-: Wireless considerations. 2 2
In this video, we're going to discuss wireless considerations 3 3
that you need to think about 4 4
as you start to troubleshoot your wireless networks. 5 5
We're going to begin by discussing antennas, 6 6
their placement, their type, and their polarization. 7 7
Then, we're going to cover channel utilization, 8 8
site surveys and the types of information 9 9
that those things can help us with 10 10
as we troubleshoot our networks. 11 11
Finally, we're going to talk about wireless access points 12 12
or AP association times, and how association actually works. 13 13
First, let's talk all about antennas. 14 14
Antennas can come in multiple different types, 15 15
and each one has its own purpose. 16 16
By default, most wireless access points 17 17
are going to use omnidirectional antennas. 18 18
And normally they're going to be located 19 19
in a vertical form factor. 20 20
Now for a vertical antenna, 21 21
the radiation pattern for the radio frequency waves 22 22
is going to extend outward in all directions, 23 23
away from that antenna and away 24 24
from the wireless access point at an equal power level. 25 25
As the radio waves travel further and further 26 26
from that antenna, 27 27
the signal is going to get lost a little bit 28 28
and the power and strength is going to decrease. 29 29
Now, if you're monitoring this from your client, 30 30
you're going to see that you have more negative RSSIs 31 31
as you go further from the wireless access point 32 32
and it's antenna. 33 33
Now, for example, 34 34
let's say you're sitting right next to the antenna. 35 35
You might have an RSSI of negative 30 to negative 40 36 36
decibels, but if you move out a hundred feet away, 37 37
that signal strength may now be at an RSSI 38 38
of negative 65 or negative 70 decibels. 39 39
Another type of antenna you might come across 40 40
is known as a dipole antenna or a bi-directional antenna. 41 41
With a dipole antenna, 42 42
the antenna is going to produce a radio-frequency wave 43 43
that extends outward in two directions, 44 44
away from the antenna. 45 45
With a dipole antenna, 46 46
you're normally going to see a higher RSSI 47 47
in the client device, 48 48
even when it's located further from the access point, 49 49
because this antenna can now focus its power 50 50
in only two directions instead of the entire 360 degrees 51 51
around itself, like an omnidirectional antenna produces. 52 52
So if you need the radio frequency waves 53 53
to only go in two directions, 54 54
a dipole is going to be a good option for you, 55 55
but they're not heavily used in wireless networks 56 56
for the most part. 57 57
Next, we have this thing known as a Yagi antenna. 58 58
Now a Yagi antenna is the type of unidirectional antenna 59 59
that sends the radio frequency waves only in one direction. 60 60
Due to this focusing of the radio frequency 61 61
in a singular direction, 62 62
a Yagi antenna can send the radio frequency waves 63 63
further in a single direction using less power 64 64
than an omnidirectional antenna will. 65 65
Normally you're going to see a Yagi antenna used 66 66
when you're trying to connect to different sites 67 67
using a wireless link. 68 68
For example, 69 69
you may want to connect to office buildings 70 70
on a college campus using a wireless link 71 71
instead of running a buried fiber optic cable. 72 72
In this case, 73 73
you're going to use a Yagi antenna to do that. 74 74
Finally, we have a parabolic grid or disc antenna. 75 75
These antennas are unidirectional antennas, 76 76
just like a Yagi antenna is, 77 77
but they're built a little bit differently. 78 78
Now, parabolic grid antennas are most commonly used 79 79
for site-to-site applications 80 80
where you need to connect buildings 81 81
over a longer distance than you would with a Yagi antenna. 82 82
Like a Yagi antenna, 83 83
a parabolic antenna is going to allow the radio wave 84 84
to be transmitted in only one direction, 85 85
making it a unidirectional antenna. 86 86
Now, when it comes to placing your antennas, 87 87
this is going to be dependent 88 88
on where you're going to place them 89 89
inside or outside of your building 90 90
to create the coverage that you desire. 91 91
If you're setting up a site-to-site 92 92
or building-to-building connection, 93 93
you're going to want to use a unidirectional antenna 94 94
like a parabolic or Yagi antenna. 95 95
Now you're going to take that and mount it 96 96
on the outside of your building. 97 97
And usually you're going to attach it to the roof 98 98
