-: Coverage and interference. 2 2
In this lesson, we're going to discuss 3 3
coverage and interference in wireless networks. 4 4
These are two of the most common issues 5 5
we're going to come across when we start troubleshooting 6 6
our wireless network connections in the field. 7 7
First, let's talk about coverage. 8 8
Coverage is a measure of how large of an area 9 9
around a wireless transmitter 10 10
there is sufficient signal strength 11 11
for a wireless device to utilize. 12 12
Basically, it comes down to how much physical area 13 13
you can use with your wireless devices 14 14
before you get outside the coverage area 15 15
and you lose signal. 16 16
To determine your coverage area, 17 17
you're going to conduct a wireless site survey. 18 18
This'll generate a heat map showing the areas 19 19
with the highest level signals in green, 20 20
then as they get weaker, it goes to yellow, 21 21
orange and red with the weakest signals. 22 22
Now, the signal is always getting measured from your client, 23 23
using the RSSI. 24 24
And this is measured in decibels. 25 25
If you're going to measure it from your access point, 26 26
you're going to measure it in EIRP 27 27
and you're going to do that in dBI. 28 28
Now, one of the most common issues experienced 29 29
by Wi-Fi users is insufficient wireless coverage. 30 30
For example, let's say you have a three-story townhouse 31 31
and you only have a single wireless access point 32 32
on the main floor of that home. 33 33
Well, it's going to be unlikely to provide you 34 34
with sufficient coverage all the way up 35 35
on the third story of your townhouse. 36 36
In this case, if you're using your laptop 37 37
on the first floor, everything's going to work just fine 38 38
because you have really high RSSI value. 39 39
But when you go up to the second floor, 40 40
that RSSI value is going to lower 41 41
because the signal got weaker as you got further away, 42 42
and the signal has to now go through the ceiling 43 43
of the first floor to get into 44 44
that second floor you're sitting at. 45 45
Now, if you go up another level to the third floor, 46 46
that signal may be too weak to even register 47 47
by your wireless client, or it can drop you completely. 48 48
Now, if this happens, you're going to need to find a way 49 49
to boost up that signal and make it stronger 50 50
so you can increase your coverage area. 51 51
To do this, you can use a signal booster to raise the power, 52 52
you can use an antenna with a stronger DB rating, 53 53
you can use a wireless repeater, 54 54
or you can add a second wireless access point 55 55
that can operate in an extended server set 56 56
or ESS configuration with your initial access point. 57 57
Remember, the coverage area for a single 58 58
wireless access point is affected by two main factors, 59 59
the amount of power that the transmitter is sending out, 60 60
and the size of your antenna. 61 61
If you replace a five decibel antenna 62 62
with a nine decibel antenna, 63 63
you're going to effectively double your range 64 64
under the right conditions. 65 65
As for the power being sent out 66 66
unfortunately, you're normally going to be limited 67 67
in your ability to control that 68 68
because there's a maximum amount of power 69 69
that the FCC and regulators allow us to use 70 70
inside of the wireless frequency range, 71 71
but you could use a wireless repeater. 72 72
Now, a wireless repeater is simply a layer one device 73 73
that has two radios built into it. 74 74
You plug this device into a power outlet 75 75
and you configure it to connect to your wireless network. 76 76
Now, one of the radios grabs the signal 77 77
from your wireless network. 78 78
Then it's going to boost up the signal 79 79
and send it out again off its other radio. 80 80
This way, when the signal comes in 81 81
on one side of the device, it's going to be repeated out 82 82
at full strength on the other radio too. 83 83
This will extend that wireless network 84 84
into an additional area of space 85 85
because you have this new signal going out 86 86
from this repeater. 87 87
For example, let's say if this repeater 88 88
receives a signal at -70 RSSI. 89 89
It can retransmit it to get it out on the other side 90 90
at -30 RSSI. 91 91
Effectively making it a brand new signal. 92 92
So, in the example of a three-story townhouse, 93 93
I might want to have my first wireless access point 94 94
on the first floor. 95 95
Then I can have a repeater on the second floor. 96 96
Now, at this point, 97 97
we'll probably have some signal on the third floor, 98 98
or we could put another repeater on the third floor 99 99
to boost that signal even more 100 100
when it reaches it from the second floor. 101 101
Finally, we could use an extended server set 102 102
to cover the entire building properly. 103 103
In one of my old houses, 104 104
I actually ran ethernet twisted pair cable 105 105
from the basement to the second floor of my house, 106 106
and then I connected a second wireless access point there. 107 107
These two wireless access points work together 108 108
to provide coverage over the entire home. 109 109
One from the top down from the second floor, 110 110
