1 1 00:00:00,170 --> 00:00:02,290 Wireless frequencies. 2 2 00:00:02,290 --> 00:00:03,620 We've talked about antennas 3 3 00:00:03,620 --> 00:00:05,800 and we've talked about the basics of wireless. 4 4 00:00:05,800 --> 00:00:08,180 Now it's time for us to dig a little bit deeper 5 5 00:00:08,180 --> 00:00:10,350 into the specific frequencies that are being used 6 6 00:00:10,350 --> 00:00:12,130 in this wireless spectrum. 7 7 00:00:12,130 --> 00:00:13,960 First, I want to talk about spread 8 8 00:00:13,960 --> 00:00:15,810 spectrum wireless transmission. 9 9 00:00:15,810 --> 00:00:18,160 There are three main ways that we can do this. 10 10 00:00:18,160 --> 00:00:22,700 The first is DSSS or direct sequence spread spectrum, 11 11 00:00:22,700 --> 00:00:26,860 the next is FHSS or frequency hopping spread spectrum 12 12 00:00:26,860 --> 00:00:28,940 and the third is OFDM 13 13 00:00:28,940 --> 00:00:31,870 or orthogonal frequency division multiplexing. 14 14 00:00:31,870 --> 00:00:33,060 Now in today's networks, 15 15 00:00:33,060 --> 00:00:35,490 we don't rely as much on frequency hopping, 16 16 00:00:35,490 --> 00:00:37,680 instead, we like to use direct sequence 17 17 00:00:37,680 --> 00:00:39,540 or orthogonal frequency division. 18 18 00:00:39,540 --> 00:00:40,500 Now we're going to talk about this 19 19 00:00:40,500 --> 00:00:43,610 as we go through each of the next parts of this lesson. 20 20 00:00:43,610 --> 00:00:48,020 First, we have DSSS or direct sequence spread spectrum. 21 21 00:00:48,020 --> 00:00:49,470 This is going to modulate your data 22 22 00:00:49,470 --> 00:00:51,410 over the entire range of frequencies, 23 23 00:00:51,410 --> 00:00:54,600 using a series of signals, which are called chips. 24 24 00:00:54,600 --> 00:00:56,240 Now these chips are more susceptible 25 25 00:00:56,240 --> 00:00:58,890 to electrical interference and environmental interference, 26 26 00:00:58,890 --> 00:01:01,420 and that's going to cause us to have slower bandwidth. 27 27 00:01:01,420 --> 00:01:03,770 For this reason, we don't use it very often. 28 28 00:01:03,770 --> 00:01:06,510 Also, it's going to use the entire frequency of the spectrum 29 29 00:01:06,510 --> 00:01:07,900 to transmit signal. 30 30 00:01:07,900 --> 00:01:10,110 This is very self optimal for us. 31 31 00:01:10,110 --> 00:01:12,200 So for example, if I'm using channel one 32 32 00:01:12,200 --> 00:01:14,230 or channel six or channel 11, 33 33 00:01:14,230 --> 00:01:16,820 you can see here on the screen that I have large portions 34 34 00:01:16,820 --> 00:01:18,970 of that frequency band being used. 35 35 00:01:18,970 --> 00:01:20,930 Now to have no overlapping channels 36 36 00:01:20,930 --> 00:01:22,080 and prevent interference, 37 37 00:01:22,080 --> 00:01:24,650 I have to use channels one, six and 11, 38 38 00:01:24,650 --> 00:01:27,420 but that means I'm giving up all the other channels, 39 39 00:01:27,420 --> 00:01:32,020 two, three, four, five, seven, eight, nine, and 10. 40 40 00:01:32,020 --> 00:01:34,440 You can see this is a ton of wasted space here 41 41 00:01:34,440 --> 00:01:37,120 because we're using DSSS. 42 42 00:01:37,120 --> 00:01:41,170 On the other hand, FHSS or frequency hopping spread spectrum 43 43 00:01:41,170 --> 00:01:42,760 is going to allow devices to hop 44 44 00:01:42,760 --> 00:01:44,720 between predetermined frequencies. 45 45 00:01:44,720 --> 00:01:46,090 Now, this makes it harder to guess 46 46 00:01:46,090 --> 00:01:47,880 where the frequency actually is, 47 47 00:01:47,880 --> 00:01:50,700 depending on the algorithms being used by your protocol. 48 48 00:01:50,700 --> 00:01:53,100 Now, frequency hopping is used as a security measure 49 49 00:01:53,100 --> 00:01:54,370 in some networks, 50 50 00:01:54,370 --> 00:01:57,240 but in most commercial grade wireless networks, 51 51 00:01:57,240 --> 00:01:58,540 we're not going to be using it, 52 52 00:01:58,540 --> 00:02:01,450 because it slows down our ability to use all the bandwidth 53 53 00:02:01,450 --> 00:02:03,990 and reduces the amount of spectrum you have available 54 54 00:02:03,990 --> 00:02:05,080 to use for bandwidth. 55 55 00:02:05,080 --> 00:02:07,330 And so this is going to start slowing down your network, 56 56 00:02:07,330 --> 00:02:09,640 although it does increase security. 57 57 00:02:09,640 --> 00:02:13,290 So using it is a trade off, if you decide to use it. 58 58 00:02:13,290 --> 00:02:15,890 Our next and our most common one that we use nowadays, 59 59 00:02:15,890 --> 00:02:17,780 is known as OFDM, 60 60 00:02:17,780 --> 00:02:20,270 orthogonal frequency division multiplexing. 61 61 00:02:20,270 --> 00:02:23,380 Now, OFDM is going to use a slow modulation rate 62 62 00:02:23,380 --> 00:02:24,960 with simultaneous transmissions 63 63 00:02:24,960 --> 00:02:27,350 over 52 different data streams. 64 64 00:02:27,350 --> 00:02:29,410 By doing this with these small chunks, 65 65 00:02:29,410 --> 00:02:32,120 we're able to actually take a larger piece of the spectrum 66 66 00:02:32,120 --> 00:02:33,890 and give us more bandwidth. 67 67 00:02:33,890 --> 00:02:35,490 Now, this gives us higher data rates 68 68 00:02:35,490 --> 00:02:37,770 while at the same time resisting interference, 69 69 00:02:37,770 --> 00:02:40,850 because these data streams are small little chunks. 70 70 00:02:40,850 --> 00:02:42,360 Now, if we compare OFDM, 71 71 00:02:42,360 --> 00:02:44,107 that's used by wireless G and wireless N 72 72 00:02:44,107 --> 00:02:46,030 and we can see how these differ. 73 73 00:02:46,030 --> 00:02:47,570 When we use it with wireless G, 74 74 00:02:47,570 --> 00:02:50,320 we're going to be using it with a 22 megahertz spectrum, 75 75 00:02:50,320 --> 00:02:52,190 and these chunks are going to take place 76 76 00:02:52,190 --> 00:02:54,530 on channels one, six, and 11. 77 77 00:02:54,530 --> 00:02:55,900 Now, if I move into wireless N, 78 78 00:02:55,900 --> 00:02:57,560 in the 5 GHz spectrum, 79 79 00:02:57,560 --> 00:03:00,200 we're now going to have a 40 megahertz chunk. 