1 1 00:00:00,682 --> 00:00:02,570 The OSI model. 2 2 00:00:02,570 --> 00:00:04,640 What is the OSI model? 3 3 00:00:04,640 --> 00:00:07,850 Well, it stands for the Open Systems Interconnection model, 4 4 00:00:07,850 --> 00:00:10,980 and it was developed all the way back in 1977 5 5 00:00:10,980 --> 00:00:14,050 by the International Organization for Standardization. 6 6 00:00:14,050 --> 00:00:15,530 This organization is responsible 7 7 00:00:15,530 --> 00:00:16,940 for creating different standards, 8 8 00:00:16,940 --> 00:00:19,420 which we refer to as the ISO. 9 9 00:00:19,420 --> 00:00:21,680 And there's some number usually behind it. 10 10 00:00:21,680 --> 00:00:25,240 For example, if you see ISO 7498, 11 11 00:00:25,240 --> 00:00:28,480 that's the standard we use to refer to the OSI model. 12 12 00:00:28,480 --> 00:00:29,580 Now for the exam, 13 13 00:00:29,580 --> 00:00:30,460 you don't have to go 14 14 00:00:30,460 --> 00:00:32,930 and worry about memorizing the ISO number, 15 15 00:00:32,930 --> 00:00:34,990 but I did want to introduce you to this concept 16 16 00:00:34,990 --> 00:00:37,120 because in everything we cover in computing, 17 17 00:00:37,120 --> 00:00:39,580 there's some sort of standard associated with it. 18 18 00:00:39,580 --> 00:00:42,590 Now, in the case of the Open Systems Interconnection model, 19 19 00:00:42,590 --> 00:00:45,480 most people simply call this the OSI model, 20 20 00:00:45,480 --> 00:00:49,440 but you may sometimes hear it referred to as the OSI stack. 21 21 00:00:49,440 --> 00:00:53,160 Either way, we're talking about exactly the same thing here. 22 22 00:00:53,160 --> 00:00:55,360 The OSI model is extremely important 23 23 00:00:55,360 --> 00:00:56,780 to the network plus exam, 24 24 00:00:56,780 --> 00:00:59,190 because it's a fundamental thing that we use to discuss 25 25 00:00:59,190 --> 00:01:01,540 all the pieces and parts of a network. 26 26 00:01:01,540 --> 00:01:02,770 It's so important, 27 27 00:01:02,770 --> 00:01:04,070 that you're going to see a lot of questions 28 28 00:01:04,070 --> 00:01:06,450 on the network plus exam about this concept 29 29 00:01:06,450 --> 00:01:08,740 and how it relates to other things. 30 30 00:01:08,740 --> 00:01:11,830 The OSI model is made up of seven different layers, 31 31 00:01:11,830 --> 00:01:14,080 and we're going to talk about each of these seven layers 32 32 00:01:14,080 --> 00:01:17,340 in the next seven videos covering one layer per video, 33 33 00:01:17,340 --> 00:01:19,720 because this model is just that important 34 34 00:01:19,720 --> 00:01:22,020 to understanding our networks. 35 35 00:01:22,020 --> 00:01:24,100 Now, as we work through this section of the course, 36 36 00:01:24,100 --> 00:01:26,550 we're going to be focused mainly on domain one, 37 37 00:01:26,550 --> 00:01:30,390 networking fundamentals, specifically, objective 1.1, 38 38 00:01:30,390 --> 00:01:32,880 which states that we must be able to compare and contrast 39 39 00:01:32,880 --> 00:01:36,170 the Open Systems Interconnection or OSI model layers 40 40 00:01:36,170 --> 00:01:38,660 and the encapsulation concepts. 41 41 00:01:38,660 --> 00:01:41,150 We're also going to briefly jump into domain five, 42 42 00:01:41,150 --> 00:01:43,710 network troubleshooting, towards the end of this section, 43 43 00:01:43,710 --> 00:01:46,310 because I'm going to use a network tool called Wireshark 44 44 00:01:46,310 --> 00:01:48,660 to help show you how we can break down the data 45 45 00:01:48,660 --> 00:01:50,260 that's transferring over our networks 46 46 00:01:50,260 --> 00:01:52,870 into various layers of the OSI model. 47 47 00:01:52,870 --> 00:01:54,570 This tool, Wireshark, 48 48 00:01:54,570 --> 00:01:57,760 is specifically listed under objective 5.3, 49 49 00:01:57,760 --> 00:01:59,310 which states "given a scenario, 50 50 00:01:59,310 --> 00:02:02,720 use the appropriate network software tools and commands". 