-: Wireless frequencies. 2 2
We've talked about antennas 3 3
and we've talked about the basics of wireless. 4 4
Now it's time for us to dig a little bit deeper 5 5
into the specific frequencies that are being used 6 6
in this wireless spectrum. 7 7
First, I want to talk about spread 8 8
spectrum wireless transmission. 9 9
There are three main ways that we can do this. 10 10
The first is DSSS or direct sequence spread spectrum, 11 11
the next is FHSS or frequency hopping spread spectrum 12 12
and the third is OFDM 13 13
or orthogonal frequency division multiplexing. 14 14
Now in today's networks, 15 15
we don't rely as much on frequency hopping, 16 16
instead, we like to use direct sequence 17 17
or orthogonal frequency division. 18 18
Now we're going to talk about this 19 19
as we go through each of the next parts of this lesson. 20 20
First, we have DSSS or direct sequence spread spectrum. 21 21
This is going to modulate your data 22 22
over the entire range of frequencies, 23 23
using a series of signals, which are called chips. 24 24
Now these chips are more susceptible 25 25
to electrical interference and environmental interference, 26 26
and that's going to cause us to have slower bandwidth. 27 27
For this reason, we don't use it very often. 28 28
Also, it's going to use the entire frequency of the spectrum 29 29
to transmit signal. 30 30
This is very self optimal for us. 31 31
So for example, if I'm using channel one 32 32
or channel six or channel 11, 33 33
you can see here on the screen that I have large portions 34 34
of that frequency band being used. 35 35
Now to have no overlapping channels 36 36
and prevent interference, 37 37
I have to use channels one, six and 11, 38 38
but that means I'm giving up all the other channels, 39 39
two, three, four, five, seven, eight, nine, and 10. 40 40
You can see this is a ton of wasted space here 41 41
because we're using DSSS. 42 42
On the other hand, FHSS or frequency hopping spread spectrum 43 43
is going to allow devices to hop 44 44
between predetermined frequencies. 45 45
Now, this makes it harder to guess 46 46
where the frequency actually is, 47 47
depending on the algorithms being used by your protocol. 48 48
Now, frequency hopping is used as a security measure 49 49
in some networks, 50 50
but in most commercial grade wireless networks, 51 51
we're not going to be using it, 52 52
because it slows down our ability to use all the bandwidth 53 53
and reduces the amount of spectrum you have available 54 54
to use for bandwidth. 55 55
And so this is going to start slowing down your network, 56 56
although it does increase security. 57 57
So using it is a trade off, if you decide to use it. 58 58
Our next and our most common one that we use nowadays, 59 59
is known as OFDM, 60 60
orthogonal frequency division multiplexing. 61 61
Now, OFDM is going to use a slow modulation rate 62 62
with simultaneous transmissions 63 63
over 52 different data streams. 64 64
By doing this with these small chunks, 65 65
we're able to actually take a larger piece of the spectrum 66 66
and give us more bandwidth. 67 67
Now, this gives us higher data rates 68 68
while at the same time resisting interference, 69 69
because these data streams are small little chunks. 70 70
Now, if we compare OFDM, 71 71
that's used by wireless G and wireless N 72 72
and we can see how these differ. 73 73
When we use it with wireless G, 74 74
we're going to be using it with a 22 megahertz spectrum, 75 75
and these chunks are going to take place 76 76
on channels one, six, and 11. 77 77
Now, if I move into wireless N, 78 78
in the 5 GHz spectrum, 79 79
we're now going to have a 40 megahertz chunk. 80 80
That's going to give us the ability and additional bandwidth 81 81
to increase our speeds in wireless N, 82 82
and following protocols like wireless AC and wireless AX. 83 83
Now, before we go further, I do want to point out 84 84
that for the exam, you do not need to go in-depth in DSSS, 85 85
frequency hopping, orthogonal division. 86 86
Instead, you really just need to know these three terms, 87 87
and when you see them, they're referring to something 88 88
in the wireless networking world, 89 89
if you know that you'll be able to pick out 90 90
