WEBVTT 00:00:00.000 --> 00:00:03.950 Welcome back to CSE 316 — Data Communication and Networking. 00:00:04.000 --> 00:00:07.380 This is the detailed video version of Session 2. 00:00:07.430 --> 00:00:16.380 It runs a little over an hour and a half, in six sections, and it is meant to be used the way you would use a textbook chapter — in pieces, with the pause button, and again before an exam. 00:00:21.210 --> 00:00:27.520 There is a checkpoint at the end of each section so you can tell whether to go on or go back. 00:00:27.570 --> 00:00:30.660 Here is the shape of it. 00:00:30.710 --> 00:00:38.400 First we make the word 'network' precise — four terms, and three tests every network must pass. 00:00:38.450 --> 00:00:44.790 Then the heart of the session: the four physical topologies — mesh, star, bus, ring. 00:00:44.840 --> 00:00:53.220 Then we zoom out to LAN, WAN and the Internet, and meet the two ways data is switched across all of it. 00:00:53.270 --> 00:00:57.380 And at the end, we answer the question I am about to ask you. 00:00:57.430 --> 00:01:03.370 Last session ended with a task: find out what topology the GPL lab uses. 00:01:03.420 --> 00:01:11.517 Keep your answer in hand — this session is where you find out whether you were right. 00:01:11.567 --> 00:01:14.727 Before I start, picture the GPL lab downstairs. 00:01:14.777 --> 00:01:16.647 Forty computers, in rows. 00:01:16.697 --> 00:01:20.377 Here is the question. 00:01:20.427 --> 00:01:28.807 Suppose we gave every computer its own direct cable to every other computer — no sharing, every pair wired privately. 00:01:28.857 --> 00:01:31.887 How many cables would that take? 00:01:31.937 --> 00:01:35.267 And second: how many cables does the lab actually use? 00:01:35.317 --> 00:01:44.127 Pause the video and commit to two numbers — better, write them down somewhere. 00:01:44.177 --> 00:01:53.127 Section six computes both numbers exactly, so you can check your guess: one of them grows quadratically with the number of machines, the other linearly. 00:01:54.337 --> 00:01:58.547 The gap between those two numbers is the whole session. 00:01:58.597 --> 00:02:07.547 Today a network stops being a vague cloud and becomes a shape — and the shape decides two things: what the network costs to build, and what dies when something breaks. 00:02:09.277 --> 00:02:18.227 Every network you will ever design or debug is a bet between those two, and this session prices the bet. 00:02:19.091 --> 00:02:20.351 Section one. 00:02:20.401 --> 00:02:22.351 A network, precisely. 00:02:22.401 --> 00:02:31.351 Four words you will use every week for twelve weeks — network, host, connecting device, link — and then the three tests every network must pass. 00:02:32.511 --> 00:02:40.065 As always, I will use these words exactly, and so will the exam. 00:02:40.115 --> 00:02:45.395 Let us start with the noun itself, from Forouzan section 1.2. 00:02:45.445 --> 00:02:51.465 A network is the interconnection of a set of devices capable of communication. 00:02:51.515 --> 00:02:53.605 Notice how little it demands. 00:02:53.655 --> 00:02:58.925 Two computers and a plug-and-play router in your flat — that is already a network. 00:02:58.975 --> 00:03:00.535 Small, but legal. 00:03:00.585 --> 00:03:05.485 Nothing about size, nothing about the Internet, nothing about cost. 00:03:05.535 --> 00:03:13.275 Inside a network there are two kinds of citizens, and the first kind is the host — also called an end system. 00:03:13.325 --> 00:03:21.215 A host is a device that runs your programs: a desktop, a laptop, a phone, the security camera over the door. 00:03:21.265 --> 00:03:24.295 Hosts are the reason the network exists. 00:03:24.345 --> 00:03:30.905 The second kind is the connecting device — a device that exists for the network's own sake. 00:03:30.955 --> 00:03:35.395 Three of them matter now: a router joins networks to other networks. 00:03:35.445 --> 00:03:38.635 A switch joins devices inside one network. 00:03:38.685 --> 00:03:41.345 A modem changes the form of the data. 00:03:41.395 --> 00:03:47.555 Keep those three verbs — joins networks, joins devices, changes form. 00:03:47.605 --> 00:03:54.005 And the link: a communication pathway that transfers data from one device to another. 00:03:54.055 --> 00:03:59.555 Wired or wireless — a cable behind the wall or a radio channel through it. 00:03:59.605 --> 00:04:04.805 Both are links, exactly as both were transmission media in Session 1. 00:04:04.855 --> 00:04:06.005 Four terms. 00:04:06.055 --> 00:04:13.731 The next slide takes the middle two apart, because they are the pair students blur. 00:04:13.781 --> 00:04:21.011 The host column you already understand, because you own most of it: the desktop, the laptop, the phone, the camera. 00:04:21.061 --> 00:04:26.921 The test is simple — if something of yours runs on it, it is a host. 00:04:26.971 --> 00:04:32.531 Hosts are the ends of every communication we will ever draw in this course. 00:04:32.581 --> 00:04:38.101 Now the other column, and read it as a job description rather than a shopping list. 00:04:38.151 --> 00:04:41.051 A router joins networks to other networks. 00:04:41.101 --> 00:04:44.291 A switch joins devices inside one network. 00:04:44.341 --> 00:04:53.161 A modem changes the form of the data — from the digital form your computer speaks to whatever form the line outside your building carries. 00:04:53.211 --> 00:04:58.281 And the last line is the whole distinction: nothing of yours runs on any of them. 00:04:58.331 --> 00:05:00.231 So apply the test. 00:05:00.281 --> 00:05:08.681 Is a router a host? No. Nothing of yours runs on it; it exists so that other devices can reach each other. 00:05:08.731 --> 00:05:17.681 That sounds pedantic today, and it carries the whole of Chapter 2 — because a LAN (Local Area Network) connects hosts, and a WAN (Wide Area Network) interconnects connecting devices. 00:05:21.841 --> 00:05:27.431 Nail the distinction now, while it is cheap. 00:05:27.481 --> 00:05:29.441 So that is what a network is. 00:05:29.491 --> 00:05:32.881 The next question: what makes a network any good? 00:05:32.931 --> 00:05:38.331 Forouzan gives three criteria, and you should be able to name all three cold. 00:05:38.381 --> 00:05:39.881 Performance. 00:05:39.931 --> 00:05:42.261 Is it fast enough? 00:05:42.311 --> 00:05:51.261 We measure transit time and response time, and behind both sit the two working numbers we will use all term: throughput — how much data gets through per second — and delay — how long each piece takes. 00:05:55.191 --> 00:05:57.821 We want more throughput and less delay. 00:05:57.871 --> 00:05:58.681 Reliability. 00:05:58.731 --> 00:06:00.941 Is it up enough? 00:06:00.991 --> 00:06:04.271 And note this is not just 'does it work'. 00:06:04.321 --> 00:06:13.271 It is three sharper questions: how often does it fail, how fast does a link recover, and how does the design behave in a catastrophe? 00:06:13.341 --> 00:06:14.831 Security. 