1 00:00:00,090 --> 00:00:02,310 If we start on the bottom of the OSI model, 2 00:00:02,310 --> 00:00:05,460 we'll find our first layer, the physical layer. 3 00:00:05,460 --> 00:00:08,370 Now, this is where bits are transmitted across the network 4 00:00:08,370 --> 00:00:09,750 and includes all the physical 5 00:00:09,750 --> 00:00:12,270 and electrical characteristics of this network. 6 00:00:12,270 --> 00:00:13,290 So, this is going to tell us 7 00:00:13,290 --> 00:00:15,210 whether we're using an ethernet network, 8 00:00:15,210 --> 00:00:17,610 whether we're using fiber or copper cables, 9 00:00:17,610 --> 00:00:19,980 whether we're using Cat 5 or Cat 6, 10 00:00:19,980 --> 00:00:23,670 and even if we're using radio frequency in the case of Wi-Fi. 11 00:00:23,670 --> 00:00:25,380 Regardless of which method we're using 12 00:00:25,380 --> 00:00:27,630 to send our data across this first layer, 13 00:00:27,630 --> 00:00:29,610 it's always going to occur as bits, 14 00:00:29,610 --> 00:00:31,080 and these are binary bits. 15 00:00:31,080 --> 00:00:34,170 These are going to be a series of ones and zeros. 16 00:00:34,170 --> 00:00:36,120 Now, each media has a different way 17 00:00:36,120 --> 00:00:39,300 of representing these bits, these series of ones and zeros, 18 00:00:39,300 --> 00:00:41,160 because these series of ones and zeros 19 00:00:41,160 --> 00:00:43,980 are the basic building blocks of all of our data. 20 00:00:43,980 --> 00:00:46,050 For example, if I'm using a copper wire, 21 00:00:46,050 --> 00:00:48,660 such as in a Cat 5 or a Cat 6 network, 22 00:00:48,660 --> 00:00:51,030 you may see that there's zero voltage on the wire 23 00:00:51,030 --> 00:00:52,410 when you have a zero bit. 24 00:00:52,410 --> 00:00:54,180 And then if you want to represent a one, 25 00:00:54,180 --> 00:00:56,700 you might use plus five or negative five volts 26 00:00:56,700 --> 00:00:58,350 on that copper wire. 27 00:00:58,350 --> 00:01:00,390 Now, when we switch between these two modes, 28 00:01:00,390 --> 00:01:02,340 this tells us whether we should read a one 29 00:01:02,340 --> 00:01:03,840 or a zero on the network. 30 00:01:03,840 --> 00:01:06,390 And this is called Transition Modulation. 31 00:01:06,390 --> 00:01:08,370 Now, for the exam, you don't need to understand 32 00:01:08,370 --> 00:01:10,980 the specifics of transition modulation, 33 00:01:10,980 --> 00:01:13,410 but you should understand this basic concept. 34 00:01:13,410 --> 00:01:16,770 On this wire, we're going to have one level that represents one 35 00:01:16,770 --> 00:01:19,380 and another level that represents zero. 36 00:01:19,380 --> 00:01:20,970 Let me give you another example. 37 00:01:20,970 --> 00:01:24,000 This time, let's pretend we're using a fiber optic cable. 38 00:01:24,000 --> 00:01:25,320 Now, with fiber optic cables, 39 00:01:25,320 --> 00:01:27,780 we're going to use light instead of voltages. 40 00:01:27,780 --> 00:01:29,640 Now, it's similar to the way we did voltages, 41 00:01:29,640 --> 00:01:31,680 but instead when we want to represent a one, 42 00:01:31,680 --> 00:01:33,150 we turn the light on. 43 00:01:33,150 --> 00:01:35,970 If we want to represent a zero, we turn the light off. 44 00:01:35,970 --> 00:01:37,560 Now, we can just read the state of the light. 45 00:01:37,560 --> 00:01:39,240 Is it on? That's a one. 46 00:01:39,240 --> 00:01:40,950 Is it off? That's a zero. 47 00:01:40,950 --> 00:01:42,600 And when there's a transition between, 48 00:01:42,600 --> 00:01:44,790 that tells us between these two modes, 49 00:01:44,790 --> 00:01:47,640 whether we should be reading this as a one or a zero. 50 00:01:47,640 --> 00:01:49,260 Now, as we start understanding that 51 00:01:49,260 --> 00:01:51,270 we then have to look at the cables themselves, 52 00:01:51,270 --> 00:01:53,670 because this is also part of our physical layer. 