the right answer on test day. 91 91
Just recognizing those three terms are relating 92 92
to wireless networking, is really as in-depth 93 93
as you need for this particular exam. 94 94
Next, let's talk about frequencies and channels. 95 95
Now we've already touched a little bit on this, 96 96
as I started talking about 2.4 GHz and 5 GHz. 97 97
These are two different spectrums 98 98
that are used by wireless networks today. 99 99
The 2.4 GHz band, actually isn't 2.4 GHz, 100 100
it's 2.4 and 2.5 GHz, 101 101
but for the exam and anything else you see in real life, 102 102
people are just going to say 2.4 GHz, 103 103
and that's sufficient. 104 104
Now the same thing holds shoe with 5 GHz, 105 105
technically it's 5.75 to 5.875 GHz, 106 106
but everyone just calls it 5 GHz. 107 107
And for the exam, that's what they'll call it as well. 108 108
So each band here between 2.5 GHz and 5 GHz 109 109
has specific frequencies and channels 110 110
that are going to be used, 111 111
and this helps us to avoid overlapping with other signals 112 112
and causing interference. 113 113
Now, when I talk about a channel, 114 114
I'm really talking about something that's anomalous 115 115
to a physical medium. 116 116
Now, when we think about a channel, 117 117
it's essentially how we're going to transmit information 118 118
over our wireless networks. 119 119
Think about it like a virtual pipe. 120 120
It's very much like the physical cables 121 121
we use in our wired networks, 122 122
but instead of a physical copper or fiber cable, 123 123
we're using a portion of the wireless frequency 124 124
that exists to create these channels, and send our data 125 125
over these virtual pipes, over the airwaves. 126 126
Now, depending on which frequency band you're using, 127 127
you're going to have more or less channels available. 128 128
When we deal with the 2.4 GHz spectrum, 129 129
there are 11 channels or 14 channels. 130 130
Now, the reason there's a difference 131 131
is because of regulation, 132 132
depending on where you are in the world, 133 133
you'll never have access to 11 channels or 14 channels. 134 134
All wireless frequencies are regulated by the country 135 135
that you're operating in. 136 136
So if you live in the United States, you can only use 137 137
11 channels within the 2.4 GHz spectrum. 138 138
This goes from 2401 megahertz, up to 2473 megahertz. 139 139
Now, if you're operating in the rest of the world, 140 140
except Japan, you can operate from 2401 megahertz, 141 141
just like the United States, up to 2483 megahertz. 142 142
If you're operating in Japan, 143 143
you can go all the way up to 2495 megahertz. 144 144
So this means in the U.S. we only have 11 channels, 145 145
the rest of the world gets 13 channels, 146 146
and Japan has 14 channels. 147 147
Now, each of these channels is only around 22 megahertz wide 148 148
within the 2.4 GHz spectrum. 149 149
This is going to limit the amount of data 150 150
that we can send at any given time. 151 151
The other problem we have these channels, 152 152
is that they actually overlap a lot 153 153
because we only have 72 megahertz of total frequency 154 154
inside the 2.4 GHz spectrum 155 155
that's been allocated to us by the FCC 156 156
and other regulatory authorities 157 157
within our 802.11 wireless standards. 158 158
So if you're dealing with 2.4 GHz for instance, 159 159
there are going to be three channels 160 160
that you have to memorize 161 161
and use these three channels to prevent interference. 162 162
These are channels one, six, and 11. 163 163
Those three channels are truly important 164 164
because they are far enough apart from each other 165 165
to prevent any kind of interference 166 166
by giving you 22 megahertz for each of those three channels 167 167
and still fitting within the 72 megahertz 168 168
total spectrum provided. 169 169
So if you're ever asked about 170 170
how to prevent wireless interference, 171 171
and somebody asks you what channels you should use, 172 172
the answer is always going to be one, six and 11, 173 173
if we're talking about using wireless B, wireless G 174 174
or wireless N within the 2.4 GHz spectrum. 175 175
Because of this limitation, newer wireless networks 176 176