51 51 00:02:02,720 --> 00:02:04,790 Wireshark is a really helpful tool 52 52 00:02:04,790 --> 00:02:06,690 that's used a lot in troubleshooting, 53 53 00:02:06,690 --> 00:02:10,640 but even the OSI model itself can be helpful as a framework 54 54 00:02:10,640 --> 00:02:13,530 to think through when you're doing your troubleshooting too. 55 55 00:02:13,530 --> 00:02:15,330 These seven layers of the OSI model 56 56 00:02:15,330 --> 00:02:17,620 are useful when you're trying to troubleshoot a network, 57 57 00:02:17,620 --> 00:02:20,010 because if I have a problem and I can think about it 58 58 00:02:20,010 --> 00:02:21,830 from all the different seven layers, 59 59 00:02:21,830 --> 00:02:24,660 I can start to identify exactly where the problem is, 60 60 00:02:24,660 --> 00:02:27,310 and then I can troubleshoot it appropriately. 61 61 00:02:27,310 --> 00:02:28,790 This means the OSI model 62 62 00:02:28,790 --> 00:02:31,050 can also serve as a reference model. 63 63 00:02:31,050 --> 00:02:32,220 Now you may be wondering 64 64 00:02:32,220 --> 00:02:34,700 what's the exact purpose of a reference model? 65 65 00:02:34,700 --> 00:02:36,250 Well, a reference model is simply 66 66 00:02:36,250 --> 00:02:37,670 something that we use to categorize 67 67 00:02:37,670 --> 00:02:40,610 the functions of a network into a particular layer, 68 68 00:02:40,610 --> 00:02:43,000 and that's what we're going to use the OSI model for 69 69 00:02:43,000 --> 00:02:44,680 during our troubleshooting efforts. 70 70 00:02:44,680 --> 00:02:47,600 Now, when you start looking at the OSI model in depth, 71 71 00:02:47,600 --> 00:02:48,770 you're going to start to notice 72 72 00:02:48,770 --> 00:02:51,130 that the OSI model doesn't actually cleanly 73 73 00:02:51,130 --> 00:02:54,335 or easily or accurately refer to the way our modern networks 74 74 00:02:54,335 --> 00:02:56,520 actually operate these days. 75 75 00:02:56,520 --> 00:02:58,430 For example, some things are going to operate 76 76 00:02:58,430 --> 00:03:00,620 at multiple layers of the OSI model, 77 77 00:03:00,620 --> 00:03:03,650 especially when we're discussing the upper three layers. 78 78 00:03:03,650 --> 00:03:04,760 Now there's a good reason 79 79 00:03:04,760 --> 00:03:06,710 for why the OSI model isn't perfect 80 80 00:03:06,710 --> 00:03:08,880 for our way that networks operate today. 81 81 00:03:08,880 --> 00:03:11,470 And that's because our networks operate under a model 82 82 00:03:11,470 --> 00:03:13,650 known as the TCP/IP model, 83 83 00:03:13,650 --> 00:03:15,910 but the OSI model was designed 84 84 00:03:15,910 --> 00:03:18,260 to refer to how all networks operate, 85 85 00:03:18,260 --> 00:03:20,430 not just TCP/IP networks. 86 86 00:03:20,430 --> 00:03:22,420 Now I know that may be a little bit confusing, 87 87 00:03:22,420 --> 00:03:24,360 but don't worry too much about it yet. 88 88 00:03:24,360 --> 00:03:25,710 It's going to make a lot more sense 89 89 00:03:25,710 --> 00:03:29,270 as we go through the OSI model and the TCP/IP model. 90 90 00:03:29,270 --> 00:03:31,950 After we talk about all seven layers of the OSI model, 91 91 00:03:31,950 --> 00:03:35,570 we'll go back and compare the OSI model and the TCP/IP model 92 92 00:03:35,570 --> 00:03:37,110 in the next section of this course, 93 93 00:03:37,110 --> 00:03:38,550 and you'll see what I mean. 94 94 00:03:38,550 --> 00:03:40,840 But to keep things simple for right now, 95 95 00:03:40,840 --> 00:03:43,820 let's just remember that the OSI model is a reference model 96 96 00:03:43,820 --> 00:03:45,850 and we're going to use it in our network operations 97 97 00:03:45,850 --> 00:03:47,740 