and one from the bottom up going from the basement. 111 111
And this gave me adequate coverage for the whole house. 112 112
Now, each wireless access point 113 113
operate on a different channel 114 114
and I could run from upstairs to downstairs 115 115
and my devices would automatically switch 116 116
from one access point to the other 117 117
as they needed to, to maintain coverage. 118 118
These days, we also have wireless mesh systems available. 119 119
And they rely on a combination of repeaters 120 120
and access points that are put into a single device 121 121
to provide full coverage for larger homes and offices. 122 122
Instead of having to run ethernet cables to each device, 123 123
they instead rely on internal wireless radios 124 124
to send the data from one device to the other 125 125
and create a whole house mesh network 126 126
for the wireless network with sufficient coverage. 127 127
Next, let's talk about interference. 128 128
If you remember, anytime you have multiple wireless networks 129 129
communicating on the same channel, 130 130
you can have interference 131 131
because they're all talking at the same frequency. 132 132
To avoid interference in the 2.4 gigahertz spectrum, 133 133
we always use the three channels of one, six, and 11. 134 134
If I'm using channel six 135 135
and you're using channel four though, 136 136
we're going to have an overlap 137 137
and we're going to interfere with each other's networks. 138 138
This interference can lead to retransmissions 139 139
and this will slow down both of our networks. 140 140
So when you're conducting your site survey, 141 141
you're always going to want to make sure you see 142 142
what channels and frequencies 143 143
are already being used in your area. 144 144
If you create an extended service at wireless network, 145 145
you need to plan the locations of your access points 146 146
and the channels they are going to use. 147 147
Remember, in a 2.4 gigahertz network, 148 148
you always want to make sure you're using 149 149
channels one, six, and 11, 150 150
and you never want to have the same channel number 151 151
sitting next to each other and overlapping. 152 152
This way, I can make sure that I have good coverage. 153 153
For example, let's say I was installing access points 154 154
down a long hallway. 155 155
I would install them as one, six, 11, one, six, and 11. 156 156
Notice that I skipped each one to a different number. 157 157
This ensures there's no overlapping zones, 158 158
they're touching one another of the same type of number. 159 159
Also, you want to ensure you have 10 to 15% overlap 160 160
between the two access points 161 161
to make sure you have sufficient coverage 162 162
and time for the devices to hand off. 163 163
Now, if you're using a newer five gigahertz network, 164 164
you're going to utilize a honeycomb pattern 165 165
for your access points as they're being installed. 166 166
And you should always ensure that each channel 167 167
is not repeated until you're at least two zones away. 168 168
This'll ensure adequate coverage and minimize interference. 169 169
Next, let's cover attenuation. 170 170
Attenuation is a reduction in signal strength 171 171
between the transmission and receipt of the wireless signal. 172 172
Attenuation can occur for lots of different reasons, 173 173
such as increase in the distance 174 174
between the transmitter and the receiver, 175 175
hitting a wall or other physical obstacle, 176 176
or signal interference that's reducing 177 177
the overall signal strength 178 178
because of more noise in the environment 179 179
causing interference. 180 180
Attenuation can occur within the antenna's cable itself 181 181
or in the radio frequency wave 182 182
as it's traveling away from the transmitter 183 183
and towards the receiver. 184 184
When the attenuation is occurring 185 185
inside the antenna's cable, it's normally going to be caused 186 186
by the materials and the construction of that antenna. 187 187
If low quality components were used to make that antenna 188 188
or the cables inside of it, 189 189
there are going to be more resistance inside that wire, 190 190
and that leads to more attenuation. 191 191
To increase the signal strength and reduce the attenuation, 192 192
you can replace the cable or the antenna 193 193
with a higher quality, lower resistant component. 194 194
With radio-frequency wave signal attenuation, 195 195
one of the causes of this is multipath reception. 196 196
Now, this occurs when the transmitted signal 197 197
starts bouncing off walls and other physical objects, 198 198
and then it eventually gets redirected back to the receiver. 199 199
Now, for example, let's say I was standing 200 200
on one side of the room 201 201
and you're standing on the other side of the room. 202 202
And there's a big support column standing right between us. 203 203
If I wanted to send a ball to you 204 204
that's going to signify my radio waves, 205 205
I couldn't throw it directly to you. 206 206
Because if I did that, 207 207
it's going to hit the column and bounce back towards me. 208 208
So instead, I would need as an omnidirectional antenna 209 209