80 80 00:03:00,200 --> 00:03:02,740 That's going to give us the ability and additional bandwidth 81 81 00:03:02,740 --> 00:03:04,730 to increase our speeds in wireless N, 82 82 00:03:04,730 --> 00:03:08,880 and following protocols like wireless AC and wireless AX. 83 83 00:03:08,880 --> 00:03:11,010 Now, before we go further, I do want to point out 84 84 00:03:11,010 --> 00:03:15,710 that for the exam, you do not need to go in-depth in DSSS, 85 85 00:03:15,710 --> 00:03:18,190 frequency hopping, orthogonal division. 86 86 00:03:18,190 --> 00:03:21,090 Instead, you really just need to know these three terms, 87 87 00:03:21,090 --> 00:03:23,340 and when you see them, they're referring to something 88 88 00:03:23,340 --> 00:03:25,020 in the wireless networking world, 89 89 00:03:25,020 --> 00:03:26,670 if you know that you'll be able to pick out 90 90 00:03:26,670 --> 00:03:28,350 the right answer on test day. 91 91 00:03:28,350 --> 00:03:30,420 Just recognizing those three terms are relating 92 92 00:03:30,420 --> 00:03:32,700 to wireless networking, is really as in-depth 93 93 00:03:32,700 --> 00:03:35,390 as you need for this particular exam. 94 94 00:03:35,390 --> 00:03:38,250 Next, let's talk about frequencies and channels. 95 95 00:03:38,250 --> 00:03:39,980 Now we've already touched a little bit on this, 96 96 00:03:39,980 --> 00:03:43,530 as I started talking about 2.4 GHz and 5 GHz. 97 97 00:03:43,530 --> 00:03:45,200 These are two different spectrums 98 98 00:03:45,200 --> 00:03:47,330 that are used by wireless networks today. 99 99 00:03:47,330 --> 00:03:50,576 The 2.4 GHz band, actually isn't 2.4 GHz, 100 100 00:03:50,576 --> 00:03:51,667 it's 2.4 and 2.5 GHz, 101 101 00:03:53,680 --> 00:03:56,570 but for the exam and anything else you see in real life, 102 102 00:03:56,570 --> 00:03:59,030 people are just going to say 2.4 GHz, 103 103 00:03:59,030 --> 00:04:00,380 and that's sufficient. 104 104 00:04:00,380 --> 00:04:02,720 Now the same thing holds shoe with 5 GHz, 105 105 00:04:02,720 --> 00:04:07,520 technically it's 5.75 to 5.875 GHz, 106 106 00:04:07,520 --> 00:04:10,150 but everyone just calls it 5 GHz. 107 107 00:04:10,150 --> 00:04:12,980 And for the exam, that's what they'll call it as well. 108 108 00:04:12,980 --> 00:04:16,790 So each band here between 2.5 GHz and 5 GHz 109 109 00:04:16,790 --> 00:04:18,800 has specific frequencies and channels 110 110 00:04:18,800 --> 00:04:19,870 that are going to be used, 111 111 00:04:19,870 --> 00:04:22,660 and this helps us to avoid overlapping with other signals 112 112 00:04:22,660 --> 00:04:24,240 and causing interference. 113 113 00:04:24,240 --> 00:04:25,900 Now, when I talk about a channel, 114 114 00:04:25,900 --> 00:04:28,220 I'm really talking about something that's anomalous 115 115 00:04:28,220 --> 00:04:29,770 to a physical medium. 116 116 00:04:29,770 --> 00:04:31,010 Now, when we think about a channel, 117 117 00:04:31,010 --> 00:04:33,370 it's essentially how we're going to transmit information 118 118 00:04:33,370 --> 00:04:34,900 over our wireless networks. 119 119 00:04:34,900 --> 00:04:36,900 Think about it like a virtual pipe. 120 120 00:04:36,900 --> 00:04:38,510 It's very much like the physical cables 121 121 00:04:38,510 --> 00:04:39,780 we use in our wired networks, 122 122 00:04:39,780 --> 00:04:42,740 but instead of a physical copper or fiber cable, 123 123 00:04:42,740 --> 00:04:45,420 we're using a portion of the wireless frequency 124 124 00:04:45,420 --> 00:04:47,990 that exists to create these channels, and send our data 125 125 00:04:47,990 --> 00:04:51,130 over these virtual pipes, over the airwaves. 126 126 00:04:51,130 --> 00:04:53,420 Now, depending on which frequency band you're using, 127 127 00:04:53,420 --> 00:04:55,970 you're going to have more or less channels available. 128 128 00:04:55,970 --> 00:04:58,180 When we deal with the 2.4 GHz spectrum, 129 129 00:04:58,180 --> 00:05:01,010 there are 11 channels or 14 channels. 130 130 00:05:01,010 --> 00:05:02,290 Now, the reason there's a difference 131 131 00:05:02,290 --> 00:05:03,960 is because of regulation, 132 132 00:05:03,960 --> 00:05:05,580 depending on where you are in the world, 133 133 00:05:05,580 --> 00:05:08,790 you'll never have access to 11 channels or 14 channels. 134 134 00:05:08,790 --> 00:05:11,160 All wireless frequencies are regulated by the country 135 135 00:05:11,160 --> 00:05:12,480 that you're operating in. 136 136 00:05:12,480 --> 00:05:15,390 So if you live in the United States, you can only use 137 137 00:05:15,390 --> 00:05:18,260 11 channels within the 2.4 GHz spectrum. 138 138 00:05:18,260 --> 00:05:23,200 This goes from 2401 megahertz, up to 2473 megahertz. 139 139 00:05:23,200 --> 00:05:25,090 Now, if you're operating in the rest of the world, 140 140 00:05:25,090 --> 00:05:28,730 except Japan, you can operate from 2401 megahertz, 141 141 00:05:28,730 --> 00:05:32,440 just like the United States, up to 2483 megahertz. 142 142 00:05:32,440 --> 00:05:33,760 If you're operating in Japan, 143 143 00:05:33,760 --> 00:05:36,980 you can go all the way up to 2495 megahertz. 144 144 00:05:36,980 --> 00:05:40,220 So this means in the U.S. we only have 11 channels, 145 145 00:05:40,220 --> 00:05:42,540 the rest of the world gets 13 channels, 146 146 00:05:42,540 --> 00:05:45,410 and Japan has 14 channels. 147 147 00:05:45,410 --> 00:05:49,520 Now, each of these channels is only around 22 megahertz wide 148 148 00:05:49,520 --> 00:05:51,950 within the 2.4 GHz spectrum. 149 149 00:05:51,950 --> 00:05:53,550 This is going to limit the amount of data 150 150 00:05:53,550 --> 00:05:55,600 that we can send at any given time. 151 151 00:05:55,600 --> 00:05:57,160 The other problem we have these channels, 152 152 00:05:57,160 --> 00:05:59,100 is that they actually overlap a lot 153 153 00:05:59,100 --> 00:06:02,510 because we only have 72 megahertz of total frequency 154 154 00:06:02,510 --> 00:06:04,640 inside the 2.4 GHz spectrum 155 155 00:06:04,640 --> 00:06:07,000 that's been allocated to us by the FCC 156 156 00:06:07,000 --> 00:06:08,730 and other regulatory authorities 157 157 00:06:08,730 --> 00:06:11,370 within our 802.11 wireless standards. 