-: If we start on the bottom of the OSI Model, 2 2
we'll find our first layer, the physical layer. 3 3
Now, this is where bits are transmitted across the network 4 4
and includes all of the physical 5 5
and electrical characteristics of this network. 6 6
So, this is going to tell us 7 7
whether we're using an Ethernet network, 8 8
whether we're using fiber or copper cables, 9 9
whether we're using Cat5 or Cat6, 10 10
and even if we're using radio frequency 11 11
in the case of Wi-Fi. 12 12
Regardless of which method we're using 13 13
to send our data across this first layer, 14 14
it's always going to occur as bits, 15 15
and these are binary bits. 16 16
These are going to be a series of ones and zeros. 17 17
Now, each media has a different way 18 18
of representing these bits, these series of ones and zeros, 19 19
because these series of ones and zeros 20 20
are the basic building blocks of all of our data. 21 21
For example, if I'm using a copper wire, 22 22
such as in a Cat5 or a Cat6 network, 23 23
you may see that there's zero voltage on the wire 24 24
when you have a zero bit, 25 25
and then if you want to represent a one, 26 26
you might use plus 5 or negative five volts 27 27
on that copper wire. 28 28
Now, when we switch between these two modes, 29 29
this tells us whether we should read a one 30 30
or a zero on the network, 31 31
and this is called transition modulation. 32 32
Now, for the exam, you don't need to understand 33 33
the specifics of transition modulation. 34 34
But you should understand this basic concept 35 35
that on this wire, 36 36
we're going to have one level that represents a one 37 37
and another level that represents a zero. 38 38
Let me give you another example. 39 39
This time, let's pretend we're using a fiber optic cable. 40 40
Now, with fiber optic cables, 41 41
we're going to use light instead of voltages. 42 42
Now, it's similar to the way we did voltages, 43 43
but instead, when we want to represent a one, 44 44
we turn the light on. 45 45
If we want to represent a zero, we turn the light off. 46 46
Now, we can just read the state of light. 47 47
Is it on? 48 48
That's a one. 49 49
Is it off? 50 50
That's a zero. 51 51
And when there's a transition between, 52 52
that tells us between these two modes 53 53
whether we should be reading this as a one or zero. 54 54
Now, as we start understanding that, 55 55
we then have to look at the cables themself 56 56
because this is also part of our physical layer. 57 57
If we're using something like a Cat5 or a Cat6 cable, 58 58
we may have a certain connector on the end 59 59
called an RJ45, which allows us to plug that cable 60 60
into the back of a computer or into a switch. 61 61
Now, the way that connector is wired 62 62
is based on a certain standard. 63 63
We use two standards inside our network: 64 64
TIA/EIA-568A and TIA/EIA-568B. 65 65
Now, we'll talk about these 66 66
and which way these pins actually are set up 67 67
inside, this connector in a future lesson. 68 68
This is going to be important 69 69
because as we start talking about these, 70 70
this is going to tell us whether or not we're using 71 71
crossover cables or straight-thru cables. 72 72
If we use a crossover cable, 73 73
we're actually going to flip the transmission 74 74
and receive bits on the end of the cable. 75 75
So, one end will be the A standard 76 76
and one end will be the B standard. 77 77
But if we're using a straight-thru cable or a patch cable, 78 78
we're going to have the B standard on both sides. 79 79
Now, again, it's important to understand 80 80
these wiring standards, 81 81
so, we're going to spend some time on them in a future lesson 82 82
because on the exam, you may be asked 83 83
to wire up an Ethernet jack. 84 84
Maybe they're going to tell you to make a crossover cable 85 85
and you have to drag the right colors to the right pins. 86 86
That kind of a thing. 87 87
And so, to make sure you're ready for that, 88 88
we're going to cover that in a separate lesson. 89 89
Now, at this point, we've talked about having our cables, 90 90
we've talked about how we represent bits on those cables, 91 91
and we talked about how we're going to set up 92 92
the connectors of those cables. 93 93
But there's another thing we have to think about 94 94
at the physical layer, 95 95
and that's the topology of the network. 96 96
How are we actually running these cables 97 97
to physically connect the different devices together? 98 98
Well, we look at this from a Layer 1 perspective, 99 99
we can look at this based on the things we talked about 100 100
in the last section of the course. 101 101
Is it a bus, is it a ring, is it a star? 102 102
Is it a hub and spoke? 103 103
How about a full mesh, a partial mesh, 104 104
or any other topology that we discussed. 105 105
When it comes to figuring this out, 106 106
you're going to look at how they're physically cabled, 107 107
and if you drew them out, 108 108
does it make a line like a bus, 109 109
