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