are going to operate in the 5 GHz spectrum instead. 177 177
In the 5 GHz spectrums, 178 178
regulators have given us from 5.725 GHz 179 179
all the way up to 5.875 GHz. 180 180
This allows us to run our wireless networks 181 181
within that range. 182 182
Now, if we keep with the 20 megahertz wide channels 183 183
that we're using with 2.4 GHz, 184 184
we are now going to have 24 non-overlapping channels, 185 185
which is a huge improvement 186 186
over the older 2.4 GHz networks, 187 187
which only had the three non-overlapping channels 188 188
of one, six and 11. 189 189
Now, inside of our 5 GHz networks, 190 190
we can also make wider channels 191 191
than just 20 megahertz though. 192 192
Starting with wireless N networks, 193 193
there's an option to perform, 194 194
what's known as channel bonding, 195 195
and this was increased in wireless AC 196 196
to allow 480 megahertz channels 197 197
and 160 megahertz channels too. 198 198
So, what is channel bonding? 199 199
Well, bonding a channel allows you to create 200 200
a wider channel by merging neighboring channels into one. 201 201
Think about it as if we have these virtual pipes 202 202
and we put them all together. 203 203
That's going to allow us to push more data through 204 204
at the same time. 205 205
So, instead of only taking up 120 megahertz area 206 206
for a single channel, 207 207
we can now take two 20 megahertz channels 208 208
to give us a 40 megahertz bonded channel, 209 209
or I can combine eight of these channels 210 210
and get 160 megahertz channel. 211 211
By having this wider channel, 212 212
I can push more data across the network at one time, 213 213
leading to increased speeds and additional bandwidth. 214 214
Now, the only challenge with channel bonding, 215 215
is that now increases the probability 216 216
that you can experience interference, 217 217
because you're now reducing the number 218 218
of non-overlapping channels, 219 219
because you've taken up more of the spectrum 220 220
by combining these channels together. 221 221
Remember with 5 GHz networks, 222 222
we have 24 non-overlapping channels of 20 megahertz each, 223 223
but if I created a bonded channel of 160 megahertz, 224 224
I just took up the equivalent of eight 225 225
of those 24 non-overlapping channels. 226 226
This could lead other wireless network devices 227 227
near my access point, 228 228
to start causing interference with my network. 229 229
Now, for the exam, you don't need to memorize 230 230
all the different frequencies for the different channels, 231 231
instead, you should be aware of the standard channel size 232 232
being 20 megahertz for both 2.4 GHz 233 233
and 5 GHz networks. 234 234
But if you use channel bonding 235 235
with the 5 GHz network, you can make them larger. 236 236
You can make them two times, 237 237
four times or eight times as wide. 238 238
Now, when you do that, 239 239
you can reach higher network speeds, 240 240
but you also risk more interference too. 241 241
So it is a balancing act between these. 242 242
Now up to this point, 243 243
I've mentioned a few wireless standards like B and G, 244 244
N and AC. 245 245
Next, I'm going to provide you 246 246
with a nice little summary chart, 247 247
that's going to cover all the wireless networking standards 248 248
that you need to memorize for the exam. 249 249
This is one that I would print out and memorize. 250 250
You need to know the standard and you need to know the band, 251 251
and you need to know the maximum bandwidth. 252 252
These three pieces of information are very important. 253 253
Now, when we start out with wireless networks 254 254
all the way back in the early 1990s, 255 255
we only had the 802.11 standard. 256 256
This standard though, was not commercially viable, 257 257
and it was essentially a big proof of concept. 258 258
It didn't really make it into the marketplace. 259 259
It operated in the 2.4 GHz spectrum, 260 260
but it only operated 261 261
at about one to two megabits per second. 262 262
Now, for your chart, 263 263
I wouldn't even bother writing that one down. 264 264
Instead, you need to know about A, B, G, N, AC, 265 265
and AIX for our exam. 266 266
Those six Wi-Fi types are the ones you need to memorize 267 267