a ring, going in a circle, 110 110
or a star pattern? 111 111
And that will tell you what physical topology you have. 112 112
This, again, is a Layer 1 issue. 113 113
Another issue that we have to concern ourself with 114 114
at Layer 1 is synchronizing our communications. 115 115
We have to ask ourself how does the receiving end know 116 116
if it's ready to accept ones and zeros 117 117
that we're going to send it? 118 118
Now, this sounds like a really easy thing to do 119 119
if we're talking to each other, 120 120
but with computers, this can get much more complicated. 121 121
So, to make sure that we understand this, 122 122
we have two things that can happen. 123 123
It can either be transmitted asynchronously 124 124
or synchronously. 125 125
Now, when I'm looking at asynchronous communication, 126 126
you should be able to consider something like a voice mail. 127 127
You call up your friend, they don't answer, 128 128
and so, you leave a message so they can listen to it later. 129 129
The communication happens out of sync or out of time. 130 130
You do it and then later on, 131 131
they can go back and listen to it. 132 132
Now, in networks, this happens via a start and stop bit. 133 133
Similarly to how your friend can press Play 134 134
on their voicemail system to listen to their message, 135 135
a network can send a start bit 136 136
when it wants to start beginning the transmission 137 137
and then a stop bit to tell the other side, 138 138
hey, I'm done transmitting, 139 139
you've gotten everything I'm going to send. 140 140
Now, if we decide to go 141 141
and do communications synchronously, on the other hand, 142 142
we have to be in the same place at the same time. 143 143
So, in our previous example, 144 144
if your friend picked up the phone 145 145
and you had a conversation, 146 146
you could talk to them and they could talk to you, 147 147
this is a synchronized conversation 148 148
because you're both talking at the same time. 149 149
Now, this communication happens in real time. 150 150
That's what's great about synchronization. 151 151
Now, as far as when we start talking about this 152 152
from a network perspective, 153 153
instead of using a start and a stop bit for synchronizing, 154 154
we would use some sort of common time source. 155 155
And so, maybe we're all going to use a clock, 156 156
and every time a second passes, 157 157
we can transmit and receive, 158 158
and that tells us that we're going to do it 159 159
on the cadence of the bit. 160 160
That would be something that is synchronous. 161 161
Now, in addition to figuring out 162 162
if it's going to be asynchronous or synchronous, 163 163
you also have to figure out how you're going to utilize 164 164
the bandwidth of the cable, 165 165
and there's two main ways that you can do this. 166 166
One is called broadband and one is called baseband. 167 167
Now, broadband is going to divide our bandwidth 168 168
into separate channels. 169 169
If you have a TV service at your house, 170 170
you're probably familiar with this 171 171
because you have a single cable coming into your house, 172 172
but it carries 200 or more channels. 173 173
The user, then, is going to choose a single channel 174 174
and the rest are going to be filtered out. 175 175
In opposition to this, we have baseband, 176 176
where you're going to use all of the frequency of the cable 177 177
all of the time. 178 178
So, a telephone, for instance, uses baseband communication, 179 179
which means that when you pick up the phone, 180 180
you're using all of the bandwidth 181 181
allocated to that phone line. 182 182
This doesn't hold true with the cable TV signal, right? 183 183
Because we had 200 channels 184 184
all using some of that bandwidth, 185 185
but we only pulled out the ones we wanted. 186 186
For this reason, 187 187
we can only make one call at a time when using a phone, 188 188
but we can have 200 channels or more on our TV. 189 189
Now, when we use baseband, 190 190
we're going to use a reference clock 191 191
that allows us to send the information 192 192
for both the sender and the receiver at the certain time. 193 193
By using this reference clock, 194 194
this is an example of using a synchronous communication. 195 195
Another good example of a baseband network 196 196
is a wired home Ethernet network 197 197
because this is going to use all of the frequency 198 198
that's available on your cable, giving you more bandwidth 199 199
than you would if you had a broadband area. 200 200
Now, in this case, if we have a single baseband 201 201
using up all of the bandwidth, 202 202
we need to figure out how to get more out of it. 203 203
And so, to do this, 204 204
we have a couple of different mechanisms we can do. 205 205
And the first one of these 206 206
is what's known as time-division multiplexing. 207 207
In this mode, each session is going to take turns 208 208
using a dedicated time slot 209 209
to get part of that bandwidth from the baseband. 210 210