158 158 00:06:11,370 --> 00:06:14,410 So if you're dealing with 2.4 GHz for instance, 159 159 00:06:14,410 --> 00:06:15,820 there are going to be three channels 160 160 00:06:15,820 --> 00:06:17,060 that you have to memorize 161 161 00:06:17,060 --> 00:06:20,030 and use these three channels to prevent interference. 162 162 00:06:20,030 --> 00:06:22,810 These are channels one, six, and 11. 163 163 00:06:22,810 --> 00:06:25,350 Those three channels are truly important 164 164 00:06:25,350 --> 00:06:27,290 because they are far enough apart from each other 165 165 00:06:27,290 --> 00:06:29,000 to prevent any kind of interference 166 166 00:06:29,000 --> 00:06:32,150 by giving you 22 megahertz for each of those three channels 167 167 00:06:32,150 --> 00:06:34,700 and still fitting within the 72 megahertz 168 168 00:06:34,700 --> 00:06:36,410 total spectrum provided. 169 169 00:06:36,410 --> 00:06:37,820 So if you're ever asked about 170 170 00:06:37,820 --> 00:06:39,530 how to prevent wireless interference, 171 171 00:06:39,530 --> 00:06:41,610 and somebody asks you what channels you should use, 172 172 00:06:41,610 --> 00:06:44,500 the answer is always going to be one, six and 11, 173 173 00:06:44,500 --> 00:06:47,330 if we're talking about using wireless B, wireless G 174 174 00:06:47,330 --> 00:06:50,800 or wireless N within the 2.4 GHz spectrum. 175 175 00:06:50,800 --> 00:06:53,240 Because of this limitation, newer wireless networks 176 176 00:06:53,240 --> 00:06:56,400 are going to operate in the 5 GHz spectrum instead. 177 177 00:06:56,400 --> 00:06:58,130 In the 5 GHz spectrums, 178 178 00:06:58,130 --> 00:07:01,800 regulators have given us from 5.725 GHz 179 179 00:07:01,800 --> 00:07:04,700 all the way up to 5.875 GHz. 180 180 00:07:04,700 --> 00:07:06,340 This allows us to run our wireless networks 181 181 00:07:06,340 --> 00:07:07,810 within that range. 182 182 00:07:07,810 --> 00:07:10,250 Now, if we keep with the 20 megahertz wide channels 183 183 00:07:10,250 --> 00:07:12,360 that we're using with 2.4 GHz, 184 184 00:07:12,360 --> 00:07:15,200 we are now going to have 24 non-overlapping channels, 185 185 00:07:15,200 --> 00:07:16,450 which is a huge improvement 186 186 00:07:16,450 --> 00:07:18,830 over the older 2.4 GHz networks, 187 187 00:07:18,830 --> 00:07:21,270 which only had the three non-overlapping channels 188 188 00:07:21,270 --> 00:07:23,580 of one, six and 11. 189 189 00:07:23,580 --> 00:07:25,680 Now, inside of our 5 GHz networks, 190 190 00:07:25,680 --> 00:07:27,650 we can also make wider channels 191 191 00:07:27,650 --> 00:07:29,610 than just 20 megahertz though. 192 192 00:07:29,610 --> 00:07:31,460 Starting with wireless N networks, 193 193 00:07:31,460 --> 00:07:32,560 there's an option to perform, 194 194 00:07:32,560 --> 00:07:34,380 what's known as channel bonding, 195 195 00:07:34,380 --> 00:07:36,570 and this was increased in wireless AC 196 196 00:07:36,570 --> 00:07:38,280 to allow 480 megahertz channels 197 197 00:07:38,280 --> 00:07:41,000 and 160 megahertz channels too. 198 198 00:07:41,000 --> 00:07:43,180 So, what is channel bonding? 199 199 00:07:43,180 --> 00:07:45,230 Well, bonding a channel allows you to create 200 200 00:07:45,230 --> 00:07:48,810 a wider channel by merging neighboring channels into one. 201 201 00:07:48,810 --> 00:07:50,950 Think about it as if we have these virtual pipes 202 202 00:07:50,950 --> 00:07:52,430 and we put them all together. 203 203 00:07:52,430 --> 00:07:54,330 That's going to allow us to push more data through 204 204 00:07:54,330 --> 00:07:55,720 at the same time. 205 205 00:07:55,720 --> 00:07:59,270 So, instead of only taking up 120 megahertz area 206 206 00:07:59,270 --> 00:08:00,370 for a single channel, 207 207 00:08:00,370 --> 00:08:02,740 we can now take two 20 megahertz channels 208 208 00:08:02,740 --> 00:08:05,210 to give us a 40 megahertz bonded channel, 209 209 00:08:05,210 --> 00:08:07,410 or I can combine eight of these channels 210 210 00:08:07,410 --> 00:08:09,760 and get 160 megahertz channel. 211 211 00:08:09,760 --> 00:08:11,310 By having this wider channel, 212 212 00:08:11,310 --> 00:08:13,970 I can push more data across the network at one time, 213 213 00:08:13,970 --> 00:08:16,830 leading to increased speeds and additional bandwidth. 214 214 00:08:16,830 --> 00:08:18,720 Now, the only challenge with channel bonding, 215 215 00:08:18,720 --> 00:08:20,650 is that now increases the probability 216 216 00:08:20,650 --> 00:08:22,360 that you can experience interference, 217 217 00:08:22,360 --> 00:08:23,720 because you're now reducing the number 218 218 00:08:23,720 --> 00:08:25,340 of non-overlapping channels, 219 219 00:08:25,340 --> 00:08:27,100 because you've taken up more of the spectrum 220 220 00:08:27,100 --> 00:08:29,230 by combining these channels together. 221 221 00:08:29,230 --> 00:08:31,320 Remember with 5 GHz networks, 222 222 00:08:31,320 --> 00:08:35,440 we have 24 non-overlapping channels of 20 megahertz each, 223 223 00:08:35,440 --> 00:08:39,560 but if I created a bonded channel of 160 megahertz, 224 224 00:08:39,560 --> 00:08:41,420 I just took up the equivalent of eight 225 225 00:08:41,420 --> 00:08:43,790 of those 24 non-overlapping channels. 226 226 00:08:43,790 --> 00:08:45,710 This could lead other wireless network devices 227 227 00:08:45,710 --> 00:08:46,910 near my access point, 228 228 00:08:46,910 --> 00:08:49,420 to start causing interference with my network. 229 229 00:08:49,420 --> 00:08:51,590 Now, for the exam, you don't need to memorize 230 230 00:08:51,590 --> 00:08:54,020 all the different frequencies for the different channels, 231 231 00:08:54,020 --> 00:08:57,240 instead, you should be aware of the standard channel size 232 232 00:08:57,240 --> 00:08:59,950 being 20 megahertz for both 2.4 GHz 233 233 00:08:59,950 --> 00:09:01,700 and 5 GHz networks. 