53 00:01:53,670 --> 00:01:56,850 If we're using something like a Cat 5 or a Cat 6 cable, 54 00:01:56,850 --> 00:02:00,660 we may have a certain connector on the end called an RJ45, 55 00:02:00,660 --> 00:02:02,190 which allows us to plug that cable 56 00:02:02,190 --> 00:02:04,860 into the back of a computer or into a switch. 57 00:02:04,860 --> 00:02:07,050 Now, the way that connector is wired 58 00:02:07,050 --> 00:02:08,940 is based on a certain standard. 59 00:02:08,940 --> 00:02:11,340 We use two standards inside our network, 60 00:02:11,340 --> 00:02:16,080 TIA/EIA-568A and TIA/EIA-568B. 61 00:02:17,190 --> 00:02:18,480 Now, we'll talk about these 62 00:02:18,480 --> 00:02:20,700 and which way these pins actually are set up 63 00:02:20,700 --> 00:02:23,370 inside this connector in a future lesson. 64 00:02:23,370 --> 00:02:24,540 This is going to be important, 65 00:02:24,540 --> 00:02:26,580 because as we start talking about these, 66 00:02:26,580 --> 00:02:27,840 this is going to tell us whether or not 67 00:02:27,840 --> 00:02:31,140 we're using crossover cables or straight-through cables. 68 00:02:31,140 --> 00:02:32,640 If we use a crossover cable, 69 00:02:32,640 --> 00:02:34,500 we're actually going to flip the transmission 70 00:02:34,500 --> 00:02:36,660 and receive bits on the end of the cable. 71 00:02:36,660 --> 00:02:38,730 So, one end will be the A standard 72 00:02:38,730 --> 00:02:40,590 and one end will be the B standard. 73 00:02:40,590 --> 00:02:41,423 But if we're using 74 00:02:41,423 --> 00:02:43,470 a straight-through cable or a patch cable, 75 00:02:43,470 --> 00:02:46,470 we're going to have the B standard on both sides. 76 00:02:46,470 --> 00:02:48,060 Now again, it's important to understand 77 00:02:48,060 --> 00:02:49,260 these wiring standards, 78 00:02:49,260 --> 00:02:51,870 so we're going to spend some time on them in a future lesson, 79 00:02:51,870 --> 00:02:54,090 because on the exam you may be asked 80 00:02:54,090 --> 00:02:56,400 to wire up an ethernet jack. 81 00:02:56,400 --> 00:02:58,860 Maybe they're going to tell you to make a crossover cable, 82 00:02:58,860 --> 00:03:01,680 And you have to drag the right colors to the right pins, 83 00:03:01,680 --> 00:03:02,790 that kind of a thing. 84 00:03:02,790 --> 00:03:04,110 And so, to make sure you're ready for that, 85 00:03:04,110 --> 00:03:06,630 we're going to cover that in a separate lesson. 86 00:03:06,630 --> 00:03:09,390 Now, at this point, we've talked about having our cables, 87 00:03:09,390 --> 00:03:12,060 we've talked about how we represent bits on those cables, 88 00:03:12,060 --> 00:03:13,470 and we talked about how we're going to set up 89 00:03:13,470 --> 00:03:15,030 the connectors of those cables, 90 00:03:15,030 --> 00:03:16,440 but there's another thing we have to think about 91 00:03:16,440 --> 00:03:17,790 at the physical layer, 92 00:03:17,790 --> 00:03:19,980 and that's the topology of the network. 93 00:03:19,980 --> 00:03:21,720 How are we actually running these cables 94 00:03:21,720 --> 00:03:24,510 to physically connect the different devices together? 95 00:03:24,510 --> 00:03:27,150 Well, when we look at this from a layer 1 perspective, 96 00:03:27,150 --> 00:03:28,770 we can look at this based on the things 97 00:03:28,770 --> 00:03:30,960 we talked about in the last section of the course. 98 00:03:30,960 --> 00:03:32,850 Is it a bus? Is it a ring? 99 00:03:32,850 --> 00:03:35,190 Is it a star? Is it a hub and spoke? 100 00:03:35,190 --> 00:03:37,410 How about a full mesh, a partial mesh, 101 00:03:37,410 --> 00:03:39,508 or any other topology that we discussed? 