Now, an easy analogy of this 211 211
is if you have a house with a single TV in it 212 212
but you have four family members. 213 213
Everyone wants to watch TV, but there's only one TV, 214 214
so they're going to have to take turns 215 215
picking what program is going to be on. 216 216
Now, in a pure time-division multiplexing environment, 217 217
each person is going to be assigned a time slot 218 218
and they can pick whatever TV show they want 219 219
during that time slot. 220 220
Now, this may or may not line up 221 221
with the time that their show is actually on 222 222
and that could cause a problem, right? 223 223
So, we have the second method 224 224
called statistical time-division multiplexing or StatTDM. 225 225
This is a more efficient version 226 226
of time-division multiplexing 227 227
because it's going to dynamically allocate these time slots 228 228
based on when people need it. 229 229
So, if we take our TV example, 230 230
maybe, for example, I want to go down 231 231
and nobody is watching TV at eight o'clock, 232 232
but it wasn't my time slot. 233 233
Well, under time-division multiplexing, 234 234
I couldn't turn on the TV 235 235
because it wasn't my time slot. 236 236
But with StatTDM, I can, 237 237
because as long as nobody is using the TV, 238 238
anyone is free to use it. 239 239
Everyone is going to take turns based on their necessity, 240 240
not based on the time itself. 241 241
So, instead, when I start watching the TV at eighth o'clock, 242 242
then after my 30 minutes is up, 243 243
I'm going to get off at 8:30 so somebody else can get on 244 244
even though my assigned time slot under TDM 245 245
might have been something like nine to 9:30. 246 246
Now, our last method is what's known 247 247
as frequency-division multiplexing or FDM. 248 248
This is going to involve taking the medium, that cable, 249 249
and splitting it up into channels 250 250
similar to the way we do in broadband. 251 251
So, if I take a single cable and I break it up 252 252
into 50, 100, or 200 different frequencies, 253 253
then each person can get a small portion 254 254
of frequency allotted to them 255 255
and they can use it as much as they want. 256 256
Now, for the exam, the good news 257 257
is you don't need to memorize TDM, StatTDM, and FDM, 258 258
but rather, you just need to understand that multiplexing 259 259
involves taking some limited amount of resource 260 260
and using it more efficiently. 261 261
In the real world, you may come across 262 262
these multiplexing techniques, 263 263
especially if you start working 264 264
as a network engineer or a network architect, 265 265
and that's why I want to introduce them to you. 266 266
But for the Network+ Exam, 267 267
just remember, multiplexing allows multiple people 268 268
to use a baseband connection at the same time. 269 269
The final thing we need to talk about in this lesson 270 270
is some examples of physical or Layer 1 devices. 271 271
The most common one is going to be a cable. 272 272
So, if I have a fiber optic cable 273 273
or an Ethernet cable or a coaxial cable, 274 274
these are all different types of media. 275 275
And if I have different types of media, 276 276
that's considered a Layer 1 device. 277 277
The reason is whatever goes in one end of the cable 278 278
is going to come out the other end of the cable. 279 279
So, if I have a fiber optic cable 280 280
and I put light in one end, 281 281
I'm going to get light out the other end. 282 282
That's a physical response, 283 283
a physical layer of the OSI Model. 284 284
Additionally, beyond wired cables, 285 285
we also have wireless things, 286 286
things like Bluetooth and Wi-Fi 287 287
and near field communication. 288 288
All of these radio frequencies make up the media 289 289
at Layer 1 for those type of networks. 290 290
The final example is infrastructure devices, 291 291
and that will be things like hubs, access points, 292 292
and media converters. 293 293
All of these devices operate at the bit layer. 294 294
This is going to be a function 295 295
to just simply repeat what they get. 296 296
So, if I have a hub, whatever goes in port one of the hub 297 297
is coming out of ports two, three, and four. 298 298
Whatever comes in gets repeated out. 299 299
The same thing with the media converter. 300 300
If I have something coming in over coaxial, 301 301
it's going to get converted through media 302 302
and pushed out over fiber optic. 303 303
That device is simply doing it at the physical layer. 304 304
Whatever comes in is going to go out. 305 305
There's no logic to it, there's no intelligence to it. 306 306
Layer 1 devices simply repeat whatever they're told. 307 307
Now, we'll talk about some other infrastructure devices 308 308
as we get into Layers 2 and 3 and 4 309 309
but for right now, I want you to remember 310 310
Layer 1 is dumb devices. 311 311
They're simply repeaters. 312 312
Whatever they take in, they send it right back out.