234 234 00:09:01,700 --> 00:09:03,400 But if you use channel bonding 235 235 00:09:03,400 --> 00:09:06,180 with the 5 GHz network, you can make them larger. 236 236 00:09:06,180 --> 00:09:07,380 You can make them two times, 237 237 00:09:07,380 --> 00:09:09,510 four times or eight times as wide. 238 238 00:09:09,510 --> 00:09:10,460 Now, when you do that, 239 239 00:09:10,460 --> 00:09:12,290 you can reach higher network speeds, 240 240 00:09:12,290 --> 00:09:14,960 but you also risk more interference too. 241 241 00:09:14,960 --> 00:09:17,550 So it is a balancing act between these. 242 242 00:09:17,550 --> 00:09:18,630 Now up to this point, 243 243 00:09:18,630 --> 00:09:21,833 I've mentioned a few wireless standards like B and G, 244 244 00:09:21,833 --> 00:09:23,530 N and AC. 245 245 00:09:23,530 --> 00:09:24,780 Next, I'm going to provide you 246 246 00:09:24,780 --> 00:09:26,280 with a nice little summary chart, 247 247 00:09:26,280 --> 00:09:28,490 that's going to cover all the wireless networking standards 248 248 00:09:28,490 --> 00:09:30,500 that you need to memorize for the exam. 249 249 00:09:30,500 --> 00:09:32,870 This is one that I would print out and memorize. 250 250 00:09:32,870 --> 00:09:36,320 You need to know the standard and you need to know the band, 251 251 00:09:36,320 --> 00:09:38,650 and you need to know the maximum bandwidth. 252 252 00:09:38,650 --> 00:09:41,580 These three pieces of information are very important. 253 253 00:09:41,580 --> 00:09:43,530 Now, when we start out with wireless networks 254 254 00:09:43,530 --> 00:09:45,570 all the way back in the early 1990s, 255 255 00:09:45,570 --> 00:09:48,480 we only had the 802.11 standard. 256 256 00:09:48,480 --> 00:09:50,627 This standard though, was not commercially viable, 257 257 00:09:50,627 --> 00:09:53,080 and it was essentially a big proof of concept. 258 258 00:09:53,080 --> 00:09:55,270 It didn't really make it into the marketplace. 259 259 00:09:55,270 --> 00:09:57,950 It operated in the 2.4 GHz spectrum, 260 260 00:09:57,950 --> 00:09:59,220 but it only operated 261 261 00:09:59,220 --> 00:10:01,800 at about one to two megabits per second. 262 262 00:10:01,800 --> 00:10:02,790 Now, for your chart, 263 263 00:10:02,790 --> 00:10:05,260 I wouldn't even bother writing that one down. 264 264 00:10:05,260 --> 00:10:10,260 Instead, you need to know about A, B, G, N, AC, 265 265 00:10:10,580 --> 00:10:12,680 and AIX for our exam. 266 266 00:10:12,680 --> 00:10:15,090 Those six Wi-Fi types are the ones you need to memorize 267 267 00:10:15,090 --> 00:10:15,923 for the exam, 268 268 00:10:15,923 --> 00:10:18,130 with those three pieces of critical information 269 269 00:10:18,130 --> 00:10:20,100 to make sure you're successful on the exam 270 270 00:10:20,100 --> 00:10:21,900 for wireless networking questions. 271 271 00:10:21,900 --> 00:10:23,590 Let's talk about each one now. 272 272 00:10:23,590 --> 00:10:27,500 First, let's talk about wireless A, or 802.11a. 273 273 00:10:27,500 --> 00:10:29,780 This operate in the 5 GHz spectrum, 274 274 00:10:29,780 --> 00:10:32,150 which was a very expensive radio to build a manufacturer 275 275 00:10:32,150 --> 00:10:33,330 at that time, 276 276 00:10:33,330 --> 00:10:35,780 but it did give us a good amount of speed 277 277 00:10:35,780 --> 00:10:38,670 because it operated at 54 megabits per second. 278 278 00:10:38,670 --> 00:10:40,870 This was really good in the late nineties, 279 279 00:10:40,870 --> 00:10:43,640 but again, unfortunately it costs a lot of money 280 280 00:10:43,640 --> 00:10:46,120 because of that high cost, only business users 281 281 00:10:46,120 --> 00:10:47,160 really ended up using it, 282 282 00:10:47,160 --> 00:10:49,160 and it wasn't really that commercially viable 283 283 00:10:49,160 --> 00:10:50,730 in the mainstream market. 284 284 00:10:50,730 --> 00:10:52,500 Now, since it wasn't getting as much traction 285 285 00:10:52,500 --> 00:10:53,730 in the commercial markets, 286 286 00:10:53,730 --> 00:10:56,210 they decided to make something cheaper and easier. 287 287 00:10:56,210 --> 00:10:59,130 So the manufacturers decided to create wireless B, 288 288 00:10:59,130 --> 00:11:01,860 which operates in the 2.4 GHz spectrum. 289 289 00:11:01,860 --> 00:11:03,920 Now, this frequency range is commonly used 290 290 00:11:03,920 --> 00:11:05,850 by a lot of other household devices, 291 291 00:11:05,850 --> 00:11:08,180 things like security cameras, walkie-talkies, 292 292 00:11:08,180 --> 00:11:10,690 baby monitors, microwaves, and more. 293 293 00:11:10,690 --> 00:11:13,820 Now, this made the radios and the 802.11b wireless devices 294 294 00:11:13,820 --> 00:11:15,420 very cheap and easy to get 295 295 00:11:15,420 --> 00:11:18,800 and it led to widespread adoption of Wi-Fi throughout homes, 296 296 00:11:18,800 --> 00:11:21,850 businesses, and schools bringing us to where we are today. 297 297 00:11:21,850 --> 00:11:23,730 Now using this cheaper chip set 298 298 00:11:23,730 --> 00:11:25,120 and the way the frequencies work, 299 299 00:11:25,120 --> 00:11:26,820 actually slowed down our networks. 300 300 00:11:26,820 --> 00:11:28,880 So we went from 54 megabits per second, 301 301 00:11:28,880 --> 00:11:30,930 down to 11 megabits per second, 302 302 00:11:30,930 --> 00:11:33,230 which today sounds extremely slow. 303 303 00:11:33,230 --> 00:11:35,970 But again, we're talking about the late 1990s here, 304 304 00:11:35,970 --> 00:11:37,630 and we weren't doing a lot of streaming video, 305 305 00:11:37,630 --> 00:11:40,290 and so 11 megabits per second was actually fast enough 306 306 00:11:40,290 --> 00:11:41,920 for most home users. 307 307 00:11:41,920 --> 00:11:44,250 Now over time though, networks got faster 308 308 00:11:44,250 --> 00:11:45,520 and we wanted more speed. 