102 00:03:39,508 --> 00:03:41,310 When it comes to figuring this out, 103 00:03:41,310 --> 00:03:43,620 you're going to look at how they're physically cabled, 104 00:03:43,620 --> 00:03:47,310 if you drew them out, does it make a line like a bus, 105 00:03:47,310 --> 00:03:50,070 a ring going in a circle, or a star pattern? 106 00:03:50,070 --> 00:03:52,680 And that'll tell you what physical topology you have. 107 00:03:52,680 --> 00:03:55,290 This again, is a layer 1 issue. 108 00:03:55,290 --> 00:03:57,270 Another issue that we have to concern ourself with 109 00:03:57,270 --> 00:04:00,270 at layer 1 is synchronizing our communications. 110 00:04:00,270 --> 00:04:03,510 We have to ask ourself, how does the receiving end know 111 00:04:03,510 --> 00:04:05,370 if it's ready to accept ones and zeros 112 00:04:05,370 --> 00:04:06,840 that we're going to send it? 113 00:04:06,840 --> 00:04:09,030 Now, this sounds like a really easy thing to do, 114 00:04:09,030 --> 00:04:10,080 if we're talking to each other, 115 00:04:10,080 --> 00:04:12,930 but with computers this can get much more complicated. 116 00:04:12,930 --> 00:04:14,730 So, to make sure that we understand this, 117 00:04:14,730 --> 00:04:16,529 we have two things that can happen. 118 00:04:16,529 --> 00:04:17,730 It can either be transmitted 119 00:04:17,730 --> 00:04:20,579 asynchronously or synchronously. 120 00:04:20,579 --> 00:04:23,100 Now, when I'm looking at asynchronous communication, 121 00:04:23,100 --> 00:04:25,650 you should be able to consider something like a voicemail. 122 00:04:25,650 --> 00:04:27,510 You call up your friend, they don't answer, 123 00:04:27,510 --> 00:04:30,240 and so you leave a message so they can listen to it later, 124 00:04:30,240 --> 00:04:33,360 the communication happens out of sync or out of time. 125 00:04:33,360 --> 00:04:34,890 You do it and then later on 126 00:04:34,890 --> 00:04:36,780 they can go back and listen to it. 127 00:04:36,780 --> 00:04:40,350 Now in networks, this happens via a start and stop bit, 128 00:04:40,350 --> 00:04:41,520 similarly to how your friend 129 00:04:41,520 --> 00:04:43,140 can press play on their voicemail system 130 00:04:43,140 --> 00:04:44,400 to listen to their message, 131 00:04:44,400 --> 00:04:46,230 a network can send a start bit 132 00:04:46,230 --> 00:04:48,300 when it wants to start beginning the transmission, 133 00:04:48,300 --> 00:04:50,467 and then a stop bit to tell the other side, 134 00:04:50,467 --> 00:04:52,200 "Hey, I'm done transmitting. 135 00:04:52,200 --> 00:04:54,420 You've gotten everything I'm going to send." 136 00:04:54,420 --> 00:04:55,890 Now, if we decide to go and do 137 00:04:55,890 --> 00:04:58,290 communications synchronously, on the other hand, 138 00:04:58,290 --> 00:05:01,140 we have to be in the same place at the same time. 139 00:05:01,140 --> 00:05:02,700 So, in our previous example, 140 00:05:02,700 --> 00:05:04,230 if your friend picked up the phone 141 00:05:04,230 --> 00:05:05,730 and you had a conversation, 142 00:05:05,730 --> 00:05:07,800 you could talk to them and they could talk to you. 143 00:05:07,800 --> 00:05:09,660 This is a synchronized conversation, 144 00:05:09,660 --> 00:05:12,480 because you're both talking at the same time. 145 00:05:12,480 --> 00:05:14,580 Now, this communication happens in real time. 146 00:05:14,580 --> 00:05:16,800 That's what's great about synchronization. 147 00:05:16,800 --> 00:05:18,330 Now, as far as when we start talking 148 00:05:18,330 --> 00:05:20,280 about this from a network perspective, 149 00:05:20,280 --> 00:05:23,610 instead of using a start and a stop bit for synchronizing, 150 00:05:23,610 --> 00:05:26,580 we would use some sort of common time source. 151 00:05:26,580 --> 00:05:28,770 And so, maybe we're all going to use a clock, 152 00:05:28,770 --> 00:05:33,090 and every time a second passes, we can transmit and receive, 153 00:05:33,090 --> 00:05:34,770 and that tells us that we're going to do it 154 00:05:34,770 --> 00:05:36,630 on the cadence of the beat. 155 00:05:36,630 --> 00:05:38,850 That would be something that is synchronous. 