00:06:14.881 --> 00:06:17.151 Is it safe enough? 00:06:17.201 --> 00:06:26.151 Protect data from unauthorized access, protect it from damage — and the part everyone forgets: have policies for recovery after a breach. 00:06:26.621 --> 00:06:28.441 Assume the breach. 00:06:28.491 --> 00:06:31.241 And now the part that costs you. 00:06:31.291 --> 00:06:33.291 Throughput and delay fight. 00:06:33.341 --> 00:06:40.121 Push more data into the network and throughput may rise — but the traffic jams push the delay up with it. 00:06:40.171 --> 00:06:44.051 The two numbers we want to improve are often contradictory. 00:06:44.101 --> 00:06:53.051 It returns in Session 6 with formulas attached, and it is the reason network design is engineering rather than shopping. 00:06:54.778 --> 00:06:59.948 One more distinction before the shapes, because every shape is built out of it. 00:06:59.998 --> 00:07:03.008 There are two ways for devices to own a link. 00:07:03.058 --> 00:07:04.528 Point-to-point. 00:07:04.578 --> 00:07:10.788 A link between exactly two devices — A and B — and the entire capacity of the link is reserved for them. 00:07:10.838 --> 00:07:14.148 One hundred percent, all the time. 00:07:14.198 --> 00:07:17.138 Nobody else may touch it. 00:07:17.188 --> 00:07:20.548 Keep the word 'dedicated'; the mesh leans on it. 00:07:20.598 --> 00:07:22.108 Multipoint. 00:07:22.158 --> 00:07:30.288 More than two devices share one link — here, a mainframe and three stations hanging off the same line. 00:07:30.338 --> 00:07:39.288 Now the capacity is shared, and there are only two ways to share it: spatially, if several devices use the link simultaneously, or temporally, if they take turns. 00:07:42.018 --> 00:07:48.148 In Session 1 we asked who is allowed to talk on a link and when — simplex, half-duplex, full-duplex. 00:07:48.198 --> 00:07:54.398 Same flavor of question, one level up: who owns the channel, and when. 00:07:54.448 --> 00:08:03.398 Every topology you are about to meet is a policy on this question: who owns the wire, and when. 00:08:05.102 --> 00:08:06.422 First checkpoint. 00:08:06.472 --> 00:08:11.182 Pause the video and answer these three on paper, without scrolling back. 00:08:11.232 --> 00:08:13.572 One. 00:08:13.622 --> 00:08:19.112 Two PCs and a plug-and-play router in your flat — is that a network? 00:08:19.162 --> 00:08:19.942 Two. 00:08:19.992 --> 00:08:21.612 Is a router a host? 00:08:21.662 --> 00:08:25.522 Give the reason, not just the answer. 00:08:25.572 --> 00:08:25.842 Three. 00:08:25.892 --> 00:08:30.132 You push more data into a network and throughput rises. 00:08:30.182 --> 00:08:34.662 What tends to happen to delay — and why is that pair called contradictory? 00:08:34.712 --> 00:08:38.752 Pause now. 00:08:38.802 --> 00:08:39.882 Answers. 00:08:39.932 --> 00:08:41.892 The first: yes. 00:08:41.942 --> 00:08:49.612 A network is the interconnection of a set of devices capable of communication — small, but legal. 00:08:49.662 --> 00:08:51.332 The second: no. 00:08:51.382 --> 00:08:57.802 A host runs your programs; a router exists for the network's sake, and nothing of yours runs on it. 00:08:57.852 --> 00:09:00.602 And the third: delay rises too. 00:09:00.652 --> 00:09:03.992 More traffic means congestion, and packets wait. 00:09:04.042 --> 00:09:09.002 We want throughput up and delay down — and the same action pushes both up. 00:09:09.052 --> 00:09:13.256 That is the fight. 00:09:13.306 --> 00:09:14.606 Section two. 00:09:14.656 --> 00:09:16.296 The four topologies. 00:09:16.346 --> 00:09:21.936 Two or more devices connect to a link; two or more links form a topology. 00:09:21.986 --> 00:09:26.676 The geometry decides the cost — and what dies when something breaks. 00:09:26.726 --> 00:09:32.156 This section is the heart of the session. 00:09:32.386 --> 00:09:34.856 First the definition, precisely. 00:09:34.906 --> 00:09:43.466 The topology of a network is the geometric relationship of all the links and the nodes — the way the network is laid out physically. 00:09:43.516 --> 00:09:49.116 Two or more devices connect to a link; two or more links form a topology. 00:09:49.166 --> 00:09:53.586 There are four basic ones, and here they are in one line each. 00:09:53.636 --> 00:09:57.826 Mesh — every pair of devices gets its own cable. 00:09:57.876 --> 00:10:01.596 A dedicated link between every two devices. 00:10:01.646 --> 00:10:04.426 Star — everyone to one. 00:10:04.476 --> 00:10:08.886 Each device links only to a central controller, the hub. 00:10:08.936 --> 00:10:12.196 Bus — everyone onto one cable. 00:10:12.246 --> 00:10:15.646 One backbone, and every device taps on. 00:10:15.696 --> 00:10:18.636 Ring — everyone in a circle. 00:10:18.686 --> 00:10:23.426 Each device links to its two neighbors, and the signal goes around. 00:10:23.476 --> 00:10:31.436 We will take them in that order, starting from the most expensive — and for each one I will ask the same three questions. 00:10:31.486 --> 00:10:32.826 How is it wired? 00:10:32.876 --> 00:10:34.706 What does that wiring buy you? 00:10:34.756 --> 00:10:36.996 And what dies when something breaks? 00:10:37.046 --> 00:10:45.112 The same three questions, four times — and the habit of asking them is the point. 00:10:45.162 --> 00:10:46.512 The mesh. 00:10:46.562 --> 00:10:48.962 Five devices on this slide. 00:10:49.012 --> 00:10:54.882 Every device holds a dedicated point-to-point link to every other device. 00:10:54.932 --> 00:10:58.912 Five devices, ten links — count them on the diagram. 00:10:58.962 --> 00:11:07.632 And 'dedicated' means exactly what it meant three slides ago: each link carries traffic only between the two devices it connects. 00:11:07.682 --> 00:11:10.982 No third device ever touches your bits. 00:11:11.032 --> 00:11:13.572 What does all that wire buy? 00:11:13.622 --> 00:11:14.812 Three things. 00:11:14.862 --> 00:11:22.972 Robust — one link going down never strands the rest of the network; every other pair still holds its own private cable. 00:11:23.022 --> 00:11:29.822 Private — your traffic touches no third device, which is security by construction rather than by policy. 00:11:29.872 --> 00:11:38.822 And faults are easy to identify and isolate — when a cable dies, you know exactly whose cable it is, because it only ever belonged to one pair. 00:11:40.242 --> 00:11:42.522 And nobody builds it. 00:11:42.572 --> 00:11:43.392 Why? 00:11:43.442 --> 00:11:47.812 Because of the price: cabling, and I/O (input/output) ports. 00:11:47.862 --> 00:11:56.812 Every device must connect to every other one — installation is hard, the sheer bulk of wiring can beat the space in your walls, and the hardware cost is prohibitive. 00:11:59.702 --> 00:12:01.622 How prohibitive, exactly? 00:12:01.672 --> 00:12:06.479 Next slide — we count. 00:12:06.529 --> 00:12:09.219 Here is the only formula of the day. 00:12:09.269 --> 00:12:17.609 Five devices, full mesh, duplex links — meaning each cable carries both directions, so one cable per pair is enough. 