and make sure there's a clear line of sight 99 99
between both antennas on each of those buildings. 100 100
Now, one of the common issues you're going to experience 101 101
with wireless antennas for a site-to-site or 102 102
building-to-building connection is that the connection 103 103
may deteriorate and slow down or completely stop over time. 104 104
If this occurs, 105 105
you need to ensure that both the antennas 106 106
still have a clear sight between them. 107 107
For example, maybe you have two office buildings 108 108
and they are only one or two stories high. 109 109
You've gone ahead and installed the antennas in the fall. 110 110
And there was no issues at all. 111 111
You had a perfectly clear line of sight, 112 112
but it's been about six months, 113 113
and now the seasons have changed and spring appears 114 114
and your wireless connection 115 115
may start to be affected by trees 116 116
because they've sprouted new leaves 117 117
and they are now blocking your line of sight clearly 118 118
between those two buildings. 119 119
Similarly, if in the winter time there's a lot of snow, 120 120
this can also block your signal, 121 121
or if it rains heavily in the summer, 122 122
you could also have a deteriorating signal 123 123
because the water droplets will block 124 124
the wireless communication between these two antennas. 125 125
So if you're working with antennas, 126 126
you need to think about these things 127 127
when you're working outside. 128 128
Now, on the other hand, 129 129
if you're working inside your building, 130 130
you're going to be more likely to use omni-directional 131 131
or unidirectional patch antennas to control the signal 132 132
being radiated by your wireless networks, 133 133
to each of your clients. 134 134
With omnidirectional antennas, 135 135
it's going to be common to place this on the ceiling 136 136
in the middle of the office or classroom. 137 137
For a unidirectional patch antenna, 138 138
you're usually going to place this 139 139
on the outer wall of the building facing inward, 140 140
thereby directing all those radiofrequency waves 141 141
back inward towards the office or the classrooms. 142 142
Now, the last thing we need to discuss with antennas 143 143
is the concept of polarization. 144 144
Polarization is the orientation of the electric field 145 145
or transmissions that are occurring from that antenna. 146 146
Basically, as the radio frequency waves leave the antenna, 147 147
how is it going to be oriented? 148 148
Every antenna has a polarization associated with it. 149 149
And for wifi networks, 150 150
our intent is usually you're going to have 151 151
a vertical or horizontal polarization associated with them. 152 152
Now, most wifi clients that have external antennas 153 153
are going to use vertical polarization, 154 154
but your wireless access point 155 155
could use vertical or horizontal polarization 156 156
depending on what type of antenna it has. 157 157
Now, if you see a problem with some clients 158 158
getting a poor RSSI, 159 159
even though they're pretty close to your access point, 160 160
this may indicate you have a polarization issue. 161 161
If this is the case, 162 162
I want you to try flipping 163 163
the wireless access point antenna 164 164
to allow the clients to better connect 165 165
to that device if they support bending the antenna upwards, 166 166
or if you're using wireless access points 167 167
that are sitting on the floor, 168 168
you want to make sure you're using vertical polarization 169 169
and radiating antennas to increase your coverage area. 170 170
If you're using horizontal, 171 171
it's going to keep sending those radio waves 172 172
out horizontally 173 173
right along the floor and up to the desk level 174 174
where everybody else is working. 175 175
Remember, most wifi networks use vertical polarization. 176 176
So your intentions should be sticking upward 177 177
if you're using an omnidirectional antenna 178 178
with your wifi networks. 179 179
All right, next, we have channel utilization. 180 180
Channel utilization is a statistic or measure of the amount 181 181
of airtime utilization that occurs 182 182
for a particular frequency or channel. 183 183
If there's a higher rate of channel utilization, 184 184
then that means there's more traffic being transmitted 185 185
over that particular frequency. 186 186
In order to have a fast wireless network, 187 187
you want to keep your channel utilization under around 30%. 188 188
Now, channel utilization is not something 189 189
you alone can control though, 190 190
