-: In this lesson, we're going to talk about network topologies. 2 2
And when we talk about typologies for networks, 3 3
we're going to be talking about it in one of two ways. 4 4
First, we can talk about it physically. 5 5
How are these devices physically cabled 6 6
and connected together using various types of media. 7 7
And the second is how it is done logically. 8 8
We talk about logically, we're talking about how the traffic 9 9
is actually going to flow in that network. 10 10
So, here on the screen, you can see 11 11
I have a logical diagram of what a network looks like. 12 12
You could see where the workstations are, the routers, 13 13
the switches, all of that stuff, but this is not 14 14
how the network actually looks in the real world. 15 15
For example, that Windows 7 machine in the upper left 16 16
might be on the third floor to the building, 17 17
while the Windows 2012 machine on the upper right 18 18
may be all the way down in our basement. 19 19
You really don't know based on this diagram 20 20
where these things are physically, 21 21
because this is a logical topology, 22 22
I'm just concerned with the way the data flows, 23 23
not the way it's actually cabled. 24 24
Now, here we're going to learn how to read these diagrams 25 25
throughout the rest of this course and what each of these 26 26
icons mean, but right now that's not important. 27 27
I just want you to realize that there's a difference 28 28
between a logical and a physical topology 29 29
and that difference is one is focused on the logical 30 30
or network flow and the other is focused 31 31
on the physical layout of the cabling. 32 32
So, what are some of these options that we have 33 33
for different topologies? 34 34
Well, the first one we have is what's known 35 35
as a bus topology. 36 36
This is where you use a single cable that runs the length 37 37
of the entire area that needs network connectivity. 38 38
Each machine, whether 't's a laptop or a desktop 39 39
or a server, will then tap into that cable 40 40
using either a T-connector or a vampire tap. 41 41
Now, you may be wondering what's a vampire tap, 42 42
well, it was an old way of connecting networks. 43 43
Essentially, we would take a big thick metal cable 44 44
and run it down the room where all the computers 45 45
were to be connected and then each computer 46 46
would have a clamp that would bite into the cable 47 47
and it would actually make the connection to the network 48 48
that way and that's why we called it a vampire tap. 49 49
Now, this is a very old technology, it was used 50 50
back in the eighties and nineties, we don't use this today, 51 51
but, we still do have some bus typologies out there. 52 52
We don't use them very often, but they do still exist. 53 53
If you see a bus topology, all the devices on this cable 54 54
would form what's known as a single collision domain. 55 55
This means, as you can see here, there are six 56 56
different devices here trying to talk. 57 57
If they all try to talk at the same time, 58 58
you just have a collision, because they're all sharing 59 59
the same cable, so they'd have to instead take turns. 60 60
The next topology we're going to discuss 61 61
is known as a ring topology and this uses 62 62
a single cable like a bus does, but instead of it being 63 63
in a straight line, it's going to run in a complete circle. 64 64
Each device in the ring can then talk on that cable, 65 65
but again, they have to wait their turn 66 66
or you're going to have a collision. 67 67
Data would travel around this in a single way, 68 68
either clockwise or counterclockwise, 69 69
depending on the configuration of your network. 70 70
Now, we had these, because of that collision 71 71
we wanted to overcome, what we did was we had what's called 72 72
a token ring and a token was just 73 73
an electronic tag essentially, 74 74
that was passed around logically from computer to computer, 75 75
as they were going to talk. 76 76
This would allow them to take turns. 77 77
So, if you think back to when you're in elementary school 78 78
and everyone's sitting around a circle and there's 20 kids 79 79
and one teacher, the teacher may have had the talking stick 80 80
and they gave the stick to a child 81 81
and that child can then talk, then they would take 82 82
that stick away and give it to a different child 83 83
and then that child could talk. 84 84
And this way, everyone could hear what everybody else 85 85
was saying, without any collisions or anyone talking 86 86
over each other, that's the idea of a token ring network. 87 87
This eliminates those collisions that you had 88 88
back on the bus network. 89 89
Now, when you had these rings, they still were vulnerable 90 90
to being broken. 91 91
If you cut the cable, that would take down the network, 92 92
so there was no redundancy. 93 93
Now, there is something known as a FDDI ring, 94 94
which is a fiber distribution network. 95 95
A FDDI ring actually uses two rings one on top of the other, 96 96
one operates in a clockwise direction 97 97
and the other operates in a counterclockwise direction. 98 98
When you use a FDDI ring, you do get redundancy, 99 99
because if one of those rings is broken, 100 100
the other one would take over the network load. 101 101
Now, for the network plus exam, if you see the word ring, 102 102
I want you to think about a FDDI ring, 103 103
those fiber optic distribution rings, 104 104
because these days, that's the only type of ring topology 105 105
we really use in our networks. 106 106
For the exam, anytime they ask you a question 107 107
and ring is an option, I want you to think 108 108
ring equals redundancy. 109 109
Now, in the real world, if you had a standard token ring 110 110
with a single ring, there's no redundancy there, 111 111
but for the exam, I'm telling you, 112 112
they're talking about FDDI rings. 113 113
So, when you see ring, think about redundancy. 114 114
You might get a question that says, 115 115
which of these typologies is redundant 116 116
and they're going to give you options like star and bus 117 117
and ring and the answer is going to be ring. 118 118
All right, I think I stress that and repeat that enough, 119 119
you're going to remember, ring equals redundancy 120 120
for the exam now, right? 121 121
All right, let's move on to the next topology. 122 122
The next one we have is called a star topology. 123 123
You'll notice here, all of the outlying machines 124 124
are talking to this central point. 125 125
This central point is normally going to be something 126 126
like a switch. 127 127