00:12:17.659 --> 00:12:19.049 How many cables? 00:12:19.099 --> 00:12:20.949 And how many ports? 00:12:20.999 --> 00:12:23.589 Count the cables first. 00:12:23.639 --> 00:12:28.889 Each of the five devices links to the other four — that suggests five times four, twenty. 00:12:28.939 --> 00:12:34.799 But look at any single cable: it was counted twice, once from each end. 00:12:34.849 --> 00:12:39.459 The cable from D1 to D2 is the same cable as the cable from D2 to D1. 00:12:39.509 --> 00:12:40.659 So halve it. 00:12:40.709 --> 00:12:47.119 Links equal n times n minus one, over two. 00:12:47.169 --> 00:12:50.249 Five times four over two — ten cables. 00:12:50.299 --> 00:12:53.939 Exactly the ten you counted on the previous slide. 00:12:53.989 --> 00:12:59.539 Now the ports — and halving these is the most common error on this question. 00:12:59.589 --> 00:13:03.819 A port sits at each cable end — and nobody halves the ends. 00:13:03.869 --> 00:13:07.509 A cable has two ends, and you buy a port for each. 00:13:07.559 --> 00:13:11.919 Each device needs one port per neighbor: n minus one. 00:13:11.969 --> 00:13:15.059 So per device, four ports. 00:13:15.109 --> 00:13:19.649 In total, five times four — twenty ports, not ten. 00:13:19.699 --> 00:13:28.649 The twenty we refused to halve for cables is precisely the right count for ports, because ports count cable-ends, and there really are twenty cable-ends. 00:13:30.469 --> 00:13:38.509 Say the result as one line: an n equals five mesh is ten links, four ports per device, twenty ports total. 00:13:38.559 --> 00:13:46.299 And say the rule underneath it even more firmly: the divide-by-two belongs to the links, never to the ports. 00:13:46.349 --> 00:13:50.469 If you remember one algebraic fact from today, that is the one. 00:13:50.519 --> 00:13:54.799 Before you go on — do n equals seven yourself, on paper. 00:13:54.849 --> 00:13:56.839 Links, and total ports. 00:13:56.889 --> 00:14:05.805 Pause now; the next slide will confirm you. 00:14:05.855 --> 00:14:08.875 Here is the same formula, five times. 00:14:08.925 --> 00:14:15.665 The n equals five row you have: ten links, four ports each, twenty in total. 00:14:15.715 --> 00:14:21.175 Six devices: six times five over two — fifteen links. 00:14:21.225 --> 00:14:25.305 Five ports on every device, thirty ports in total. 00:14:25.355 --> 00:14:28.795 Seven — the one you just did on paper. 00:14:28.845 --> 00:14:34.275 Seven times six over two: twenty-one links, and forty-two ports in total. 00:14:34.325 --> 00:14:43.275 If you wrote twenty-one for both, you halved the ports too — you have just demonstrated the classic error, and better here than on the midterm. 00:14:44.135 --> 00:14:51.805 Ten devices: ten times nine over two — forty-five links, nine ports per device, ninety in total. 00:14:51.855 --> 00:14:55.075 And forty — the GPL lab. 00:14:55.125 --> 00:14:59.735 Forty times thirty-nine over two: seven hundred and eighty cables. 00:14:59.785 --> 00:15:07.085 Thirty-nine ports on the back of every single machine, one thousand five hundred and sixty ports in total. 00:15:07.135 --> 00:15:15.635 Now name the shape of that growth. n times n minus one over two grows like n squared over two — quadratically. 00:15:15.685 --> 00:15:19.395 Double the devices and you roughly quadruple the cables. 00:15:19.445 --> 00:15:22.955 Linear things get bigger; quadratic things explode. 00:15:23.005 --> 00:15:31.955 Seven hundred and eighty cables through the walls of one teaching lab — that is why you have never seen a mesh LAN, and why the mesh survives only where the traffic justifies it: a handful of core routers, not forty student PCs. 00:15:37.585 --> 00:15:44.085 Open the topology explorer on the course page and load the mesh preset with n equals six. 00:15:44.135 --> 00:15:49.865 The live counters read fifteen links and thirty port-ends — exactly our table. 00:15:49.915 --> 00:15:55.745 Then drag n upward and watch the link counter outrun everything else on the screen. 00:15:55.795 --> 00:16:01.305 The formula is the animation's slope. 00:16:01.355 --> 00:16:02.675 Topology two: the star. 00:16:02.725 --> 00:16:07.865 Six devices this time, and the filled box in the middle is new. 00:16:07.915 --> 00:16:15.195 It is not a device like the others; it is a hub — a central controller that exists only for the network's sake. 00:16:15.245 --> 00:16:18.565 That is a connecting device, from section one. 00:16:18.615 --> 00:16:25.145 Each device gets exactly one dedicated link — to the hub, and to nothing else. 00:16:25.195 --> 00:16:27.625 Six devices, six cables. 00:16:27.675 --> 00:16:30.105 And devices never talk directly. 00:16:30.155 --> 00:16:37.145 If D1 wants to send to D4, the message goes D1 to the hub, and the hub relays it to D4. 00:16:37.195 --> 00:16:41.625 Every conversation in the room passes through that one box. 00:16:41.675 --> 00:16:46.145 The hub relays everything — that is the entire design. 00:16:46.195 --> 00:16:51.775 That is a single point of failure, and slide 15 grades it. First, what it buys. 00:16:51.825 --> 00:16:53.375 Three things. 00:16:53.425 --> 00:17:02.375 Cheaper — n links and one port per device, far less cabling than a mesh; the wiring bill collapses from quadratic to linear. 00:17:03.345 --> 00:17:10.745 Easy — to install and to reconfigure: a new machine arrives, you pull one cable to the hub, done. 00:17:10.795 --> 00:17:17.945 And robust, of a very specific kind — if one link fails, only that link's device is affected. 00:17:17.995 --> 00:17:20.365 Everyone else does not even notice. 00:17:20.415 --> 00:17:27.225 And yes — this is the topology in the GPL lab, which most of you worked out from last session's task. 00:17:27.275 --> 00:17:31.795 It is also the topology of almost all wired enterprise LANs. 00:17:31.845 --> 00:17:37.823 The next slide grades the star's two failures. 00:17:37.873 --> 00:17:41.603 Let us grade the star's failures honestly, both of them. 00:17:41.653 --> 00:17:43.543 One cable fails. 00:17:43.593 --> 00:17:48.823 One device is lost — that cable's device, and nobody else. 00:17:48.873 --> 00:17:55.383 The lab keeps working while you fetch a new cable, and the dead machine tells you exactly which cable to fetch. 00:17:55.433 --> 00:18:00.433 As failures go, this is about as kind as networking ever gets. 00:18:00.483 --> 00:18:02.333 Now the hub. 00:18:02.383 --> 00:18:05.663 If the hub goes down, the whole system is dead. 00:18:05.713 --> 00:18:09.393 Every path ran through it, so no path survives it. 00:18:09.443 --> 00:18:14.343 The star has a single point of failure, and it is exactly one box. 00:18:14.393 --> 00:18:18.993 So here is the trade, and it is a trade rather than a flaw. 00:18:19.043 --> 00:18:24.743 The star does not remove the fragility of the network — it collects it into one box. 00:18:24.793 --> 00:18:31.043 And then we guard that box: a padlocked cabinet, spare power, a spare switch on the shelf. 