and troubleshooting because again, 98 98 00:03:47,740 --> 00:03:49,150 it's generic in nature 99 99 00:03:49,150 --> 00:03:50,930 and it's going to allow us to work for any 100 100 00:03:50,930 --> 00:03:52,980 and all networks we may have. 101 101 00:03:52,980 --> 00:03:56,730 Not just our most common networks, which are TCP/IP. 102 102 00:03:56,730 --> 00:03:59,120 Now, one of the benefits of using a reference model 103 103 00:03:59,120 --> 00:04:02,040 like the OSI model is that it can be used equivalently 104 104 00:04:02,040 --> 00:04:04,100 across lots of different technologies 105 105 00:04:04,100 --> 00:04:06,400 and a lot of different manufacturers as well. 106 106 00:04:06,400 --> 00:04:07,720 So if I'm going to look at a particular 107 107 00:04:07,720 --> 00:04:09,760 wireless network card from my computer, 108 108 00:04:09,760 --> 00:04:11,040 I can compare how it operates 109 109 00:04:11,040 --> 00:04:13,000 through the seven layers of the OSI model, 110 110 00:04:13,000 --> 00:04:15,560 and then compare it equivalently to a different 111 111 00:04:15,560 --> 00:04:17,310 manufacturer's card. 112 112 00:04:17,310 --> 00:04:18,840 When you can understand the functions 113 113 00:04:18,840 --> 00:04:21,000 at each and every layer that's being performed, 114 114 00:04:21,000 --> 00:04:22,420 it's going to help you better understand 115 115 00:04:22,420 --> 00:04:25,090 the flow of data in the network and through that card 116 116 00:04:25,090 --> 00:04:28,060 and how it communicates and how you can troubleshoot it. 117 117 00:04:28,060 --> 00:04:29,990 Now I've mentioned that there are seven layers 118 118 00:04:29,990 --> 00:04:32,820 of the OSI model, several times in this video already, 119 119 00:04:32,820 --> 00:04:35,200 but I still haven't even showed you what they are. 120 120 00:04:35,200 --> 00:04:38,920 So let's take a look at the seven layers of the OSI model. 121 121 00:04:38,920 --> 00:04:40,970 First, we start at the bottom layer 122 122 00:04:40,970 --> 00:04:42,440 working our way from layer one, 123 123 00:04:42,440 --> 00:04:45,610 all the way to the top layer, which is known as layer seven. 124 124 00:04:45,610 --> 00:04:48,540 These are the physical layer, the data link layer, 125 125 00:04:48,540 --> 00:04:51,840 the network layer, the transport layer, the session layer, 126 126 00:04:51,840 --> 00:04:54,780 the presentation layer and the application layer. 127 127 00:04:54,780 --> 00:04:57,100 For the exam, you need to be able to remember 128 128 00:04:57,100 --> 00:04:59,290 these seven layers in the proper order 129 129 00:04:59,290 --> 00:05:02,040 going from bottom to top and top to bottom. 130 130 00:05:02,040 --> 00:05:04,640 Now, to help you with that, I like to use a memory aid 131 131 00:05:04,640 --> 00:05:06,090 known as a pneumonic. 132 132 00:05:06,090 --> 00:05:07,810 In the case of the OSI model, 133 133 00:05:07,810 --> 00:05:10,190 I like to think about my favorite kind of pizza 134 134 00:05:10,190 --> 00:05:11,040 when I'm listing things 135 135 00:05:11,040 --> 00:05:13,150 from the bottom layer to the top layer. 136 136 00:05:13,150 --> 00:05:15,750 Now my favorite kind of pizza is sausage pizza. 137 137 00:05:15,750 --> 00:05:17,730 So if you want to remember these seven layers 138 138 00:05:17,730 --> 00:05:20,430 of the OSI model, just remember what I always say 139 139 00:05:20,430 --> 00:05:22,170 about sausage pizza. 140 140 00:05:22,170 --> 00:05:24,640 Please do not throw sausage pizza away. 141 141 00:05:24,640 --> 00:05:28,120 After all, I think sausage pizza is pretty tasty, right? 142 142 00:05:28,120 --> 00:05:30,580 And so you shouldn't throw it away and waste it. 143 143 00:05:30,580 --> 00:05:32,220 Now by using this sentence, 144 144 00:05:32,220 --> 00:05:35,200 you can remember the seven layers from bottom to top. 145 145 00:05:35,200 --> 00:05:37,020 Just take the letters from this sentence. 