309 309 00:11:45,520 --> 00:11:49,300 And so wireless G came out as a replacement for wireless B. 310 310 00:11:49,300 --> 00:11:54,050 Now wireless 802.11g is also in the 2.4 GHz spectrum, 311 311 00:11:54,050 --> 00:11:57,150 but it operates at 54 megabits per second. 312 312 00:11:57,150 --> 00:11:59,710 Now, eventually we wanted to go even faster than this, 313 313 00:11:59,710 --> 00:12:01,660 so engineers kept working on new solutions 314 314 00:12:01,660 --> 00:12:03,710 and new ways to manipulate the frequencies. 315 315 00:12:03,710 --> 00:12:06,050 And eventually they came up with wireless N, 316 316 00:12:06,050 --> 00:12:08,220 which is also called Wi-Fi 4, 317 317 00:12:08,220 --> 00:12:10,730 since it was the fourth generation of Wi-Fi. 318 318 00:12:10,730 --> 00:12:13,740 Now 802.11n really wanted to increase speed. 319 319 00:12:13,740 --> 00:12:16,300 So moved back to the 5 GHz spectrum again, 320 320 00:12:16,300 --> 00:12:17,640 and this allowed to get up to speeds 321 321 00:12:17,640 --> 00:12:20,600 of 300 to 600 megabits per second. 322 322 00:12:20,600 --> 00:12:22,660 This allowed for really fast networks, 323 323 00:12:22,660 --> 00:12:24,100 but the big problem 324 324 00:12:24,100 --> 00:12:26,130 is that this newer 5 GHz spectrum 325 325 00:12:26,130 --> 00:12:28,340 wasn't compatible with all the existing devices 326 326 00:12:28,340 --> 00:12:30,790 are out there, because they were wireless B and G, 327 327 00:12:30,790 --> 00:12:33,050 and they operate at 2.4 GHz. 328 328 00:12:33,050 --> 00:12:36,640 So people were resistant to buying wireless N at first. 329 329 00:12:36,640 --> 00:12:38,780 Now to overcome this manufacturer started 330 330 00:12:38,780 --> 00:12:41,450 making hybrid devices that were market under the name 331 331 00:12:41,450 --> 00:12:43,950 wireless N, and these types of devices 332 332 00:12:43,950 --> 00:12:47,020 had a wireless access point with two sets of radios in them. 333 333 00:12:47,020 --> 00:12:49,277 One was for the 2.4 GHz spectrum, 334 334 00:12:49,277 --> 00:12:51,640 and one for the 5 GHz spectrum. 335 335 00:12:51,640 --> 00:12:55,010 This way, if you had a mixture devices that were 802.11b, 336 336 00:12:55,010 --> 00:12:57,070 and G, and N, you could connect 337 337 00:12:57,070 --> 00:12:59,300 to the slower 2.4 GHz spectrum, 338 338 00:12:59,300 --> 00:13:01,900 and it would support wireless B speeds, wireless G speeds, 339 339 00:13:01,900 --> 00:13:03,670 or newer wireless N speeds 340 340 00:13:03,670 --> 00:13:06,340 that went up to about 150 megabits per second. 341 341 00:13:06,340 --> 00:13:07,610 Now, if someone connected 342 342 00:13:07,610 --> 00:13:09,670 to the more modern wireless N radios 343 343 00:13:09,670 --> 00:13:12,140 using the 5 GHz spectrum, they could actually 344 344 00:13:12,140 --> 00:13:15,070 reach speeds up to 600 megabits per second, 345 345 00:13:15,070 --> 00:13:17,727 by using a technology known as MIMO. 346 346 00:13:17,727 --> 00:13:20,640 MIMO stands for multiple input and multiple output, 347 347 00:13:20,640 --> 00:13:21,920 which means that the access point 348 348 00:13:21,920 --> 00:13:24,790 could use multiple antennas to send and receive data, 349 349 00:13:24,790 --> 00:13:27,180 instead of putting it all through a single antenna, 350 350 00:13:27,180 --> 00:13:29,160 essentially your data was going to be split 351 351 00:13:29,160 --> 00:13:30,480 across multiple antennas, 352 352 00:13:30,480 --> 00:13:32,040 and it was received on the other end, 353 353 00:13:32,040 --> 00:13:34,420 it was multiplex back into a single data stream 354 354 00:13:34,420 --> 00:13:35,620 for processing. 355 355 00:13:35,620 --> 00:13:38,020 This is why you can see wireless and access points 356 356 00:13:38,020 --> 00:13:41,330 that have one, two, three, or even five antennas, 357 357 00:13:41,330 --> 00:13:42,850 because the more intense you had, 358 358 00:13:42,850 --> 00:13:46,030 the more data transfer they could support simultaneously. 359 359 00:13:46,030 --> 00:13:50,070 Next, we have wireless AC, which is also called Wi-Fi 5, 360 360 00:13:50,070 --> 00:13:52,450 or 802 11 AC. 361 361 00:13:52,450 --> 00:13:54,780 This was the fifth generation of Wi-Fi. 362 362 00:13:54,780 --> 00:13:57,150 Now wireless AC operates exclusively 363 363 00:13:57,150 --> 00:13:58,780 in the 5 GHz spectrum, 364 364 00:13:58,780 --> 00:14:00,700 and technically it does not provide 365 365 00:14:00,700 --> 00:14:02,610 any kind of backward compatibility. 366 366 00:14:02,610 --> 00:14:05,390 These 802.11ac networks can operate 367 367 00:14:05,390 --> 00:14:08,540 at speeds up to three gigabits per second or more. 368 368 00:14:08,540 --> 00:14:11,400 These networks are really fast in theory. 369 369 00:14:11,400 --> 00:14:13,200 Now to achieve these higher speeds, 370 370 00:14:13,200 --> 00:14:18,200 802.11ac networks, uses the technology known as MU-MIMO, 371 371 00:14:18,270 --> 00:14:21,420 which has multiple user, multiple input, multiple output. 372 372 00:14:21,420 --> 00:14:23,840 It's a newer variation of the MIMO technology 373 373 00:14:23,840 --> 00:14:27,400 that was first developed back with 802.11n networks. 374 374 00:14:27,400 --> 00:14:31,610 Now MU-MIMO is a multipath wireless communication technology 375 375 00:14:31,610 --> 00:14:34,210 that allows multiple users to access the wireless network 376 376 00:14:34,210 --> 00:14:36,560 and access point at the same time. 377 377 00:14:36,560 --> 00:14:38,460 This is different than a regular MIMO, 378 378 00:14:38,460 --> 00:14:40,930 where a single user supported at one time. 379 379 00:14:40,930 --> 00:14:43,220 And the access point switches between users 380 380 00:14:43,220 --> 00:14:45,400 to share the bandwidth across all the users 381 381 00:14:45,400 --> 00:14:46,840 who are requesting services. 