00:18:31.093 --> 00:18:33.853 Concentrate the risk where you can defend it. 00:18:33.903 --> 00:18:42.853 That is why the hub in the GPL lab has its own locked cabinet, and it is the shape of most engineering — you will meet the same move in databases, in power grids, in aircraft. 00:18:45.163 --> 00:18:49.863 In the topology explorer, run the scenario where the star's hub dies. 00:18:49.913 --> 00:18:53.813 One box goes dark, and eight devices go red at once. 00:18:53.863 --> 00:19:00.473 Then cut a single spoke instead, and watch exactly one device drop while the rest keep talking. 00:19:00.523 --> 00:19:08.908 The two failures could not look more different, and the demo makes that impossible to forget. 00:19:08.958 --> 00:19:17.078 Topology three: the bus — and this is the corridor network from the question at the start, so pay attention to its price tag. 00:19:17.128 --> 00:19:21.388 One long cable — the backbone — is the whole infrastructure. 00:19:21.438 --> 00:19:30.388 It is on the slide already, before any device, because it exists before any device: you lay it once, along the most efficient path through the building. 00:19:32.158 --> 00:19:35.038 Then every device taps on. 00:19:35.088 --> 00:19:40.728 A tap clamps onto the backbone, and a drop line of whatever length runs to the machine. 00:19:40.778 --> 00:19:45.918 Four devices here — one cable, four taps, four short drop lines. 00:19:45.968 --> 00:19:54.918 And note the vocabulary: this is a multipoint connection — many devices sharing one link — which makes the bus the odd one out, because mesh and star were built entirely from dedicated point-to-point links. 00:20:00.198 --> 00:20:08.648 The advantage is exactly what it looks like: ease of installation, and the least cabling of all four topologies. 00:20:08.698 --> 00:20:13.658 One backbone down the corridor, drop lines of any length hanging off it. 00:20:13.708 --> 00:20:15.458 Nothing is cheaper. 00:20:15.508 --> 00:20:22.588 And now the failure, and Forouzan's sentence here needs the explanation that comes in section three. 00:20:22.638 --> 00:20:30.278 A fault or break in the bus cable stops all transmission — even between devices on the same side of the break. 00:20:30.328 --> 00:20:35.738 The devices on the same side — their cable is intact, and they still cannot talk. 00:20:35.788 --> 00:20:44.738 There are also two smaller costs: every tap degrades the signal a little, which caps how many devices one bus can carry, and when something does go wrong, finding which tap or which meter of cable is at fault is genuinely hard. 00:20:50.508 --> 00:20:58.474 Why the same side dies too is a physics story, and slide 22 tells it. 00:20:58.524 --> 00:21:00.844 Topology four: the ring. 00:21:00.894 --> 00:21:02.674 Six devices. 00:21:02.724 --> 00:21:09.184 Each device connects to exactly its two neighbors, and the links close into a circle. 00:21:09.234 --> 00:21:12.654 And the label in the middle is the key fact: one way. 00:21:12.704 --> 00:21:16.874 The signal travels around the ring in one direction only. 00:21:16.924 --> 00:21:19.594 So how does a frame get anywhere? 00:21:19.644 --> 00:21:25.704 It passes device to device, in that one direction, until it finds its destination. 00:21:25.754 --> 00:21:29.944 D1 sending to D4 goes through D2 and D3 — there is no shortcut. 00:21:29.994 --> 00:21:38.944 And each device contains a repeater: it does not just pass the signal along, it regenerates the bits, so the signal arrives at the next hop as fresh as it left the sender. 00:21:41.484 --> 00:21:48.374 Remember regeneration from Session 1's accuracy discussion — this is it, built into every station. 00:21:48.424 --> 00:21:51.034 What the circle buys. 00:21:51.084 --> 00:21:59.284 Cheap to change — adding or deleting a device means touching exactly two connections, the two on either side of it. 00:21:59.334 --> 00:22:08.284 And the ring is self-diagnosing: the signal circulates at all times, so a device that hears nothing for too long knows something is wrong and raises an alarm. 00:22:09.884 --> 00:22:13.854 No other topology monitors itself this way for free. 00:22:13.904 --> 00:22:20.864 And the fragility, which by now you can predict from the one-way arrow. 00:22:20.914 --> 00:22:23.634 One break can disable the entire ring. 00:22:23.684 --> 00:22:26.384 Not one device — the entire ring. 00:22:26.434 --> 00:22:33.934 Traffic is unidirectional, so there is no second way around; a single break blocks the loop for everyone. 00:22:33.984 --> 00:22:42.541 The fixes — a dual ring, and a switch that closes off the break — are section four. 00:22:42.591 --> 00:22:44.151 Second checkpoint. 00:22:44.201 --> 00:22:48.381 Same rule — pause, paper, no scrolling back. 00:22:48.431 --> 00:22:49.641 One. 00:22:49.691 --> 00:22:55.791 A full mesh of seven devices — how many links, and how many ports in total? 00:22:55.841 --> 00:22:56.991 Two. 00:22:57.041 --> 00:23:00.881 In a star, D1 wants to send to D4. 00:23:00.931 --> 00:23:03.631 Describe the path. 00:23:03.681 --> 00:23:04.031 Three. 00:23:04.081 --> 00:23:11.301 Which of the four topologies is the only multipoint one — and what are the links in the other three? 00:23:11.351 --> 00:23:15.791 Pause now. 00:23:15.841 --> 00:23:16.561 Answers. 00:23:16.611 --> 00:23:20.851 The first: seven times six over two is twenty-one links. 00:23:20.901 --> 00:23:24.681 Ports are not halved: seven times six, forty-two. 00:23:24.731 --> 00:23:28.891 The second: D1 to the hub, then the hub to D4. 00:23:28.941 --> 00:23:33.791 Devices never talk directly in a star; the hub relays everything. 00:23:33.841 --> 00:23:38.591 And the third: the bus — many devices sharing one backbone. 00:23:38.641 --> 00:23:47.591 Mesh, star and ring are all built from dedicated point-to-point links; the ring's are point-to-point too, each device to its neighbor. 00:23:50.637 --> 00:23:52.097 Section three. 00:23:52.147 --> 00:23:53.587 Comparing the four. 00:23:53.637 --> 00:23:56.237 You now know how each shape is wired. 00:23:56.287 --> 00:24:04.987 This section lines them up: what each costs to build, what one break costs you — and why those two questions pull in opposite directions. 00:24:05.037 --> 00:24:11.520 Every topology is a bet on which failure you can afford. 00:24:11.570 --> 00:24:15.800 Build cost first, cheapest at the top. 00:24:15.850 --> 00:24:19.580 The bus: one cable, plus n cheap taps. 00:24:19.630 --> 00:24:20.820 Nothing else. 00:24:20.870 --> 00:24:27.520 This is the floor — no topology will ever undercut one shared conductor down the corridor. 00:24:27.570 --> 00:24:36.520 The ring: n links, but short ones — each device reaches only its next-door neighbor, so the cable runs are the shortest of any shape. 00:24:38.320 --> 00:24:47.270 The star: n cables — but now each is a home run, all the way back to the cabinet — plus the hub itself, a box you must buy, power and guard. 