Now, this switch is where all of the things 128 128
are going to connect back to and this happens 129 129
in most of our networks these days. 130 130
You can use it with fiber or copper or even wireless, 131 131
if you wanted to and this is going to be using a star topology. 132 132
We call it a star because everything is bursting out 133 133
like a star pattern. 134 134
But, the problem with this is that you have 135 135
this one central device and so if I have a switch here 136 136
in the middle and the switch fails because it loses power, 137 137
or we cut the cable or something like that, 138 138
the entire network is going to fail, 139 139
because it's a single point of failure. 140 140
So, remember with a star that you always have 141 141
this single point of failure, even though these are 142 142
very, very common to use and very inexpensive to use, 143 143
we do have this single point of failure 144 144
sitting right there in the middle. 145 145
Now, the next topology, we have tries to overcome that 146 146
and it's called a hub-and-spoke. 147 147
Now, a hub-and-spoke to topology is used 148 148
for connecting multiple sites together. 149 149
The reason we call it a hub-and-spoke, 150 150
is if you think about it, the way the airlines operate, 151 151
it looks like a hub-and-spoke. 152 152
These usually have hubs, like for instance where I live 153 153
in Baltimore, there's a hub for Southwest Airlines 154 154
and if I wanted to go to Connecticut and go to California, 155 155
you'd actually fly from Connecticut to Baltimore, 156 156
which is the hub and then go out over to California. 157 157
This allows them to centralize their operations 158 158
into hub cities. 159 159
There's usually each airline has two or three or four hubs 160 160
and all the flights go through those. 161 161
So, because you go to the hub first and then you go out 162 162
to the spoke cities, 163 163
this is the same concept with the network. 164 164
It's very similar to the star, but there's multiple 165 165
WAN links and multiple places that these hubs can exist. 166 166
So, it's almost like a hybrid of taking those stars 167 167
and connecting them together. 168 168
So, as you can see here on the screen, 169 169
I have Denver and Los Angeles as the hubs in my network 170 170
and everyone else is going to be a spoke. 171 171
So, if I wanted to go from Atlanta to Seattle, 172 172
I have to go from Atlanta to Denver and Denver to Seattle. 173 173
Now, if I wanted to go from Minneapolis to San Francisco, 174 174
I could go from Minneapolis to Denver and Denver to LA 175 175
and LA to San Francisco. 176 176
No matter which way I go, I have to go through either LA 177 177
or Denver, because those are the hub nodes. 178 178
It's not redundant fully, because if one of those 179 179
central offices like Denver or LA fails, 180 180
we're going to lose large portions of our network, 181 181
but it is better than a single star. 182 182
Now, because of this, losing half of our network 183 183
is a bad thing, right, this is a problem. 184 184
If we're looking for full redundancy, we would want to go 185 185
to what's called a full-mesh topology. 186 186
Now, full-mesh is awesome when you talk about redundancy. 187 187
Every single node or device is connected 188 188
to every other node or device. 189 189
This works really, really well if you have a small network, 190 190
two or three machines, but as I start increasing 191 191
the number of machines, it gets us get 192 192
pretty crazy pretty quickly. 193 193
Optimal routing is always available when you're dealing 194 194
with full-mesh, because every machine can go direct 195 195
with one jump over to the machine they want to get to. 196 196
So, in this example you see, 197 197
I have six machines on the network. 198 198
Now, every single one of them is tied to each other, 199 199
so, if I was doing this physically, I would need to have 200 200
five network cards for each of these machines 201 201
and five cables going from each of those machines. 202 202
That's a lot of technology that I have to add 203 203
to be able to connect these machines together, right? 204 204
And so, if I counted up the number of black lines there, 205 205
how many different connections are there? 206 206
How many lines would I actually have to run? 207 207
Well, there's going to be six machines times five cables each, 208 208
which is 30, divided by two, which is 15. 209 209
So, there's actually going to be, for six machines, 210 210
it's going to take me 15 cables. 211 211
Now, if I go to seven machines or 10 machines, 212 212
it gets pretty crazy and gets really expensive 213 213
really quickly and this is why 214 214
you're never really going to see a full-mesh 215 215
in a physical network. 216 216
Instead, you're usually going to see full-mesh 217 217
more as a logical method. 218 218
If you have something like nuclear command and control, 219 219
they may want to have full-mesh and they'll go through 220 220
the expense, because they have a zero defect 221 221
mentality there, but in your office and business networks, 222 222
you're not going to be seeing full-mesh, 223 223
it's just too expensive and too complicated. 224 224
Now, what might you see in your business networks? 225 225
Well, you might see a partial mesh network. 226 226
Now, a partial mesh network is hybrid of the full-mesh 227 227
and a hub-and-spoke. 228 228
Using this design, we can provide optimal routing 229 229
between some sites, but not all of the sites 230 230
and so to get this right, you have to do a good survey 231 231
and figure out where the busiest sites are 232 232
and where the slow sites are. 233 233
So, as you could see here on the diagram, 234 234
if you start drawing your finger around, 235 235
you can get to everywhere else, at least going through 236 236
one or two different ways. 237 237
Sometimes you're going to have to go through 238 238
one of those central sites and if one of those central sites 239 239
goes down, you can still get to everywhere else, 240 240
because we have the right partial mesh set up. 241 241
So, this partial mesh works really well 242 242
and gives you that additional redundancy you're seeking. 243 243
Now, to get it right though, you do have to understand 244 244
your traffic patterns and know where to put those main hubs 245 245
and it basically then becomes a modified hub-and-spoke, 246 246
where we have more hubs than we did before 247 247
and this allows us to have a better redundancy 248 248
than a single hub-and-spoke configuration.