382 382 00:14:46,840 --> 00:14:48,950 So if you only have one person requesting services, 383 383 00:14:48,950 --> 00:14:51,640 they get a really fast network, but if you have, or three, 384 384 00:14:51,640 --> 00:14:54,260 it starts slowing down because it just share the bandwidth. 385 385 00:14:54,260 --> 00:14:55,720 Essentially with MIMO, 386 386 00:14:55,720 --> 00:14:58,000 the wireless network acts more like a hub, 387 387 00:14:58,000 --> 00:15:00,890 but with MU-MIMO, it begins to act more like a switch 388 388 00:15:00,890 --> 00:15:03,460 and helps avoid collisions and congestion. 389 389 00:15:03,460 --> 00:15:05,290 Now, when it comes to wireless AC, 390 390 00:15:05,290 --> 00:15:08,730 some of the original and older AC devices actually still use 391 391 00:15:08,730 --> 00:15:10,370 the older MIMO technology. 392 392 00:15:10,370 --> 00:15:13,490 Whereas the newer wireless AC devices, will use the MU-MIMO 393 393 00:15:13,490 --> 00:15:14,990 for faster speeds. 394 394 00:15:14,990 --> 00:15:16,860 Now, this brings us to the latest generation 395 395 00:15:16,860 --> 00:15:20,190 of wireless networks, 802.11ax. 396 396 00:15:20,190 --> 00:15:22,960 Wireless AX is known as Wi-Fi 6, 397 397 00:15:22,960 --> 00:15:25,730 because it's the sixth generation of wireless networks. 398 398 00:15:25,730 --> 00:15:28,570 This was introduced in 2021, and it can be used 399 399 00:15:28,570 --> 00:15:31,490 in the 2.4 GHz and 5 GHz spectrum 400 400 00:15:31,490 --> 00:15:33,810 under the marketing term, Wi-Fi 6 401 401 00:15:33,810 --> 00:15:37,020 or in the newer and faster six GHz spectrum 402 402 00:15:37,020 --> 00:15:39,650 under the marketing term, Wi-Fi 6E 403 403 00:15:39,650 --> 00:15:41,740 or high efficiency Wi-Fi. 404 404 00:15:41,740 --> 00:15:44,870 Now these Wi-Fi 6 and Wi-Fi 6E networks, 405 405 00:15:44,870 --> 00:15:46,860 is 802.11ax networks, 406 406 00:15:46,860 --> 00:15:50,500 can reach speeds up to 9.6 gigabits per second, 407 407 00:15:50,500 --> 00:15:52,730 using MU-MIMO technology. 408 408 00:15:52,730 --> 00:15:55,984 Also, because these access points have both the 2.4 GHz 409 409 00:15:55,984 --> 00:15:58,090 and 5 GHz radios inside them, 410 410 00:15:58,090 --> 00:16:01,020 they are fully backwards compatible with all devices, 411 411 00:16:01,020 --> 00:16:05,870 including wireless A, B, G, N and AC. 412 412 00:16:05,870 --> 00:16:08,360 All right, for the exam, I want you to remember 413 413 00:16:08,360 --> 00:16:10,320 there are different wireless networks out there. 414 414 00:16:10,320 --> 00:16:15,300 These include A, B, G, N, AC and AX. 415 415 00:16:15,300 --> 00:16:18,140 You also need to remember that if it's a B, G, 416 416 00:16:18,140 --> 00:16:19,740 N or AX network, 417 417 00:16:19,740 --> 00:16:22,830 it's going to support 2.4 GHz as a spectrum. 418 418 00:16:22,830 --> 00:16:25,460 If it's A, N, AC or AX, 419 419 00:16:25,460 --> 00:16:28,050 it supports 5 GHz as a spectrum. 420 420 00:16:28,050 --> 00:16:30,020 You also need to remember the relative speeds 421 421 00:16:30,020 --> 00:16:31,800 of these different wireless devices, 422 422 00:16:31,800 --> 00:16:34,490 going from 11 megabits per second for wireless B, 423 423 00:16:34,490 --> 00:16:36,620 all the way up to the gigabits per second use 424 424 00:16:36,620 --> 00:16:38,900 in AC and AX networks. 425 425 00:16:38,900 --> 00:16:41,660 This is important for the exam, because on test day, 426 426 00:16:41,660 --> 00:16:43,407 you may get questions about frequencies, 427 427 00:16:43,407 --> 00:16:45,490 things like which of these frequencies 428 428 00:16:45,490 --> 00:16:47,480 do not support 5 GHz? 429 429 00:16:47,480 --> 00:16:50,080 And the answer would have to be either B or G, 430 430 00:16:50,080 --> 00:16:52,200 for wireless B and wireless G. 431 431 00:16:52,200 --> 00:16:54,310 Now, you may get a question asking you to select 432 432 00:16:54,310 --> 00:16:57,100 which wireless standard doesn't support 2.4 GHz. 433 433 00:16:57,100 --> 00:16:57,980 And in this case, 434 434 00:16:57,980 --> 00:17:01,110 you need to select wireless A or wireless AC. 435 435 00:17:01,110 --> 00:17:03,110 If they wanted to make it more difficult for you, 436 436 00:17:03,110 --> 00:17:04,110 they can ask the question 437 437 00:17:04,110 --> 00:17:06,970 as more of a troubleshooting scenario, for example, 438 438 00:17:06,970 --> 00:17:09,830 you're working as a network technician on an older laptop, 439 439 00:17:09,830 --> 00:17:12,600 and it's failing to connect to your wireless AC network, 440 440 00:17:12,600 --> 00:17:13,910 you check the laptop and see 441 441 00:17:13,910 --> 00:17:15,900 that it has a wireless B network card. 442 442 00:17:15,900 --> 00:17:17,160 What is the problem? 443 443 00:17:17,160 --> 00:17:18,780 Then you're going to find the answer that has something 444 444 00:17:18,780 --> 00:17:21,200 to do with the fact that there's a frequency mismatch, 445 445 00:17:21,200 --> 00:17:23,780 because wireless AC supports 5 GHz 446 446 00:17:23,780 --> 00:17:26,310 and wireless B supports 2.4 GHz. 447 447 00:17:26,310 --> 00:17:28,422 And therefore you can't connect to the network. 448 448 00:17:28,422 --> 00:17:31,160 Now, one more thing to keep in mind as you're studying, 449 449 00:17:31,160 --> 00:17:33,630 is that marketers sometimes mislabel things 450 450 00:17:33,630 --> 00:17:35,430 to make it easier for our consumers. 451 451 00:17:35,430 --> 00:17:36,430 But on test day, 452 452 00:17:36,430 --> 00:17:38,740 you have to go by the official standards. 453 453 00:17:38,740 --> 00:17:41,200 A great example of this is wireless AC, 454 454 00:17:41,200 --> 00:17:43,490 the 802.11ac standard. 