00:24:50.730 --> 00:24:54.330 And the mesh: n times n minus one over two. 00:24:54.380 --> 00:24:57.710 At the lab's size, seven hundred and eighty cables. 00:24:57.760 --> 00:25:04.610 The ladder does not just rise from bus to mesh — the last rung changes from linear to quadratic. 00:25:04.660 --> 00:25:09.420 Here is why the gap explodes, in one sentence of algebra. 00:25:09.470 --> 00:25:13.310 The star's cost grows in step with n — linear. 00:25:13.360 --> 00:25:16.740 The mesh's grows with n squared — quadratic. 00:25:16.790 --> 00:25:24.930 At n equals five the mesh costs ten cables to the star's five: twice as much, annoying but conceivable. 00:25:24.980 --> 00:25:30.770 At n equals forty it is seven hundred and eighty to forty: nearly twenty times. 00:25:30.820 --> 00:25:39.670 Quadratic always overtakes linear — the only question is when, and for network cabling the answer is: by the second row of the lab. 00:25:39.720 --> 00:25:48.670 This crossover argument is worth internalizing, because it comes back every time computer science compares two costs. 00:25:50.257 --> 00:25:54.177 Now the other column of the bet: what one failure costs. 00:25:54.227 --> 00:26:03.177 Read this table with two questions in mind — what happens when one device's own attachment fails, and what happens when the shared part fails. 00:26:04.337 --> 00:26:04.607 Mesh. 00:26:04.657 --> 00:26:11.857 Cut one link, and exactly one pair loses exactly one thing: their dedicated direct path. 00:26:11.907 --> 00:26:20.857 That is a real loss — the private cable those two paid for is gone, and unless the network has been set up to route their traffic through intermediate devices, those two are out of touch with each other. 00:26:24.167 --> 00:26:29.237 But every other link is untouched, and no third party even notices. 00:26:29.287 --> 00:26:32.877 And the second column: there is no shared part to fail. 00:26:32.927 --> 00:26:36.137 That is what all the money bought. 00:26:36.187 --> 00:26:36.447 Star. 00:26:36.497 --> 00:26:42.787 One cable down isolates exactly one device — everyone else does not even notice. 00:26:42.837 --> 00:26:49.417 But the shared part is the hub, and if it dies, the whole system dies with it. 00:26:49.467 --> 00:26:49.647 Bus. 00:26:49.697 --> 00:26:53.647 A single tap failing drops one device — mild. 00:26:53.697 --> 00:27:00.877 But the shared part is the backbone itself, and a break in it stops all transmission, on both sides of the cut. 00:27:00.927 --> 00:27:04.257 The bus's shared part is its entire anatomy. 00:27:04.307 --> 00:27:05.527 Ring. 00:27:05.577 --> 00:27:14.527 Here even the first column is grim: one dead station blocks the one-way loop, so everyone loses, not just its neighbors. 00:27:14.887 --> 00:27:21.767 And the shared part is any single link — in a simple ring, one break disables the entire ring. 00:27:21.817 --> 00:27:28.987 In the topology explorer, cut one link in each of the four shapes in turn — mesh, star, bus, ring. 00:27:29.037 --> 00:27:37.987 Start with the mesh, and be precise with the words: D1 and D2 lose their dedicated channel and that pair's direct communication fails — the other fourteen links are untouched. 00:27:41.947 --> 00:27:46.127 A loss for one pair, invisibility for everyone else. 00:27:46.177 --> 00:27:55.127 Then the star's hub, the bus cut, the ring break — each one a different answer to 'who pays for this failure?' 00:27:56.291 --> 00:28:05.241 Now the strange sentence from section two: a break in the bus stops all transmission, even between devices on the same side of the break. 00:28:05.381 --> 00:28:09.481 Time to explain it, because the intuition says otherwise. 00:28:09.531 --> 00:28:17.411 The intuition says: cut a cable and you get two shorter cables — two smaller networks, each working locally. 00:28:17.461 --> 00:28:20.071 Here is why the intuition is wrong. 00:28:20.121 --> 00:28:23.611 The break happens — and look at the arrows. 00:28:23.661 --> 00:28:32.261 At the damaged point, the cable's electrical properties change abruptly, and a signal that reaches the break does not simply stop there. 00:28:32.311 --> 00:28:33.661 It reflects. 00:28:33.711 --> 00:28:40.741 It travels back down the cable it came from — in both directions, from both faces of the break. 00:28:40.791 --> 00:28:46.991 First half of the story: the bus is cheapest because everyone shares one conductor. 00:28:47.041 --> 00:28:52.251 One conductor means one path, so when it breaks, there is no second path. 00:28:52.301 --> 00:28:55.391 That much you expected. 00:28:55.441 --> 00:29:04.391 Second half: the reflected signals do not carry anything useful — they arrive as noise, colliding with every legitimate transmission on the segment. 00:29:05.781 --> 00:29:08.061 So look at the right half of the diagram. 00:29:08.111 --> 00:29:10.331 Its copper is perfectly healthy. 00:29:10.381 --> 00:29:12.751 Its devices are perfectly healthy. 00:29:12.801 --> 00:29:20.841 And it is unusable — because every frame its devices send comes back at them, garbled, off the face of the break. 00:29:20.891 --> 00:29:24.691 So a cut bus is not two smaller buses. 00:29:24.741 --> 00:29:26.421 It is zero buses. 00:29:26.471 --> 00:29:32.151 The healthy half does not survive on its own — it drowns in the echo of the break. 00:29:32.201 --> 00:29:40.881 When the exam offers you 'cut a bus and each half keeps working', that is the trap, and now you know the physics behind the right answer. 00:29:40.931 --> 00:29:44.231 In the topology explorer, cut the bus cable. 00:29:44.281 --> 00:29:45.631 Both halves go red. 00:29:45.681 --> 00:29:51.571 Before you accept it, ask: the right half's cable is perfectly healthy — why is it dead? 00:29:51.621 --> 00:29:58.882 If your answer contains the word 'reflections', you have this slide. 00:29:58.932 --> 00:30:06.072 One more slide before the checkpoint. Real buildings do not pick one of the four shapes. 00:30:06.122 --> 00:30:14.472 Look at the diagram: three rooms, and inside each room a star — a hub with its own little cluster of machines. 00:30:14.522 --> 00:30:20.552 And between the rooms, a backbone — a spine joining the hubs to each other. 00:30:20.602 --> 00:30:28.452 Now trace what happened: at the edge, where devices are many and cables are cheap, the star does what stars do. 00:30:28.502 --> 00:30:37.452 Between rooms and floors, where runs are long and few, a single backbone does what a bus does best — and because that spine touches only hubs, sits in protected risers, and is guarded like the hubs are, its fragility is contained. 00:30:44.472 --> 00:30:45.792 One shape per job. 00:30:45.842 --> 00:30:53.422 Inside a room: a star — one cheap cable per machine, and a cut costs exactly one machine. 00:30:53.472 --> 00:31:01.372 Between rooms: a spine — few links, longer runs, the shared-conductor idea kept where a break is rare and defended. 