for the exam, 268 268
with those three pieces of critical information 269 269
to make sure you're successful on the exam 270 270
for wireless networking questions. 271 271
Let's talk about each one now. 272 272
First, let's talk about wireless A, or 802.11a. 273 273
This operate in the 5 GHz spectrum, 274 274
which was a very expensive radio to build a manufacturer 275 275
at that time, 276 276
but it did give us a good amount of speed 277 277
because it operated at 54 megabits per second. 278 278
This was really good in the late nineties, 279 279
but again, unfortunately it costs a lot of money 280 280
because of that high cost, only business users 281 281
really ended up using it, 282 282
and it wasn't really that commercially viable 283 283
in the mainstream market. 284 284
Now, since it wasn't getting as much traction 285 285
in the commercial markets, 286 286
they decided to make something cheaper and easier. 287 287
So the manufacturers decided to create wireless B, 288 288
which operates in the 2.4 GHz spectrum. 289 289
Now, this frequency range is commonly used 290 290
by a lot of other household devices, 291 291
things like security cameras, walkie-talkies, 292 292
baby monitors, microwaves, and more. 293 293
Now, this made the radios and the 802.11b wireless devices 294 294
very cheap and easy to get 295 295
and it led to widespread adoption of Wi-Fi throughout homes, 296 296
businesses, and schools bringing us to where we are today. 297 297
Now using this cheaper chip set 298 298
and the way the frequencies work, 299 299
actually slowed down our networks. 300 300
So we went from 54 megabits per second, 301 301
down to 11 megabits per second, 302 302
which today sounds extremely slow. 303 303
But again, we're talking about the late 1990s here, 304 304
and we weren't doing a lot of streaming video, 305 305
and so 11 megabits per second was actually fast enough 306 306
for most home users. 307 307
Now over time though, networks got faster 308 308
and we wanted more speed. 309 309
And so wireless G came out as a replacement for wireless B. 310 310
Now wireless 802.11g is also in the 2.4 GHz spectrum, 311 311
but it operates at 54 megabits per second. 312 312
Now, eventually we wanted to go even faster than this, 313 313
so engineers kept working on new solutions 314 314
and new ways to manipulate the frequencies. 315 315
And eventually they came up with wireless N, 316 316
which is also called Wi-Fi 4, 317 317
since it was the fourth generation of Wi-Fi. 318 318
Now 802.11n really wanted to increase speed. 319 319
So moved back to the 5 GHz spectrum again, 320 320
and this allowed to get up to speeds 321 321
of 300 to 600 megabits per second. 322 322
This allowed for really fast networks, 323 323
but the big problem 324 324
is that this newer 5 GHz spectrum 325 325
wasn't compatible with all the existing devices 326 326
are out there, because they were wireless B and G, 327 327
and they operate at 2.4 GHz. 328 328
So people were resistant to buying wireless N at first. 329 329
Now to overcome this manufacturer started 330 330
making hybrid devices that were market under the name 331 331
wireless N, and these types of devices 332 332
had a wireless access point with two sets of radios in them. 333 333
One was for the 2.4 GHz spectrum, 334 334
and one for the 5 GHz spectrum. 335 335
This way, if you had a mixture devices that were 802.11b, 336 336
and G, and N, you could connect 337 337
to the slower 2.4 GHz spectrum, 338 338
and it would support wireless B speeds, wireless G speeds, 339 339
or newer wireless N speeds 340 340
that went up to about 150 megabits per second. 341 341
Now, if someone connected 342 342
to the more modern wireless N radios 343 343
using the 5 GHz spectrum, they could actually 344 344
reach speeds up to 600 megabits per second, 345 345
by using a technology known as MIMO. 346 346
MIMO stands for multiple input and multiple output, 347 347
which means that the access point 348 348
could use multiple antennas to send and receive data, 349 349
instead of putting it all through a single antenna, 350 350
essentially your data was going to be split 351 351
across multiple antennas, 352 352
and it was received on the other end, 353 353