455 455 00:17:43,490 --> 00:17:45,160 It only specifies operation 456 456 00:17:45,160 --> 00:17:47,290 in the 5 GHz frequency band. 457 457 00:17:47,290 --> 00:17:49,040 But if you go to the store 458 458 00:17:49,040 --> 00:17:51,340 and you find a wireless AC access point, 459 459 00:17:51,340 --> 00:17:53,748 the box will tell you, it supports both 5 GHz 460 460 00:17:53,748 --> 00:17:54,748 and 2.4 GHz. 461 461 00:17:55,630 --> 00:17:58,550 This is a lie, and you will get in trouble on the exam, 462 462 00:17:58,550 --> 00:18:01,300 if you pick this answer, because you think it's dual band, 463 463 00:18:01,300 --> 00:18:04,200 and it's not the truth is wireless AC, 464 464 00:18:04,200 --> 00:18:06,940 only operates in the 5 GHz spectrum. 465 465 00:18:06,940 --> 00:18:09,010 When you're buying that wireless AC access point 466 466 00:18:09,010 --> 00:18:11,417 at the store, and it says it supports both frequencies, 467 467 00:18:11,417 --> 00:18:15,070 it's actually a wireless access point with two radios in it. 468 468 00:18:15,070 --> 00:18:17,850 One radio is 5 GHz for wireless AC 469 469 00:18:17,850 --> 00:18:20,950 at speeds up to about 1300 megabits per second. 470 470 00:18:20,950 --> 00:18:24,440 The other one is a 2.4 GHz radio for wireless N, 471 471 00:18:24,440 --> 00:18:26,830 at speeds of up to 600 megabits per second, 472 472 00:18:26,830 --> 00:18:29,040 with a MIMO antenna configuration. 473 473 00:18:29,040 --> 00:18:31,880 Now, while in real life, your users really don't care, 474 474 00:18:31,880 --> 00:18:34,430 and they just say, hey, I have a wireless AC access point, 475 475 00:18:34,430 --> 00:18:37,630 and they think it supports both 5 GHz and 2.4 GHz, 476 476 00:18:37,630 --> 00:18:40,030 on the exam, you will get the question wrong, 477 477 00:18:40,030 --> 00:18:43,360 if you select 2.4 GHz for wireless AC. 478 478 00:18:43,360 --> 00:18:46,670 Remember wireless AC only supports 5 GHz 479 479 00:18:46,670 --> 00:18:48,140 for its operations. 480 480 00:18:48,140 --> 00:18:51,090 The only dual band standards we have are wireless N 481 481 00:18:51,090 --> 00:18:52,570 and wireless AX. 482 482 00:18:52,570 --> 00:18:55,245 Both of those support, both 2.4 GHz 483 483 00:18:55,245 --> 00:18:59,540 and 5 GHz frequency bands per the 802.11 standards. 484 484 00:18:59,540 --> 00:19:02,410 Now, let's talk about radio frequency interference, 485 485 00:19:02,410 --> 00:19:04,470 or RFI for a minute here. 486 486 00:19:04,470 --> 00:19:06,310 Radio frequency interference is caused 487 487 00:19:06,310 --> 00:19:07,720 when there are similar frequencies 488 488 00:19:07,720 --> 00:19:09,810 to wireless networks in your area. 489 489 00:19:09,810 --> 00:19:12,380 For example, I mentioned earlier that one of the reasons 490 490 00:19:12,380 --> 00:19:15,210 we went to 2.4 GHz for Wi-Fi B, 491 491 00:19:15,210 --> 00:19:17,100 was the fact that there was other videos out there 492 492 00:19:17,100 --> 00:19:18,030 that already used it. 493 493 00:19:18,030 --> 00:19:20,340 Things like baby monitors and cordless phones, 494 494 00:19:20,340 --> 00:19:23,040 and microwave ovens and other security devices. 495 495 00:19:23,040 --> 00:19:25,770 Now, this means that 2.4 GHz as a spectrum 496 496 00:19:25,770 --> 00:19:27,270 is fairly crowded. 497 497 00:19:27,270 --> 00:19:28,950 This is what made the radios cheap, 498 498 00:19:28,950 --> 00:19:30,503 but it made it very difficult for us 499 499 00:19:30,503 --> 00:19:33,120 because it causes a lot of interference. 500 500 00:19:33,120 --> 00:19:35,110 Over time as more and more devices moved 501 501 00:19:35,110 --> 00:19:36,800 into the 5 GHz spectrum, 502 502 00:19:36,800 --> 00:19:39,490 there's also more interference in that area too. 503 503 00:19:39,490 --> 00:19:41,640 All of these other electronics can cause interference 504 504 00:19:41,640 --> 00:19:43,230 with your wireless networks, 505 505 00:19:43,230 --> 00:19:45,600 so you have to think about these things as you're developing 506 506 00:19:45,600 --> 00:19:47,710 your networks and troubleshooting your networks. 507 507 00:19:47,710 --> 00:19:49,960 For example, if you have a 2.4 GHz 508 508 00:19:49,960 --> 00:19:52,680 wireless G network in use, and the access point happens 509 509 00:19:52,680 --> 00:19:54,480 to be sitting in the break room at the office, 510 510 00:19:54,480 --> 00:19:56,240 and every time somebody turns on the microwave 511 511 00:19:56,240 --> 00:19:58,660 to reheat their burrito, the network drops, 512 512 00:19:58,660 --> 00:20:01,075 this is probably because the 2.4 GHz frequency 513 513 00:20:01,075 --> 00:20:02,650 is being interfered with, 514 514 00:20:02,650 --> 00:20:04,390 by those microwaves that are operating 515 515 00:20:04,390 --> 00:20:06,420 in that same frequency band. 516 516 00:20:06,420 --> 00:20:08,560 In addition to all this frequency interference, 517 517 00:20:08,560 --> 00:20:11,200 you might also see things like physical interference. 518 518 00:20:11,200 --> 00:20:12,340 This is where physical things 519 519 00:20:12,340 --> 00:20:14,170 can block your wireless signals. 520 520 00:20:14,170 --> 00:20:15,980 For instance, I live in Puerto Rico 521 521 00:20:15,980 --> 00:20:18,910 and the walls in my house are solid concrete. 522 522 00:20:18,910 --> 00:20:21,150 I also have a refrigerator inside my kitchen, 523 523 00:20:21,150 --> 00:20:23,810 I have kitchen cabinets and those block the signal, 524 524 00:20:23,810 --> 00:20:26,560 all these things can cause signal strength issues for you. 525 525 00:20:26,560 --> 00:20:28,110 If your signals are too weak, 526 526 00:20:28,110 --> 00:20:30,490 and it can't make it around a corner or through a wall, 527 527 00:20:30,490 --> 00:20:32,150 that signal is going to get blocked, 528 528 00:20:32,150 --> 00:20:34,940 or it suffers what's known as a tenuation. 529 529 00:20:34,940 --> 00:20:36,790 All of these things can lead to interference, 530 530 00:20:36,790 --> 00:20:38,910 which will slow down your ability for your network 531 531 00:20:38,910 --> 00:20:40,710 to operate at top speed. 532 532 00:20:40,710 --> 00:20:42,530 As your signal decreases in strength 533 533 00:20:42,530 --> 00:20:44,440 or interference increases, 534 534 00:20:44,440 --> 00:20:46,950 we get a worse signal-to-noise ratio. 