00:31:01.422 --> 00:31:08.772 Every real campus LAN you will ever touch — including this campus — is a hybrid of exactly this kind. 00:31:08.822 --> 00:31:16.253 The four pure shapes are the vocabulary; real buildings combine them. 00:31:16.303 --> 00:31:17.793 Third checkpoint. 00:31:17.843 --> 00:31:19.653 Pause and answer on paper. 00:31:19.703 --> 00:31:20.923 One. 00:31:20.973 --> 00:31:24.263 One cable is cut in a full mesh. 00:31:24.313 --> 00:31:28.373 Say exactly who is affected, and exactly what they lost. 00:31:28.423 --> 00:31:29.583 Two. 00:31:29.633 --> 00:31:33.883 One cable is cut in a star — who is affected? 00:31:33.933 --> 00:31:35.863 And if the hub dies instead? 00:31:35.913 --> 00:31:37.093 Three. 00:31:37.143 --> 00:31:42.613 Why does a break in a bus stop even the devices on the same side of the break? 00:31:42.663 --> 00:31:46.713 Pause now. 00:31:46.763 --> 00:31:48.193 Answers. 00:31:48.243 --> 00:31:57.193 The first: only the pair that owned that link, and what they lost is their dedicated direct path — every other link still works, and nobody else notices. 00:31:58.823 --> 00:32:03.873 It is a real loss for that pair; never write 'nothing dies'. 00:32:03.923 --> 00:32:07.623 The second: one cable cut isolates exactly one device. 00:32:07.673 --> 00:32:13.843 The hub dying kills the whole system — that is the star's single point of failure. 00:32:13.893 --> 00:32:19.733 And the third: the damaged point reflects signals back down the cable as noise, in both directions. 00:32:19.783 --> 00:32:28.586 The same-side devices have healthy copper and no usable channel — they drown in the echo. 00:32:28.636 --> 00:32:30.016 Section four. 00:32:30.066 --> 00:32:31.586 Ring resilience. 00:32:31.636 --> 00:32:35.866 One break kills a simple ring. That is not the end of the story. 00:32:35.916 --> 00:32:44.866 Real rings were sold for decades, in banks and telephone companies, precisely because the ring's weakness has two clean fixes: a second ring, and a switch. 00:32:46.566 --> 00:32:53.253 Both are examinable, and both are genuinely elegant. 00:32:53.303 --> 00:33:00.493 First, be sure you believe the failure, because the fixes only make sense if you do. 00:33:00.543 --> 00:33:04.543 Cut one link — the one between D1 and D2, say. 00:33:04.593 --> 00:33:09.553 And the tag at the bottom is not an exaggeration: everyone is down. 00:33:09.603 --> 00:33:11.433 Here is why. 00:33:11.483 --> 00:33:13.663 Traffic is unidirectional. 00:33:13.713 --> 00:33:22.663 D6 reaches D2 by passing through D1 — every frame, from every station, travels the same circle, so every path uses every link and every station. 00:33:24.153 --> 00:33:30.383 The loop is not a convenience of the ring; the loop is the medium. 00:33:30.433 --> 00:33:35.733 Cut one link and no frame, from anywhere, can complete the circle. 00:33:35.783 --> 00:33:41.563 Compare the mesh for one second: there, a cut cost one pair their private path. 00:33:41.613 --> 00:33:45.363 Here, a cut costs the entire population everything. 00:33:45.413 --> 00:33:48.713 Same event, opposite blast radius. 00:33:48.763 --> 00:33:53.903 Now, before the fixes, separate two failures that look alike. 00:33:53.953 --> 00:33:57.743 A cable can break — a backhoe, a loose connector. 00:33:57.793 --> 00:34:03.473 And a station can die — a machine is powered off, or simply removed. 00:34:03.523 --> 00:34:12.473 The second is easier to miss: every station is a repeater, so a dead station repeats nothing, and it blocks the loop exactly as a cut cable would. 00:34:13.183 --> 00:34:22.133 Two different failures, and the industry answered with two different mechanisms — one each, on the next two slides. 00:34:23.469 --> 00:34:27.739 Fix number one, for the broken cable: the dual ring. 00:34:27.789 --> 00:34:33.789 Run a second ring alongside the first — same stations, opposite direction. 00:34:33.839 --> 00:34:40.689 The outer ring carries the traffic clockwise; the inner ring is wired to carry it counter-clockwise. 00:34:40.739 --> 00:34:43.039 Every device sits on both. 00:34:43.089 --> 00:34:47.239 And in normal operation, the second ring carries nothing at all. 00:34:47.289 --> 00:34:52.029 It is pure insurance — capacity you build and leave idle. 00:34:52.079 --> 00:34:58.279 Now break a cable on the main ring, and follow the two stations on either side of the break. 00:34:58.329 --> 00:35:03.899 Each one wraps: it connects its side of the broken outer ring onto the inner ring. 00:35:03.949 --> 00:35:12.899 Follow the path after the wrap — traffic runs along the outer ring, reaches the break, turns onto the inner ring, runs back around the other way, and rejoins. 00:35:14.449 --> 00:35:20.689 The two rings have spliced into one longer working ring, and every station is still connected. 00:35:20.739 --> 00:35:24.879 Nothing was rerouted by a human; the wrap is automatic. 00:35:24.929 --> 00:35:28.739 The ring keeps running, and the technician can come on Monday. 00:35:28.789 --> 00:35:37.739 This is how the classic fiber ring designs survived cable cuts for decades: FDDI (Fiber Distributed Data Interface) in campus backbones, and the telephone companies' fiber rings, which is why a backhoe through one cable so rarely took a city offline. 00:35:46.039 --> 00:35:50.249 In the topology explorer, switch to the dual-ring scenario. 00:35:50.299 --> 00:35:59.249 Cut a cable and watch the two neighbors wrap, the arrows reverse on the inner ring, and the counter in the corner confirm every station still reachable. 00:36:02.600 --> 00:36:07.100 Fix number two, for the dead station: the bypass switch. 00:36:07.150 --> 00:36:10.420 State the problem precisely first. 00:36:10.470 --> 00:36:19.420 Every station is a repeater — the signal does not pass by a device, it passes through it: in one port, regenerated, out the other. 00:36:20.000 --> 00:36:25.990 So a powered-off station repeats nothing, and to the ring, a dead station is a break. 00:36:26.040 --> 00:36:34.990 The dual ring does not really solve this one. Wrapping around a dead station works, but stations die far more often than cables get cut, and wrapping the whole ring every time somebody switches off a workstation is not workable. 00:36:40.600 --> 00:36:46.870 So the fix sits at the station's attachment point: a relay — an automatic switch. 00:36:46.920 --> 00:36:53.210 While the station is alive, the relay routes the ring through it, and the station repeats as normal. 00:36:53.260 --> 00:37:02.210 The moment the station dies, the relay closes, and the ring bypasses it — look at the diagram: the bits flow past S2 without ever entering it. 00:37:03.550 --> 00:37:07.190 The station is off the network; the network is fine. 00:37:07.240 --> 00:37:14.860 One machine's death now costs exactly one machine, which is the same kind answer the star gave for a cut cable. 00:37:14.910 --> 00:37:17.890 Keep the two fixes distinct. 