146 146 00:05:37,020 --> 00:05:42,020 Please do not throw sausage pizza away, or P D N T S P A, 147 147 00:05:42,200 --> 00:05:45,920 and replace it with the words, physical, data link, network, 148 148 00:05:45,920 --> 00:05:49,290 transport, session, presentation, and application. 149 149 00:05:49,290 --> 00:05:52,500 And there you have it, the seven layers of the OSI model. 150 150 00:05:52,500 --> 00:05:54,550 Well, at least that's how I remember the seven layers 151 151 00:05:54,550 --> 00:05:57,870 of the OSI model, but you can create your own pneumonic too. 152 152 00:05:57,870 --> 00:05:59,800 I've heard several interesting ones over the years 153 153 00:05:59,800 --> 00:06:00,860 from my students. 154 154 00:06:00,860 --> 00:06:03,440 Once I was teaching this class of Navy Sailors 155 155 00:06:03,440 --> 00:06:05,050 and they told me, they're saying was, 156 156 00:06:05,050 --> 00:06:07,460 please do not tell shore patrol anything. 157 157 00:06:07,460 --> 00:06:08,540 For those of you who don't know, 158 158 00:06:08,540 --> 00:06:11,410 shore patrol is basically like the military police officers. 159 159 00:06:11,410 --> 00:06:13,230 So I'm guessing they didn't want to get in trouble 160 160 00:06:13,230 --> 00:06:14,990 with their ships captain so they said, 161 161 00:06:14,990 --> 00:06:16,870 please don't tell shore patrol anything. 162 162 00:06:16,870 --> 00:06:19,330 Now, another one of my students told me his pneumonic was, 163 163 00:06:19,330 --> 00:06:22,070 please do not teach students pointless abbreviations, 164 164 00:06:22,070 --> 00:06:23,880 which is kind of funny since the abbreviation 165 165 00:06:23,880 --> 00:06:25,400 is what we're trying to remember here, 166 166 00:06:25,400 --> 00:06:27,030 to remember the seven layers. 167 167 00:06:27,030 --> 00:06:29,270 Whatever works for you is fine with me. 168 168 00:06:29,270 --> 00:06:31,950 Just remember that you need to memorize these seven layers 169 169 00:06:31,950 --> 00:06:34,500 before you take the exam, because on the exam, 170 170 00:06:34,500 --> 00:06:37,400 you're going to get questions about these seven layers. 171 171 00:06:37,400 --> 00:06:39,427 Now you might see some easy questions, like, 172 172 00:06:39,427 --> 00:06:41,800 "what is the seventh layer of the OSI model?" 173 173 00:06:41,800 --> 00:06:43,970 and then you'd pick out the word application. 174 174 00:06:43,970 --> 00:06:46,950 Or they might say, "what layer is the session layer at?" 175 175 00:06:46,950 --> 00:06:49,110 and you'll have to answer layer five. 176 176 00:06:49,110 --> 00:06:51,600 Now you might also get some more complex things 177 177 00:06:51,600 --> 00:06:53,110 asking you about a device 178 178 00:06:53,110 --> 00:06:55,720 and figuring out what layer that device operates in. 179 179 00:06:55,720 --> 00:06:58,250 For example, a router operates at layer three 180 180 00:06:58,250 --> 00:07:01,290 or the network layer, but a hub is a physical device 181 181 00:07:01,290 --> 00:07:04,290 and it operates at layer one, the physical layer. 182 182 00:07:04,290 --> 00:07:06,170 Because of this, we're going to cover each 183 183 00:07:06,170 --> 00:07:07,600 and every layer of the OSI model 184 184 00:07:07,600 --> 00:07:09,040 in this section of the course, 185 185 00:07:09,040 --> 00:07:11,010 so you can learn which devices are used there 186 186 00:07:11,010 --> 00:07:14,050 and which protocols and other information like that. 187 187 00:07:14,050 --> 00:07:16,100 All right, before we finish up this lesson, 188 188 00:07:16,100 --> 00:07:19,470 we need to talk about one more thing, and that's data, 189 189 00:07:19,470 --> 00:07:22,570 because networks are all about communicating data. 190 190 00:07:22,570 --> 00:07:24,210 Well, we're going to specifically talk about 191 191 00:07:24,210 --> 00:07:26,510 how data is going to be called different things 192 192 00:07:26,510 --> 00:07:27,910 as it flows through our network 193 193 00:07:27,910 --> 00:07:29,540 and as it goes through different names, 194 194 00:07:29,540 --> 00:07:32,460 as it goes through those different layers of the OSI model. 