535 535 00:20:46,950 --> 00:20:49,140 This is going to cause additional retransmissions 536 536 00:20:49,140 --> 00:20:52,270 because most of the time we're sending things over TCP, 537 537 00:20:52,270 --> 00:20:55,870 when TCP retransmit, this creates additional network baggage 538 538 00:20:55,870 --> 00:20:58,210 that's being taken up and bandwidth is being used 539 539 00:20:58,210 --> 00:20:59,870 for all these retransmissions. 540 540 00:20:59,870 --> 00:21:02,440 And this slows down the network even more. 541 541 00:21:02,440 --> 00:21:03,850 You want to make sure you have good signal 542 542 00:21:03,850 --> 00:21:05,480 throughout your entire structure 543 543 00:21:05,480 --> 00:21:07,760 to increase the efficiency of your network. 544 544 00:21:07,760 --> 00:21:10,340 To do this, you would do what's called a site survey 545 545 00:21:10,340 --> 00:21:12,680 where you check the signal strength in different areas 546 546 00:21:12,680 --> 00:21:14,040 and make sure you have the right antennas 547 547 00:21:14,040 --> 00:21:16,470 and the right repeaters throughout the building. 548 548 00:21:16,470 --> 00:21:19,010 Finally, let's talk about how we actually send data 549 549 00:21:19,010 --> 00:21:20,850 over one of these wireless networks. 550 550 00:21:20,850 --> 00:21:22,590 With Ethernet, we talked about the fact 551 551 00:21:22,590 --> 00:21:25,250 that we use CSMA/CD, which was 552 552 00:21:25,250 --> 00:21:28,070 carrier sense multiple access/ collision detection. 553 553 00:21:28,070 --> 00:21:30,040 With wireless networks, we're going to use something 554 554 00:21:30,040 --> 00:21:32,290 known as CSMA/CA, 555 555 00:21:32,290 --> 00:21:35,820 which has carrier sense multiple access/collision avoidance. 556 556 00:21:35,820 --> 00:21:37,740 See here we've changed collision detection 557 557 00:21:37,740 --> 00:21:40,840 to collision avoidance, once we went to the wireless domain. 558 558 00:21:40,840 --> 00:21:44,450 Both CD and CA are going to start out the exact same way, 559 559 00:21:44,450 --> 00:21:45,790 in both of these network types, 560 560 00:21:45,790 --> 00:21:47,690 we're going to listen for transmissions. 561 561 00:21:47,690 --> 00:21:49,520 If we think the line is clear in the case 562 562 00:21:49,520 --> 00:21:53,120 of CD using Ethernet or CA we're using wireless, 563 563 00:21:53,120 --> 00:21:54,760 and the frequency has to be clear, 564 564 00:21:54,760 --> 00:21:56,530 we can then send a message. 565 565 00:21:56,530 --> 00:21:59,160 This is the carrier sense multiple access part 566 566 00:21:59,160 --> 00:22:00,330 of this stuff. 567 567 00:22:00,330 --> 00:22:01,930 Now, in the case of Ethernet, 568 568 00:22:01,930 --> 00:22:03,630 this is where we stopped doing anything. 569 569 00:22:03,630 --> 00:22:05,550 We're just going to do carrier sense multiple access 570 570 00:22:05,550 --> 00:22:06,580 collision detection. 571 571 00:22:06,580 --> 00:22:09,250 We're going to send our message and see if it crashes. 572 572 00:22:09,250 --> 00:22:10,880 This way, if there's a collision, 573 573 00:22:10,880 --> 00:22:12,620 we're going to just retransmit it. 574 574 00:22:12,620 --> 00:22:13,930 Now with wireless though, 575 575 00:22:13,930 --> 00:22:16,430 we want to try and prevent collisions ahead of time, 576 576 00:22:16,430 --> 00:22:19,730 because we said, retransmissions eat up valuable bandwidth. 577 577 00:22:19,730 --> 00:22:22,300 This is where collision avoidance comes in. 578 578 00:22:22,300 --> 00:22:23,910 As the device gets ready to transmit, 579 579 00:22:23,910 --> 00:22:25,830 it's going to listen to the frequency 580 580 00:22:25,830 --> 00:22:27,010 and make sure it's clear, 581 581 00:22:27,010 --> 00:22:28,890 and then it's going to send out a packet 582 582 00:22:28,890 --> 00:22:32,870 that's known as an RTS, which stands for Request To Send. 583 583 00:22:32,870 --> 00:22:35,460 The intended recipient usually the wireless access point 584 584 00:22:35,460 --> 00:22:38,210 on the network, will then acknowledge that Request To Send 585 585 00:22:38,210 --> 00:22:42,110 by sending a CTS packet, which stands for Clear To Send. 586 586 00:22:42,110 --> 00:22:44,860 Now, once my device sees the CTS packet, 587 587 00:22:44,860 --> 00:22:47,720 it's going to go ahead and send my data, because I was told, 588 588 00:22:47,720 --> 00:22:49,110 the whole frequency is clear, 589 589 00:22:49,110 --> 00:22:51,050 and it's ready for me to send something. 590 590 00:22:51,050 --> 00:22:53,170 Now, if we don't receive this CTS signal, 591 591 00:22:53,170 --> 00:22:54,870 this Clear To Send acknowledgement, 592 592 00:22:54,870 --> 00:22:56,990 then we're not going to start sending. 593 593 00:22:56,990 --> 00:22:59,540 Instead, I'm going to choose a random backoff timer, 594 594 00:22:59,540 --> 00:23:01,820 I'm going to wait for something like 30 milliseconds, 595 595 00:23:01,820 --> 00:23:05,170 and then I'll do another RTS or Request To Send. 596 596 00:23:05,170 --> 00:23:07,670 Now until I received that Clear To Send packet, 597 597 00:23:07,670 --> 00:23:09,900 I am not going to go and send my message, 598 598 00:23:09,900 --> 00:23:11,880 because I don't want to cause a collision. 599 599 00:23:11,880 --> 00:23:14,800 Remember every collision causes a retransmission, 600 600 00:23:14,800 --> 00:23:16,900 retransmissions take up valuable bandwidth, 601 601 00:23:16,900 --> 00:23:19,170 and that starts taking up additional resources 602 602 00:23:19,170 --> 00:23:20,460 and it becomes a negative spiral 603 603 00:23:20,460 --> 00:23:22,050 for our network's performance. 604 604 00:23:22,050 --> 00:23:26,020 So remember collision detection is used in wired networks, 605 605 00:23:26,020 --> 00:23:28,730 collision avoidance is used in wireless networks, 606 606 00:23:28,730 --> 00:23:31,990 and we won't send until we see that Clear To Send signal 607 607 00:23:31,990 --> 00:23:34,363 in response to are Ready To Send packet.