it was multiplex back into a single data stream 354 354
for processing. 355 355
This is why you can see wireless and access points 356 356
that have one, two, three, or even five antennas, 357 357
because the more intense you had, 358 358
the more data transfer they could support simultaneously. 359 359
Next, we have wireless AC, which is also called Wi-Fi 5, 360 360
or 802 11 AC. 361 361
This was the fifth generation of Wi-Fi. 362 362
Now wireless AC operates exclusively 363 363
in the 5 GHz spectrum, 364 364
and technically it does not provide 365 365
any kind of backward compatibility. 366 366
These 802.11ac networks can operate 367 367
at speeds up to three gigabits per second or more. 368 368
These networks are really fast in theory. 369 369
Now to achieve these higher speeds, 370 370
802.11ac networks, uses the technology known as MU-MIMO, 371 371
which has multiple user, multiple input, multiple output. 372 372
It's a newer variation of the MIMO technology 373 373
that was first developed back with 802.11n networks. 374 374
Now MU-MIMO is a multipath wireless communication technology 375 375
that allows multiple users to access the wireless network 376 376
and access point at the same time. 377 377
This is different than a regular MIMO, 378 378
where a single user supported at one time. 379 379
And the access point switches between users 380 380
to share the bandwidth across all the users 381 381
who are requesting services. 382 382
So if you only have one person requesting services, 383 383
they get a really fast network, but if you have, or three, 384 384
it starts slowing down because it just share the bandwidth. 385 385
Essentially with MIMO, 386 386
the wireless network acts more like a hub, 387 387
but with MU-MIMO, it begins to act more like a switch 388 388
and helps avoid collisions and congestion. 389 389
Now, when it comes to wireless AC, 390 390
some of the original and older AC devices actually still use 391 391
the older MIMO technology. 392 392
Whereas the newer wireless AC devices, will use the MU-MIMO 393 393
for faster speeds. 394 394
Now, this brings us to the latest generation 395 395
of wireless networks, 802.11ax. 396 396
Wireless AX is known as Wi-Fi 6, 397 397
because it's the sixth generation of wireless networks. 398 398
This was introduced in 2021, and it can be used 399 399
in the 2.4 GHz and 5 GHz spectrum 400 400
under the marketing term, Wi-Fi 6 401 401
or in the newer and faster six GHz spectrum 402 402
under the marketing term, Wi-Fi 6E 403 403
or high efficiency Wi-Fi. 404 404
Now these Wi-Fi 6 and Wi-Fi 6E networks, 405 405
is 802.11ax networks, 406 406
can reach speeds up to 9.6 gigabits per second, 407 407
using MU-MIMO technology. 408 408
Also, because these access points have both the 2.4 GHz 409 409
and 5 GHz radios inside them, 410 410
they are fully backwards compatible with all devices, 411 411
including wireless A, B, G, N and AC. 412 412
All right, for the exam, I want you to remember 413 413
there are different wireless networks out there. 414 414
These include A, B, G, N, AC and AX. 415 415
You also need to remember that if it's a B, G, 416 416
N or AX network, 417 417
it's going to support 2.4 GHz as a spectrum. 418 418
If it's A, N, AC or AX, 419 419
it supports 5 GHz as a spectrum. 420 420
You also need to remember the relative speeds 421 421
of these different wireless devices, 422 422
going from 11 megabits per second for wireless B, 423 423
all the way up to the gigabits per second use 424 424
in AC and AX networks. 425 425
This is important for the exam, because on test day, 426 426
you may get questions about frequencies, 427 427
things like which of these frequencies 428 428
do not support 5 GHz? 429 429
And the answer would have to be either B or G, 430 430
for wireless B and wireless G. 431 431
Now, you may get a question asking you to select 432 432
which wireless standard doesn't support 2.4 GHz. 433 433
And in this case, 434 434
you need to select wireless A or wireless AC. 435 435
If they wanted to make it more difficult for you, 436 436
they can ask the question 437 437
as more of a troubleshooting scenario, for example, 438 438
you're working as a network technician on an older laptop, 439 439