195 195 00:07:32,460 --> 00:07:34,170 Now, when we're talking about data, 196 196 00:07:34,170 --> 00:07:35,750 we're actually talking about information 197 197 00:07:35,750 --> 00:07:39,410 at layers five, six, and seven of the OSI model. 198 198 00:07:39,410 --> 00:07:41,360 Now, as it moves down through the layers, 199 199 00:07:41,360 --> 00:07:43,720 we're going to go from four to three to two to one, 200 200 00:07:43,720 --> 00:07:46,010 and we're going to change its name each time. 201 201 00:07:46,010 --> 00:07:48,350 When we're at layer four or the transport layer, 202 202 00:07:48,350 --> 00:07:50,690 we're going to call data a segment. 203 203 00:07:50,690 --> 00:07:52,800 When we go to layer three or the network layer, 204 204 00:07:52,800 --> 00:07:54,810 we refer to it as a packet. 205 205 00:07:54,810 --> 00:07:57,030 When we go into layer two or the data link layer, 206 206 00:07:57,030 --> 00:07:58,570 we call it a frame. 207 207 00:07:58,570 --> 00:08:00,610 And finally, when we're operating at layer one 208 208 00:08:00,610 --> 00:08:01,850 or the physical layer, 209 209 00:08:01,850 --> 00:08:04,620 this is where you've converted that data into ones and zeros 210 210 00:08:04,620 --> 00:08:08,000 to send it across our medium, we call this bits. 211 211 00:08:08,000 --> 00:08:10,130 Now this is something you need to memorize for the exam 212 212 00:08:10,130 --> 00:08:12,060 as well, but don't worry, 213 213 00:08:12,060 --> 00:08:14,470 I have another pneumonic to help you remember it. 214 214 00:08:14,470 --> 00:08:16,380 This one is a simple question. 215 215 00:08:16,380 --> 00:08:18,380 Do some people fear birthdays? 216 216 00:08:18,380 --> 00:08:19,740 Now, I think that's a valid question 217 217 00:08:19,740 --> 00:08:21,130 because as we get older, 218 218 00:08:21,130 --> 00:08:23,380 sometimes people start fearing their birthday 219 219 00:08:23,380 --> 00:08:25,630 because they don't want to age or appear older. 220 220 00:08:25,630 --> 00:08:27,120 Now, again, I know this is silly, 221 221 00:08:27,120 --> 00:08:28,580 but hopefully it helps you remember 222 222 00:08:28,580 --> 00:08:30,100 the four types of information 223 223 00:08:30,100 --> 00:08:32,440 as they flow through those lower layers. 224 224 00:08:32,440 --> 00:08:35,390 So if you can remember, do some people fear birthdays, 225 225 00:08:35,390 --> 00:08:38,980 that's going to give you D S P F B as your pneumonic. 226 226 00:08:38,980 --> 00:08:40,380 Now, if we add our data types, 227 227 00:08:40,380 --> 00:08:41,690 again, we always start with data 228 228 00:08:41,690 --> 00:08:43,180 and then we had the other four layers, 229 229 00:08:43,180 --> 00:08:47,580 we have data, segments, packets, frames, and bits. 230 230 00:08:47,580 --> 00:08:49,830 So for the exam, it's really important 231 231 00:08:49,830 --> 00:08:52,430 to understand the seven layers of the OSI model 232 232 00:08:52,430 --> 00:08:55,330 and how data moves up and down through the layers 233 233 00:08:55,330 --> 00:08:56,860 throughout the transmission process 234 234 00:08:56,860 --> 00:08:59,150 and what those data types are going to be called. 235 235 00:08:59,150 --> 00:09:00,700 So with that said, 236 236 00:09:00,700 --> 00:09:03,433 let's jump into our lessons on the OSI model.