is things at layer five, 186 00:06:27,180 --> 00:06:29,250 six, and seven of the OSI model 187 00:06:29,250 --> 00:06:32,070 because all three of those layers use the term data. 188 00:06:32,070 --> 00:06:33,330 As you move down the layers 189 00:06:33,330 --> 00:06:35,340 and you go from four to three to two to one, 190 00:06:35,340 --> 00:06:38,070 that name is going to change each and every time. 191 00:06:38,070 --> 00:06:39,360 Now, when we're at layer four 192 00:06:39,360 --> 00:06:42,270 or the transport layer, we are going to call it a segment. 193 00:06:42,270 --> 00:06:43,320 When we get to layer three 194 00:06:43,320 --> 00:06:45,630 or the network layer, we call it a packet. 195 00:06:45,630 --> 00:06:47,730 When we get to layer two or the data link layer, 196 00:06:47,730 --> 00:06:49,140 we call it a frame. 197 00:06:49,140 --> 00:06:51,120 And finally, if we're operating at layer one 198 00:06:51,120 --> 00:06:53,700 or the physical layer, we convert the data into ones 199 00:06:53,700 --> 00:06:55,800 and zeros and we send it across the media 200 00:06:55,800 --> 00:06:58,080 and we refer to this as bits. 201 00:06:58,080 --> 00:07:00,000 Now, this is something you're going to need to memorize 202 00:07:00,000 --> 00:07:01,920 for the exam too, but don't worry, 203 00:07:01,920 --> 00:07:04,230 I've got another pneumonic to help you remember this one. 204 00:07:04,230 --> 00:07:05,400 And this one is really just a simple 205 00:07:05,400 --> 00:07:06,780 question you can ask yourself. 206 00:07:06,780 --> 00:07:09,360 Here it is. Do some people fear birthdays? 207 00:07:09,360 --> 00:07:10,260 That's it. 208 00:07:10,260 --> 00:07:11,460 I think this is a valid question 209 00:07:11,460 --> 00:07:12,750 because as we get older, 210 00:07:12,750 --> 00:07:15,000 sometimes people start fearing their birthdays 211 00:07:15,000 --> 00:07:16,230 because they don't want to show their age 212 00:07:16,230 --> 00:07:17,550 and they don't want to appear older. 213 00:07:17,550 --> 00:07:19,530 But again, I know this is really silly pneumonic, 214 00:07:19,530 --> 00:07:21,360 but hopefully it helps you remember the four types 215 00:07:21,360 --> 00:07:23,460 of information as they flow from data 216 00:07:23,460 --> 00:07:25,680 through those upper layers and down to the lower levels, 217 00:07:25,680 --> 00:07:29,970 going from data to segments to packets, to frames to bits. 218 00:07:29,970 --> 00:07:32,610 Alright, if you can remember some people fear of birthdays, 219 00:07:32,610 --> 00:07:35,370 that's going to give you D-S-P-F-B as your pneumonic. 220 00:07:35,370 --> 00:07:38,100 And then again, we have those data types going from layers 221 00:07:38,100 --> 00:07:41,010 five, six, and seven with data moving down to segments, 222 00:07:41,010 --> 00:07:43,140 packets, frames, and bits going from layers 223 00:07:43,140 --> 00:07:44,730 four, three, two, and one. 224 00:07:44,730 --> 00:07:46,440 So one more time for the exam. 225 00:07:46,440 --> 00:07:48,510 It is really important to understand the seven layers 226 00:07:48,510 --> 00:07:50,640 of the OSI model and how the data moves up 227 00:07:50,640 --> 00:07:52,050 and down through those layers 228 00:07:52,050 --> 00:07:53,430 as you're transmitting the data 229 00:07:53,430 --> 00:07:55,440 and encapsulating and decapsulating. 230 00:07:55,440 --> 00:07:57,270 And that's what we're going to be talking about. 231 00:07:57,270 --> 00:07:59,070 Alright, as we begin this section, 232 00:07:59,070 --> 00:08:00,900 we're first going to spend one lesson on each 233 00:08:00,900 --> 00:08:03,540 of the different layers of the OSI model, as I said. 234 00:08:03,540 --> 00:08:05,940 Then we're going to move into our coverage of encapsulation 235 00:08:05,940 --> 00:08:07,470 and decapsulation. 236 00:08:07,470 --> 00:08:09,480 Encapsulation is the process of wrapping data 237 00:08:09,480 --> 00:08:12,330 with protocol information at each layer of the OSI model 238 00:08:12,330 --> 00:08:14,490 as it passes down the networking stack. 239 00:08:14,490 --> 00:08:17,160 Decapsulation, on the other hand, is the process of removing 240 00:08:17,160 --> 00:08:19,350 that protocol information layer by layer 241 00:08:19,350 --> 00:08:21,780 as the data unpacks itself on the receiving end 242 00:08:21,780 --> 00:08:23,880 as it moves up the OSI model. 243 00:08:23,880 --> 00:08:25,230 After that, I'm going to demonstrate 244 00:08:25,230 --> 00:08:27,690 how you can use a software tool known as Wireshark. 245 00:08:27,690 --> 00:08:30,690 Now, Wireshark is a network protocol analyzer that captures 246 00:08:30,690 --> 00:08:32,370 and displays data as it travels back 247 00:08:32,370 --> 00:08:33,630 and forth over a network. 248 00:08:33,630 --> 00:08:35,070 And this can be done in real time 249 00:08:35,070 --> 00:08:37,080 or captured and analyzed later. 250 00:08:37,080 --> 00:08:39,120 Now, we do this for the purposes of inspection, 251 00:08:39,120 --> 00:08:40,620 troubleshooting, and analysis. 252 00:08:40,620 --> 00:08:42,630 And in our case, we're going to use Wireshark 253 00:08:42,630 --> 00:08:44,700 to analyze a packet capture so you can better see 254 00:08:44,700 --> 00:08:46,680 how the data is broken down into these different 255 00:08:46,680 --> 00:08:48,360 layers of the OSI model. 256 00:08:48,360 --> 00:08:49,860 And then finally, at the end of the section, 257 00:08:49,860 --> 00:08:51,540 we'll take a short quiz to see if you remembered 258 00:08:51,540 --> 00:08:53,040 what you learned in this section. 259 00:08:53,040 --> 00:08:55,020 As we do that, we'll also review your answers 260 00:08:55,020 --> 00:08:56,700 to make sure you know why the right answers were right 261 00:08:56,700 --> 00:08:58,200 and the wrong answers were wrong. 262 00:08:58,200 --> 00:08:59,730 So with all that being said, 263 00:08:59,730 --> 00:09:01,370 let's jump into our lessons on the seven layers 264 00:09:01,370 --> 00:09:03,723 of the OSI model in this section of the course.