to throw out a lot of different balls simultaneously. 210 210
Some of those will bounce off the sidewalls 211 211
and they'll eventually reach you. 212 212
Some will hit the pole in front of me and bounce back to me. 213 213
But, because of all this different bounce, 214 214
we're going to have slower and weaker signals 215 215
by the time they get to you, 216 216
because I had to bounce off the wall first, right? 217 217
Well, the same thing happens with our radio frequency waves. 218 218
They may bounce around the room 219 219
and they'll eventually get to you, 220 220
but they may be attenuated and weaker than before 221 221
and it's going to lead to a lower RSSI value for you 222 222
and lower throughput for the data 223 223
being sent over these networks. 224 224
Now, finally, we need to talk about disassociation issues 225 225
within our wireless networks. 226 226
There's a lot of reasons that a client 227 227
would be disassociated from a wireless access point. 228 228
This includes an idle timeout, a session timeout, 229 229
wireless network changes, manual deletion, 230 230
authentication timeouts, or access point radio resets. 231 231
Now, a client association due to an idle timeout 232 232
is going to occur whenever a wireless client 233 233
doesn't send or receive traffic within 300 seconds, 234 234
which is five minutes. 235 235
This is the default setting on most wireless access points, 236 236
and it's used to free up the network 237 237
for other clients to join. 238 238
To prevent this from occurring, 239 239
some wireless client implementations 240 240
will send out a keep alive packet every few minutes 241 241
so they can remain connected the entire time 242 242
to that wireless network. 243 243
Now, a client association can also occur 244 244
due to a session timeout, 245 245
and this will happen after 1800 seconds. 246 246
At this point, the wireless client 247 247
should reconduct an authentication again 248 248
and reestablish their connection automatically. 249 249
This again is something that's handled 250 250
by your wireless client automatically on your behalf 251 251
in coordination with the wireless access point. 252 252
Next, we have client disassociations 253 253
that occur due to wireless network changes. 254 254
And this occurs whenever the wireless local area network 255 255
is being changed and this change 256 256
causes the wireless network to be disabled 257 257
and then reenable itself. 258 258
For example, if you're going to change the shared pass phrase 259 259
that secures the network, 260 260
it's going to reboot the wireless network 261 261
and force every device to reconnect and reauthenticate. 262 262
This makes a lot of sense because we want to make sure 263 263
they all have the right password again, right? 264 264
Another client association we can have 265 265
is caused by manual deletion. 266 266
And this occurs whenever a wireless client 267 267
is removed by an administrator. 268 268
This makes sense because if I kicked your client 269 269
off my network, that means I want you to be dissociate 270 270
and removed, and then you'd have to reconnect again. 271 271
Now, another way that client associations occur 272 272
is due to authentication timeouts. 273 273
And this occurs whenever the authentication 274 274
or key exchange process fails to finish in a given time. 275 275
When this occurs, the wireless client 276 276
is disassociated from the access point 277 277
and needs to restart the authentication process 278 278
all over again to regain access. 279 279
Another time you'll find client disassociations occurring 280 280
is when your access point radio is reset. 281 281
And this is similar to the change 282 282
that we made with the wireless network. 283 283
All of our clients are going to be disassociated, 284 284
the radio is going to be turned off and turned on 285 285
to cause that reset, and then the clients 286 286
can begin their association process once more. 287 287
Now, why is it important to understand all these times 288 288
when disassociation happens? 289 289
Well, because sometimes your clients can be disassociated 290 290
as part of a deauthentication attack 291 291
and not for one of these real legitimate reasons. 292 292
This is a common wireless attack that is used by hackers 293 293
to disassociate your wireless clients 294 294
and make them attempt to reconnect to the access point. 295 295
Now, when this occurs, 296 296
the attacker is going to attempt to capture 297 297
the packet's use in the association 298 298
and authentication processes, 299 299
and then they'll try to crack that shared pass phrase 300 300
to gain access to the network indefinitely. 301 301
So if you see a client 302 302
that's continually being deauthenticated, 303 303
you need to check your wireless gateways 304 304
and your wireless controller logs 305 305
to determine the root cause. 306 306
It could be one of the ones I just mentioned above, 307 307
and all of those are normal and expected. 308 308
But if it's not one of those, 309 309
it could be caused by an attacker. 310 310
And that's something you need to investigate further.