because there's lots of these different channels 191 191
that overlap with other people in that area. 192 192
And so the same channels and frequencies 193 193
can be used by your network and other networks too. 194 194
For example, let's say you're running a wireless B 195 195
or wireless G or wireless N network, 196 196
and you're operating in the 2.4 gigahertz spectrum. 197 197
We are going to be using the same channels 198 198
that everybody else is, channels one, six, and 11. 199 199
Now, if you have a large office building, 200 200
this won't be a big problem for you 201 201
because nobody else is going to be 202 202
in the same general area as we are. 203 203
But if you're in a crowded office building 204 204
or a shopping mall or an apartment building, 205 205
this can easily start to have a lot of 206 206
different wireless networks operating 207 207
on the same channels as ours in the same location. 208 208
Now, when wireless access points and wireless clients 209 209
are operating on the same channel 210 210
and all of those things are in 211 211
the same range of each other, 212 212
they're going to begin to form a single broadcast domain, 213 213
similar to an ethernet hub. 214 214
All of the devices can hear each other's transmissions. 215 215
And if any two devices transmit at the same time, 216 216
their radio signals will collide and it becomes garbled, 217 217
which results in data corruption or a complete frame loss. 218 218
If there's an excessive amount of collisions, 219 219
data will never be retransmitted successfully, 220 220
and the wireless network could become unusable. 221 221
So to avoid collisions 802.11 wireless devices use a 222 222
listen before they speak approach 223 223
when they're accessing the wireless medium, 224 224
which is the radio frequency, 225 225
following the CSMA CA or collision avoidance techniques. 226 226
Now with this, devices are going to perform 227 227
a clear channel assessment or CCA 228 228
by first listening to see if another device 229 229
is actively transmitting on the channel 230 230
before they attempt to send their own frames 231 231
on that given frequency. 232 232
Now, when a device detects another transmission in progress, 233 233
it's going to perform a random back-off 234 234
for a short period of time after which 235 235
it's going to perform another CCA check 236 236
before it attempts to transmit. 237 237
If the channel is clear after the check, 238 238
that device will be able to access the channel 239 239
and send some data. 240 240
Now, as the number of devices needing to transmit frames 241 241
starts increasing on a channel, 242 242
this is going to cause congestion to occur 243 243
to the point where devices can spend 244 244
a lot more time waiting to be able to send something. 245 245
This results in slower speeds, 246 246
because devices have to wait longer 247 247
before they can send their data. 248 248
This is the negative effect 249 249
of having high channel utilization 250 250
and at least a slower throughput 251 251
for our network devices. 252 252
Now, how do we solve this problem 253 253
of over channel utilization? 254 254
Well, first we're going to start 255 255
by conducting a site survey. 256 256
Now a site survey also known as a wireless survey 257 257
is the process of planning and designing 258 258
a wireless network to provide a wireless solution 259 259
that will deliver the required wireless coverage, 260 260
the data rates, network capacity, roaming capability, 261 261
and quality of service that your organization desires. 262 262
As part of the wireless survey, 263 263
you need to determine where each access point is located, 264 264
what power level it's using to transmit 265 265
by using its effective isotropic radiated power, 266 266
or EIRP, the overlapping coverage areas 267 267
for your wireless access points, 268 268
and the other wireless access points in clients 269 269
that may be operating in the same or general area 270 270
or channels as you. 271 271
Now by using this site survey information, 272 272
you can determine if you need to change the channels 273 273
you're using to a less busy, less utilized channel, 274 274
or if you may need to upgrade your networks 275 275
to a new frequency band altogether. 276 276
After all, if you decide to upgrade 277 277
to a wireless N, wireless AC or wireless AX network, 278 278
and you start using the five gigahertz spectrum, 279 279
this gives you 24 non-overlapping channels 280 280
instead of the three in the 2.4 gigahertz spectrum. 281 281