156 00:05:38,850 --> 00:05:40,140 Now, in addition to figuring out, 157 00:05:40,140 --> 00:05:42,390 if it's going to be asynchronous or synchronous, 158 00:05:42,390 --> 00:05:43,470 you also have to figure out 159 00:05:43,470 --> 00:05:46,290 how you're going to utilize the bandwidth of the cable. 160 00:05:46,290 --> 00:05:48,420 And there's two main ways that you can do this. 161 00:05:48,420 --> 00:05:51,390 One is called broadband and one is called baseband. 162 00:05:51,390 --> 00:05:53,550 Now, broadband is going to divide our bandwidth 163 00:05:53,550 --> 00:05:55,110 into separate channels. 164 00:05:55,110 --> 00:05:57,000 If you have a TV service at your house, 165 00:05:57,000 --> 00:05:58,410 you're probably familiar with this, 166 00:05:58,410 --> 00:06:00,750 because you have a single cable coming into your house, 167 00:06:00,750 --> 00:06:03,210 but it carries 200 or more channels. 168 00:06:03,210 --> 00:06:05,340 The user then is going to choose a single channel, 169 00:06:05,340 --> 00:06:07,350 and the rest are going to be filtered out. 170 00:06:07,350 --> 00:06:09,630 In opposition to this, we have baseband 171 00:06:09,630 --> 00:06:11,250 where you're going to use all of the frequency 172 00:06:11,250 --> 00:06:13,140 of the cable all of the time. 173 00:06:13,140 --> 00:06:16,620 So, a telephone for instance, uses baseband communication, 174 00:06:16,620 --> 00:06:18,360 which means that when you pick up the phone, 175 00:06:18,360 --> 00:06:21,630 you're using all the bandwidth allocated to that phone line. 176 00:06:21,630 --> 00:06:24,030 This doesn't hold true with the cable TV signal, right? 177 00:06:24,030 --> 00:06:25,380 Because we had 200 channels 178 00:06:25,380 --> 00:06:27,240 all using some of that bandwidth, 179 00:06:27,240 --> 00:06:29,246 but we only pulled out the ones we wanted. 180 00:06:29,246 --> 00:06:31,350 For this reason, we can only make 181 00:06:31,350 --> 00:06:33,180 one call at a time when using a phone, 182 00:06:33,180 --> 00:06:36,360 but we can have 200 channels or more on our TV. 183 00:06:36,360 --> 00:06:39,750 Now, when we use baseband, we're going to use a reference clock 184 00:06:39,750 --> 00:06:41,460 that allows us to send the information 185 00:06:41,460 --> 00:06:44,640 for both the sender and the receiver at this certain time. 186 00:06:44,640 --> 00:06:46,260 By using this reference clock, 187 00:06:46,260 --> 00:06:49,620 this is an example of using asynchronous communication. 188 00:06:49,620 --> 00:06:51,900 Another good example of a baseband network 189 00:06:51,900 --> 00:06:54,060 is a wired home ethernet network, 190 00:06:54,060 --> 00:06:56,460 because this is going to use all of the frequency 191 00:06:56,460 --> 00:06:58,320 that's available on your cable, 192 00:06:58,320 --> 00:07:00,330 giving you more bandwidth than you would, 193 00:07:00,330 --> 00:07:02,700 if you had a broadband area. 194 00:07:02,700 --> 00:07:05,250 Now, in this case, if we have a single baseband 195 00:07:05,250 --> 00:07:06,960 using up all of the bandwidth, 196 00:07:06,960 --> 00:07:09,270 we need to figure out how to get more out of it. 197 00:07:09,270 --> 00:07:10,110 And so to do this, 198 00:07:10,110 --> 00:07:12,510 we have a couple of different mechanisms we can do, 199 00:07:12,510 --> 00:07:13,470 and the first one of these 200 00:07:13,470 --> 00:07:16,050 is what's known as Time-Division Multiplexing. 201 00:07:16,050 --> 00:07:18,450 In this mode, each session is going to take turns 202 00:07:18,450 --> 00:07:20,910 using a dedicated time slot to get 203 00:07:20,910 --> 00:07:23,430 part of that bandwidth from the baseband. 