and it's failing to connect to your wireless AC network, 440 440
you check the laptop and see 441 441
that it has a wireless B network card. 442 442
What is the problem? 443 443
Then you're going to find the answer that has something 444 444
to do with the fact that there's a frequency mismatch, 445 445
because wireless AC supports 5 GHz 446 446
and wireless B supports 2.4 GHz. 447 447
And therefore you can't connect to the network. 448 448
Now, one more thing to keep in mind as you're studying, 449 449
is that marketers sometimes mislabel things 450 450
to make it easier for our consumers. 451 451
But on test day, 452 452
you have to go by the official standards. 453 453
A great example of this is wireless AC, 454 454
the 802.11ac standard. 455 455
It only specifies operation 456 456
in the 5 GHz frequency band. 457 457
But if you go to the store 458 458
and you find a wireless AC access point, 459 459
the box will tell you, it supports both 5 GHz 460 460
and 2.4 GHz. 461 461
This is a lie, and you will get in trouble on the exam, 462 462
if you pick this answer, because you think it's dual band, 463 463
and it's not the truth is wireless AC, 464 464
only operates in the 5 GHz spectrum. 465 465
When you're buying that wireless AC access point 466 466
at the store, and it says it supports both frequencies, 467 467
it's actually a wireless access point with two radios in it. 468 468
One radio is 5 GHz for wireless AC 469 469
at speeds up to about 1300 megabits per second. 470 470
The other one is a 2.4 GHz radio for wireless N, 471 471
at speeds of up to 600 megabits per second, 472 472
with a MIMO antenna configuration. 473 473
Now, while in real life, your users really don't care, 474 474
and they just say, hey, I have a wireless AC access point, 475 475
and they think it supports both 5 GHz and 2.4 GHz, 476 476
on the exam, you will get the question wrong, 477 477
if you select 2.4 GHz for wireless AC. 478 478
Remember wireless AC only supports 5 GHz 479 479
for its operations. 480 480
The only dual band standards we have are wireless N 481 481
and wireless AX. 482 482
Both of those support, both 2.4 GHz 483 483
and 5 GHz frequency bands per the 802.11 standards. 484 484
Now, let's talk about radio frequency interference, 485 485
or RFI for a minute here. 486 486
Radio frequency interference is caused 487 487
when there are similar frequencies 488 488
to wireless networks in your area. 489 489
For example, I mentioned earlier that one of the reasons 490 490
we went to 2.4 GHz for Wi-Fi B, 491 491
was the fact that there was other videos out there 492 492
that already used it. 493 493
Things like baby monitors and cordless phones, 494 494
and microwave ovens and other security devices. 495 495
Now, this means that 2.4 GHz as a spectrum 496 496
is fairly crowded. 497 497
This is what made the radios cheap, 498 498
but it made it very difficult for us 499 499
because it causes a lot of interference. 500 500
Over time as more and more devices moved 501 501
into the 5 GHz spectrum, 502 502
there's also more interference in that area too. 503 503
All of these other electronics can cause interference 504 504
with your wireless networks, 505 505
so you have to think about these things as you're developing 506 506
your networks and troubleshooting your networks. 507 507
For example, if you have a 2.4 GHz 508 508
wireless G network in use, and the access point happens 509 509
to be sitting in the break room at the office, 510 510
and every time somebody turns on the microwave 511 511
to reheat their burrito, the network drops, 512 512
this is probably because the 2.4 GHz frequency 513 513
is being interfered with, 514 514
by those microwaves that are operating 515 515
in that same frequency band. 516 516
In addition to all this frequency interference, 517 517
you might also see things like physical interference. 518 518
This is where physical things 519 519
can block your wireless signals. 520 520
For instance, I live in Puerto Rico 521 521
and the walls in my house are solid concrete. 522 522
I also have a refrigerator inside my kitchen, 523 523
I have kitchen cabinets and those block the signal, 524 524
all these things can cause signal strength issues for you. 525 525