00:37:17.940 --> 00:37:22.100 The dual ring survives a broken cable — it heals the path. 00:37:22.150 --> 00:37:27.570 The bypass switch survives a dead station — it removes a device from the path. 00:37:27.620 --> 00:37:29.380 Cable: dual ring. 00:37:29.430 --> 00:37:31.390 Station: bypass. 00:37:31.440 --> 00:37:33.970 Two failures, two mechanisms. 00:37:34.020 --> 00:37:38.490 In the topology explorer, switch to the bypass-switch scenario. 00:37:38.540 --> 00:37:45.560 Kill station S2 and watch the relay close: the loop shortens by one station and keeps circulating. 00:37:45.610 --> 00:37:54.560 Then kill a cable instead and watch why the bypass alone cannot save you — that one needs the dual ring. 00:37:55.742 --> 00:38:04.692 Everything in the last two sections — the formula, the scorecard, the dual ring, the bypass switch — is in one tool. 00:38:05.972 --> 00:38:08.202 Mesh, six devices. 00:38:08.252 --> 00:38:15.712 Links, fifteen — and underneath it the formula, six times five over two, doing the work in front of you. 00:38:15.762 --> 00:38:19.512 Ports per device, five, which is n minus one. 00:38:19.562 --> 00:38:21.662 Total ports to buy, thirty. 00:38:21.712 --> 00:38:27.312 Three numbers, one shape, and every one of them derived rather than remembered. 00:38:27.362 --> 00:38:29.622 Now double it to twelve. 00:38:29.672 --> 00:38:31.912 Links, sixty-six. 00:38:31.962 --> 00:38:34.042 Ports per device, eleven. 00:38:34.092 --> 00:38:36.742 Total ports, a hundred and thirty-two. 00:38:36.792 --> 00:38:45.742 Doubling the devices did not double the cable — it roughly quadrupled it, which is exactly what quadratic means, and it is the whole reason nobody wires a building this way. 00:38:47.882 --> 00:38:52.242 Now the interesting half of the tool: breaking things. 00:38:52.292 --> 00:38:54.232 Star, and I kill the hub. 00:38:54.282 --> 00:39:01.452 Every device is still perfectly healthy, every cable is still perfectly intact, and the network is gone. 00:39:01.502 --> 00:39:05.432 That is the star's bargain, drawn rather than argued. 00:39:05.482 --> 00:39:08.532 Bus, one cut in the backbone. 00:39:08.582 --> 00:39:12.612 Look carefully at what the tool says — not two smaller buses. 00:39:12.662 --> 00:39:18.882 Both halves lose the terminator, the signal reflects, and neither side works. 00:39:18.932 --> 00:39:25.972 Ring, one break, and the whole loop is dark — because traffic only travels one way round. 00:39:26.022 --> 00:39:30.062 And now the two rescues from section four, live. 00:39:30.112 --> 00:39:39.062 Dual ring: fifteen links of sixteen are up, four ports on every device instead of two, thirty-two ports in total — and all eight devices are still connected, because the stations either side of the break wrap onto the spare ring. 00:39:46.862 --> 00:39:50.342 That is FDDI and SONET, healing themselves. 00:39:50.392 --> 00:39:58.632 And the bypass switch, for the other failure: a dead station is short-circuited out of the loop and the ring closes over it. 00:39:58.682 --> 00:40:05.672 Two different faults, two different fixes, and both of them bought with hardware you had to pay for in advance. 00:40:05.722 --> 00:40:13.370 Open it after the lecture, put n at forty, and check the 780 against section six. 00:40:13.420 --> 00:40:14.840 Fourth checkpoint. 00:40:14.890 --> 00:40:17.910 Pause, paper, no scrolling back. 00:40:17.960 --> 00:40:19.160 One. 00:40:19.210 --> 00:40:23.640 A simple unidirectional ring loses one link. 00:40:23.690 --> 00:40:27.680 Who loses — and why is the answer not 'just the two neighbors'? 00:40:27.730 --> 00:40:28.880 Two. 00:40:28.930 --> 00:40:33.980 What does the dual ring add, and what happens at the moment a cable breaks? 00:40:34.030 --> 00:40:35.190 Three. 00:40:35.240 --> 00:40:38.720 A station on the ring is powered off. 00:40:38.770 --> 00:40:42.190 Which fix handles that, and what exactly does it do? 00:40:42.240 --> 00:40:46.280 Pause now. 00:40:46.330 --> 00:40:47.760 Answers. 00:40:47.810 --> 00:40:50.890 The first: everyone loses. 00:40:50.940 --> 00:40:58.120 Traffic is unidirectional and every path uses every link — the loop itself is the medium. 00:40:58.170 --> 00:41:03.660 The second: a second, counter-rotating ring that is idle in normal operation. 00:41:03.710 --> 00:41:11.740 On a break, the two devices beside it wrap onto the second ring, and the two rings splice into one longer working ring. 00:41:11.790 --> 00:41:14.390 And the third: the bypass switch. 00:41:14.440 --> 00:41:20.870 A relay at the dead station's attachment closes, so the bits flow past the station without entering it. 00:41:20.920 --> 00:41:29.870 Say the pairing once more: dead cable — dual ring; dead station — bypass switch. 00:41:30.248 --> 00:41:31.698 Section five. 00:41:31.748 --> 00:41:33.738 From the lab to the Internet. 00:41:33.788 --> 00:41:37.058 So far the whole session has lived inside one room. 00:41:37.108 --> 00:41:46.058 Now zoom out — LAN, WAN, internetwork, Internet — and then ask the question the zoom forces on us: when data has to cross all of that, how is it switched from link to link? 00:41:49.388 --> 00:41:56.290 There are exactly two answers, and the Internet had to pick one. 00:41:56.340 --> 00:42:01.110 Four terms, one ladder, each rung bigger than the last. 00:42:01.160 --> 00:42:04.630 The LAN — the local area network. 00:42:04.680 --> 00:42:08.960 Privately owned: an office, a building, a campus. 00:42:09.010 --> 00:42:12.910 And remember the precise verb: a LAN connects hosts. 00:42:12.960 --> 00:42:21.910 Once upon a time every host heard every packet and simply dropped the ones not addressed to it; today a smart switch reads the destination address and delivers each packet to exactly one host. 00:42:25.970 --> 00:42:31.550 Your lab is a LAN built on a star — both words now mean something exact. 00:42:31.600 --> 00:42:36.580 The WAN — the wide area network — spans a town, a country, the world. 00:42:36.630 --> 00:42:42.200 Two differences from the LAN, and both are exam material. 00:42:42.250 --> 00:42:49.060 First, a WAN interconnects connecting devices — switches, routers, modems — not hosts. 00:42:49.110 --> 00:42:56.660 Second, nobody in this room owns one: communication companies build them, and organizations lease them. 00:42:56.710 --> 00:43:00.640 You rent your wide-area reach the way you rent electricity. 00:43:00.690 --> 00:43:07.450 Connect two or more networks and you have an internetwork — an internet, with a small i. 00:43:07.500 --> 00:43:15.040 The canonical example: two office LANs, one on each coast, plus one leased point-to-point WAN between them. 00:43:15.090 --> 00:43:17.790 That company now runs a private internet. 00:43:17.840 --> 00:43:23.000 No permission required from anyone — that is rather the point of the design. 00:43:23.050 --> 00:43:32.000 And capitalize the I and you get the Internet — the most notable internet in the world: thousands of interconnected networks. 