So there's going to be a lot less overall utilization 282 282
on each of those 24 channels in the five gigahertz spectrum. 283 283
By conducting a site survey, 284 284
you can better understand the wireless environment 285 285
in and around your networks. 286 286
And this is going to allow you to configure your devices, 287 287
to use the less utilized channels, 288 288
ensure there's proper coverage for all of your work areas 289 289
and ensure your wireless network is not being blocked 290 290
or interfered with by physical obstacles 291 291
within the building. 292 292
Finally, we need to talk about 293 293
wireless access point association times. 294 294
Now when a wireless client attempts to connect 295 295
to a wireless network, it goes through a seven step process. 296 296
First, the wireless client is going to send 297 297
a probe request to discover any 802.11 wireless networks 298 298
in the general location that it's in. 299 299
Essentially, it's going to send a broadcast message 300 300
to the BSS ID of F F F F F F F F F F F F. 301 301
And all the access points in that area 302 302
that are using the same frequency, 303 303
either 2.4 gigahertz or five gigahertz 304 304
will respond to that client. 305 305
Second, any access points that receive this probe 306 306
will check to see if they can support the data rate 307 307
that the client is now requesting in that probe. 308 308
For example, if I'm a wireless AC access point 309 309
and you just requested to communicate using wireless B, 310 310
I'm going to ignore your request 311 311
because I'm operating at five gigahertz 312 312
and you're operating at 2.4 gigahertz. 313 313
But if we both support wireless N 314 314
then I can respond with a probe response 315 315
that provides my SS ID, my supported data rates, 316 316
my encryption type, if I'm using wireless security, 317 317
and other capabilities of my particular access point. 318 318
Third, our wireless client 319 319
will then send a low level 802.11 authentication frame 320 320
to the access point, 321 321
and this will begin the authentication handshake 322 322
between my client and the access point. 323 323
Fourth, the access point receives 324 324
this authentication frame and responds 325 325
with an acknowledgement to continue the handshake. 326 326
If the wireless access point receives anything other 327 327
than an authentication or a probe request 328 328
from this client, though, 329 329
it's going to send a de-authentication frame 330 330
and break the communication with this client 331 331
because this client is not yet part 332 332
of my authenticated network. 333 333
Fifth, the wireless client is going to choose 334 334
the access point it wants to associate with 335 335
and sends an association request 336 336
using the encryption and 802.11 capabilities 337 337
that are supported by that access point. 338 338
Sixth, the access point is going to process 339 339
the association request, 340 340
if the information sent matches its own capabilities, 341 341
and then an association ID is being created 342 342
for this wireless client and this access point, 343 343
and the access point is going to respond 344 344
with an association response of success. 345 345
This way, the client knows it is now fully connected 346 346
to this wireless access point. 347 347
Seventh, now that the client is fully connected 348 348
and associated, it can begin to connect any data transfer 349 349
it needs to, and utilize the wireless network 350 350
and its associated devices. 351 351
Now, that's how everything is supposed to operate, 352 352
but sometimes you're going to see a wireless network 353 353
that has a really long association time 354 354
for an end user's client. 355 355
This is because the client has to scan the airwaves, 356 356
find the access points it might want to connect to, 357 357
request the association, 358 358
authenticate to the access point, 359 359
and then contact the DHCP server to get an IP address, 360 360
prior to being able to actually use that wireless network 361 361
connection that we just established. 362 362
This process can take just a few seconds 363 363
in a not heavily loaded network, 364 364
but it can take up to 30 to 60 seconds 365 365
if you have a busier network. 366 366
To speed up the association process, 367 367
clients should be located in a high signal strength area. 368 368
Those with the highest RSSI during the association process 369 369
are going to give us a stronger strength 370 370
and that's going to drastically reduce the wait time 371 371
during this association.