If your signals are too weak, 526 526
and it can't make it around a corner or through a wall, 527 527
that signal is going to get blocked, 528 528
or it suffers what's known as a tenuation. 529 529
All of these things can lead to interference, 530 530
which will slow down your ability for your network 531 531
to operate at top speed. 532 532
As your signal decreases in strength 533 533
or interference increases, 534 534
we get a worse signal-to-noise ratio. 535 535
This is going to cause additional retransmissions 536 536
because most of the time we're sending things over TCP, 537 537
when TCP retransmit, this creates additional network baggage 538 538
that's being taken up and bandwidth is being used 539 539
for all these retransmissions. 540 540
And this slows down the network even more. 541 541
You want to make sure you have good signal 542 542
throughout your entire structure 543 543
to increase the efficiency of your network. 544 544
To do this, you would do what's called a site survey 545 545
where you check the signal strength in different areas 546 546
and make sure you have the right antennas 547 547
and the right repeaters throughout the building. 548 548
Finally, let's talk about how we actually send data 549 549
over one of these wireless networks. 550 550
With Ethernet, we talked about the fact 551 551
that we use CSMA/CD, which was 552 552
carrier sense multiple access/ collision detection. 553 553
With wireless networks, we're going to use something 554 554
known as CSMA/CA, 555 555
which has carrier sense multiple access/collision avoidance. 556 556
See here we've changed collision detection 557 557
to collision avoidance, once we went to the wireless domain. 558 558
Both CD and CA are going to start out the exact same way, 559 559
in both of these network types, 560 560
we're going to listen for transmissions. 561 561
If we think the line is clear in the case 562 562
of CD using Ethernet or CA we're using wireless, 563 563
and the frequency has to be clear, 564 564
we can then send a message. 565 565
This is the carrier sense multiple access part 566 566
of this stuff. 567 567
Now, in the case of Ethernet, 568 568
this is where we stopped doing anything. 569 569
We're just going to do carrier sense multiple access 570 570
collision detection. 571 571
We're going to send our message and see if it crashes. 572 572
This way, if there's a collision, 573 573
we're going to just retransmit it. 574 574
Now with wireless though, 575 575
we want to try and prevent collisions ahead of time, 576 576
because we said, retransmissions eat up valuable bandwidth. 577 577
This is where collision avoidance comes in. 578 578
As the device gets ready to transmit, 579 579
it's going to listen to the frequency 580 580
and make sure it's clear, 581 581
and then it's going to send out a packet 582 582
that's known as an RTS, which stands for Request To Send. 583 583
The intended recipient usually the wireless access point 584 584
on the network, will then acknowledge that Request To Send 585 585
by sending a CTS packet, which stands for Clear To Send. 586 586
Now, once my device sees the CTS packet, 587 587
it's going to go ahead and send my data, because I was told, 588 588
the whole frequency is clear, 589 589
and it's ready for me to send something. 590 590
Now, if we don't receive this CTS signal, 591 591
this Clear To Send acknowledgement, 592 592
then we're not going to start sending. 593 593
Instead, I'm going to choose a random backoff timer, 594 594
I'm going to wait for something like 30 milliseconds, 595 595
and then I'll do another RTS or Request To Send. 596 596
Now until I received that Clear To Send packet, 597 597
I am not going to go and send my message, 598 598
because I don't want to cause a collision. 599 599
Remember every collision causes a retransmission, 600 600
retransmissions take up valuable bandwidth, 601 601
and that starts taking up additional resources 602 602
and it becomes a negative spiral 603 603
for our network's performance. 604 604
So remember collision detection is used in wired networks, 605 605
collision avoidance is used in wireless networks, 606 606
and we won't send until we see that Clear To Send signal 607 607
in response to are Ready To Send packet.