204 00:07:23,430 --> 00:07:24,930 Now, an easy analogy to this 205 00:07:24,930 --> 00:07:27,240 is if you have a house with a single TV in it, 206 00:07:27,240 --> 00:07:29,190 but you have four family members. 207 00:07:29,190 --> 00:07:32,400 Everyone wants to watch TV, but there's only one TV, 208 00:07:32,400 --> 00:07:33,600 so they're going to have to take turns 209 00:07:33,600 --> 00:07:35,700 picking what program's going to be on. 210 00:07:35,700 --> 00:07:38,580 Now, in a pure time-division multiplexing environment, 211 00:07:38,580 --> 00:07:41,250 each person is going to be assigned a time slot 212 00:07:41,250 --> 00:07:43,050 and they can pick whatever TV show they want 213 00:07:43,050 --> 00:07:44,430 during that time slot. 214 00:07:44,430 --> 00:07:46,050 Now, this may or may not line up 215 00:07:46,050 --> 00:07:47,970 with the time that their show is actually on, 216 00:07:47,970 --> 00:07:49,830 and that could cause a problem, right? 217 00:07:49,830 --> 00:07:51,240 So, we have this second method 218 00:07:51,240 --> 00:07:55,440 called Statistical Time-Division Multiplexing, or StatTDM. 219 00:07:55,440 --> 00:07:56,910 This is a more efficient version 220 00:07:56,910 --> 00:07:58,590 of Time-Division Multiplexing, 221 00:07:58,590 --> 00:08:01,500 because it's going to dynamically allocate these time slots 222 00:08:01,500 --> 00:08:03,510 based on when people need it. 223 00:08:03,510 --> 00:08:06,690 So, if we take our TV example, maybe for example 224 00:08:06,690 --> 00:08:09,570 I want to go down and nobody's watching TV at eight o'clock, 225 00:08:09,570 --> 00:08:11,310 but it wasn't my time slot. 226 00:08:11,310 --> 00:08:13,290 Well, under Time-Division Multiplexing, 227 00:08:13,290 --> 00:08:16,350 I couldn't turn on the TV, because it wasn't my time slot, 228 00:08:16,350 --> 00:08:18,570 but with StatTDM I can, 229 00:08:18,570 --> 00:08:20,790 because as long as nobody's using the TV, 230 00:08:20,790 --> 00:08:22,380 anyone's free to use it. 231 00:08:22,380 --> 00:08:24,870 Everyone's going to take turns based on their necessity, 232 00:08:24,870 --> 00:08:26,760 not based on the time itself. 233 00:08:26,760 --> 00:08:29,550 So instead, when I start watching the TV at eight o'clock, 234 00:08:29,550 --> 00:08:30,900 then after my 30 minutes is up, 235 00:08:30,900 --> 00:08:33,600 I'm going to get off at 08:30 so somebody else can get on. 236 00:08:33,600 --> 00:08:36,299 Even though my assigned time slot under TDM 237 00:08:36,299 --> 00:08:38,640 might've been something like 09:00 to 09:30. 238 00:08:38,640 --> 00:08:40,380 Now, our last method is what's known 239 00:08:40,380 --> 00:08:43,770 as Frequency-Division Multiplexing, or FDM. 240 00:08:43,770 --> 00:08:46,560 This is going to involve taking the medium, that cable, 241 00:08:46,560 --> 00:08:48,210 and splitting it up into channels 242 00:08:48,210 --> 00:08:50,400 similar to the way we do in broadband. 243 00:08:50,400 --> 00:08:53,511 So, if I take a single cable and I break it up into 50, 244 00:08:53,511 --> 00:08:55,770 100, or 200 different frequencies, 245 00:08:55,770 --> 00:08:57,870 then each person can get a small portion 246 00:08:57,870 --> 00:08:59,340 of frequency allotted to them 247 00:08:59,340 --> 00:09:01,384 and they can use it as much as they want. 248 00:09:01,384 --> 00:09:03,990 Now, for the exam, the good news is 249 00:09:03,990 --> 00:09:07,950 you don't need to memorize TDM, StatTDM, and FDM, 250 00:09:07,950 --> 00:09:09,780 but rather you just need to understand that 251 00:09:09,780 --> 00:09:13,800 multiplexing involves taking some limited amount of resource 252 00:09:13,800 --> 00:09:15,690 and using it more efficiently. 253 00:09:15,690 --> 00:09:16,740 In the real world, 254 00:09:16,740 --> 00:09:19,110 you may come across these multiplexing techniques, 255 00:09:19,110 --> 00:09:20,460 especially if you start working 256 00:09:20,460 --> 00:09:22,920 as a network engineer or a network architect, 257 00:09:22,920 --> 00:09:24,720 and that's why I wanted to introduce 'em to you. 258 00:09:24,720 --> 00:09:26,490 But for the Network Plus exam, 259 00:09:26,490 --> 00:09:30,210 just remember, multiplexing allows multiple people 260 00:09:30,210 --> 00:09:33,831 to use a baseband connection at the same time. 261 00:09:33,831 --> 00:09:36,710 The final thing we need to talk about in this lesson 262 00:09:36,710 --> 00:09:40,500 is some examples of physical or layer 1 devices. 263 00:09:40,500 --> 00:09:42,840 The most common one is going to be a cable. 264 00:09:42,840 --> 00:09:44,520 So, if I have a fiber optic cable, 265 00:09:44,520 --> 00:09:47,370 or an ethernet cable, or a coaxial cable, 266 00:09:47,370 --> 00:09:49,590 these are all different types of media. 267 00:09:49,590 --> 00:09:50,880 And if I have different types of media, 268 00:09:50,880 --> 00:09:53,370 that's considered a layer 1 device. 269 00:09:53,370 --> 00:09:55,830 The reason is, whatever goes in one end of the cable 270 00:09:55,830 --> 00:09:57,720 is going to come out the other end of the cable. 271 00:09:57,720 --> 00:09:59,010 So, if I have a fiber optic cable 272 00:09:59,010 --> 00:10:00,330 and I put light in one end, 273 00:10:00,330 --> 00:10:02,040 I'm going to get light out the other end. 274 00:10:02,040 --> 00:10:03,720 That's a physical response, 275 00:10:03,720 --> 00:10:06,210 a physical layer of the OSI model. 276 00:10:06,210 --> 00:10:08,280 Additionally, beyond wired cables, 277 00:10:08,280 --> 00:10:10,050 we also have wireless things. 278 00:10:10,050 --> 00:10:12,120 Things like Bluetooth, and Wi-Fi, 279 00:10:12,120 --> 00:10:13,710 and Near Field Communication (NFC). 280 00:10:13,710 --> 00:10:16,530 All of these radio frequencies make up the media 281 00:10:16,530 --> 00:10:19,980 at layer 1 for those type of networks. 282 00:10:19,980 --> 00:10:22,740 The final example is infrastructure devices, 283 00:10:22,740 --> 00:10:24,390 and that would be things like hubs, 284 00:10:24,390 --> 00:10:26,790 access points, and media converters. 285 00:10:26,790 --> 00:10:30,150 All of these devices operate at the bit layer. 286 00:10:30,150 --> 00:10:31,320 This is going to be a function 287 00:10:31,320 --> 00:10:33,150 should just simply repeat what they get. 288 00:10:33,150 --> 00:10:35,670 So, if I have a hub, whatever goes in port one of the hub 289 00:10:35,670 --> 00:10:38,250 is coming out ports two, three, and four, 290 00:10:38,250 --> 00:10:40,350 whatever comes in, gets repeated out. 291 00:10:40,350 --> 00:10:42,000 The same thing with a media converter. 292 00:10:42,000 --> 00:10:44,490 If I have something coming in over coaxial, 293 00:10:44,490 --> 00:10:46,260 it's going to get converted through media 294 00:10:46,260 --> 00:10:48,150 and pushed out over fiber optic. 295 00:10:48,150 --> 00:10:51,720 That device is simply doing it at the physical layer, 296 00:10:51,720 --> 00:10:53,400 and whatever comes in is going to go out. 297 00:10:53,400 --> 00:10:54,570 There's no logic to it. 298 00:10:54,570 --> 00:10:56,220 There's no intelligence to it. 299 00:10:56,220 --> 00:10:59,850 Layer 1 devices simply repeat whatever they're told. 300 00:10:59,850 --> 00:11:02,010 Now, we'll talk about some other infrastructure devices 301 00:11:02,010 --> 00:11:04,680 as we get into layers 2 and 3 and 4, 302 00:11:04,680 --> 00:11:06,420 but for right now, I want you to remember 303 00:11:06,420 --> 00:11:10,320 layer 1 is dumb devices, they're simply repeaters. 304 00:11:10,320 --> 00:11:12,813 Whatever they take in, they send right back out.