00:43:32.200 --> 00:43:41.150 Backbones run by large communication companies, provider networks that lease capacity from them, and at the bottom the ISPs (Internet service providers) that sell you your on-ramp. 00:43:44.500 --> 00:43:53.450 Small ladder, four rungs, and every rung reuses this session's ideas — hosts, connecting devices, point-to-point links. 00:43:53.760 --> 00:43:59.009 Nothing new was invented to go big. 00:43:59.059 --> 00:44:06.719 Two of those four terms carry traps that appear on every midterm I have ever set, so let us disarm them now. 00:44:06.769 --> 00:44:14.709 Trap one: 'a WAN is just a big LAN.' It is not, and you can now say why in two clean sentences. 00:44:14.759 --> 00:44:23.329 A LAN connects hosts; a WAN interconnects connecting devices — and you lease a WAN from a communication company rather than owning it. 00:44:23.379 --> 00:44:30.699 While we are here, the two kinds: a point-to-point WAN connects two ends — that leased line between the coasts. 00:44:30.749 --> 00:44:39.699 A switched WAN is several point-to-point WANs joined by switches — and that structure is the backbone of global communication. 00:44:40.219 --> 00:44:48.779 Trap two: 'internet is just short for Internet.' No — the small i and the capital I are different words. 00:44:48.829 --> 00:44:54.719 Small i: any two or more connected networks, anywhere, built by anyone. 00:44:54.769 --> 00:45:00.189 Capital I: the specific global one everybody means. 00:45:00.239 --> 00:45:05.179 Every Internet is an internet; almost no internet is the Internet. 00:45:05.229 --> 00:45:09.219 Write the inequality in your notes: internet ≠ Internet. 00:45:09.269 --> 00:45:15.809 And one practical note to close the vocabulary: how do you personally reach the Internet? 00:45:15.859 --> 00:45:24.809 Through an ISP, over whichever on-ramp you pay for — dial-up over the old phone line, DSL (Digital Subscriber Line), cable, wireless, or a direct leased line if you are rich enough. 00:45:29.339 --> 00:45:38.289 The list is not examinable in detail; what matters is that all of them are just access — the shapes and the switching inside do not change. 00:45:41.429 --> 00:45:43.819 Now the real question of this section. 00:45:43.869 --> 00:45:50.589 A message leaving your lab crosses LANs, a WAN or three, and dozens of connecting devices. 00:45:50.639 --> 00:45:53.999 How does data actually move across all those hops? 00:45:54.049 --> 00:45:56.019 Two philosophies exist. 00:45:56.069 --> 00:45:57.559 Only two. 00:45:57.609 --> 00:46:00.879 Philosophy one: circuit switching. 00:46:00.929 --> 00:46:02.809 Reserve, then talk. 00:46:02.859 --> 00:46:11.809 Before the first word, the network sets up a dedicated circuit between the two ends — a chain of reserved capacity, held for you until the last word — and the switches along the way merely turn it on or off. 00:46:16.129 --> 00:46:20.319 The old telephone network lived exactly like this for a century. 00:46:20.369 --> 00:46:25.859 And note the built-in verdict: it is efficient only while both ends keep talking. 00:46:25.909 --> 00:46:29.239 Philosophy two: packet switching. 00:46:29.289 --> 00:46:31.059 Chop, then forward. 00:46:31.109 --> 00:46:37.509 The data is cut into independent blocks — packets — and each packet fends for itself. 00:46:37.559 --> 00:46:44.219 A switch or router receives a packet whole, can STORE it in a queue, and FORWARDS it when a line is free. 00:46:44.269 --> 00:46:49.459 No reservation, no guarantee — and nothing ever sits idle. 00:46:49.509 --> 00:46:52.319 The Internet chose packets. 00:46:52.369 --> 00:47:01.319 Everything in this course that hurts — delay, the jitter you met in Session 1, congestion, buffering — is the bill for that choice. 00:47:01.859 --> 00:47:06.889 And it is still the right choice, for one reason: computers talk in bursts. 00:47:06.939 --> 00:47:10.129 A burst, then silence, then a burst. 00:47:10.179 --> 00:47:19.079 Reserving a circuit for a burst holds the capacity idle through every gap between bursts. 00:47:19.129 --> 00:47:21.569 Here is circuit switching as a picture. 00:47:21.619 --> 00:47:30.569 A and B at the ends; three switches — S1, S2, S3 — between them; and the grey segments are the links that exist. 00:47:32.119 --> 00:47:35.759 The call begins, and the path lights up. 00:47:35.809 --> 00:47:43.749 That orange line is a reservation: capacity on every link of the chain, set aside for A and B alone, end to end. 00:47:43.799 --> 00:47:52.749 While the circuit is up, no other conversation may touch that capacity — it behaves exactly like the dedicated point-to-point links from section one, conjured on demand across a whole country. 00:47:57.579 --> 00:48:02.629 Every circuit-switched conversation has three phases, in order. 00:48:02.679 --> 00:48:11.089 Setup — the path is chosen and reserved, before the first word; that is the pause after dialing that your grandparents remember. 00:48:11.139 --> 00:48:18.999 Talk — the capacity is yours, all of it, with no queuing anywhere, which is why a landline call never stuttered. 00:48:19.049 --> 00:48:23.909 Teardown — you hang up, and the circuit is released back to the pool. 00:48:23.959 --> 00:48:27.489 And the cost is hiding in the middle phase. 00:48:27.539 --> 00:48:32.719 Pause to think, and the circuit carries nothing — yet nobody else may use it. 00:48:32.769 --> 00:48:35.729 Capacity reserved is capacity idle. 00:48:35.779 --> 00:48:43.549 The telephone network lived like this for a hundred years: efficient while both ends are talking, and idle the moment you pause. 00:48:43.599 --> 00:48:52.549 Remember the phrasing from the exam's point of view: dedicated path, capacity reserved, setup required, idle circuits waste capacity. 00:48:53.199 --> 00:48:57.913 Four facts, one picture. 00:48:57.963 --> 00:49:00.613 Same journey, other philosophy. 00:49:00.663 --> 00:49:08.453 A and B again — but now the boxes in the middle are routers, R1 to R4, and there are two ways across. 00:49:08.503 --> 00:49:13.613 The message is chopped into packets — P1, P2, P3 — and released. 00:49:13.663 --> 00:49:22.613 Look at where they are: P1 is crossing the top path, P2 chose the bottom, P3 is still leaving A. Nobody reserved anything for them. 00:49:25.763 --> 00:49:31.843 Each packet finds its own way, and different packets from the same message may take different paths. 00:49:31.893 --> 00:49:39.533 And look closely at R2 — two packets have arrived faster than its output line can drain, so a queue has formed. 00:49:39.583 --> 00:49:43.653 Packets sitting in a router's memory, waiting their turn. 00:49:43.703 --> 00:49:48.943 The mechanism has a name you should use precisely: store and forward. 00:49:48.993 --> 00:49:55.553 Each packet is received whole, stored in the router's queue, and forwarded when the output line is free. 00:49:55.603 --> 00:50:04.323 No setup phase, no reservation, no teardown — send when ready, and the network figures out the rest hop by hop. 00:50:04.373 --> 00:50:06.753 And the price is the queue. 00:50:06.803 --> 00:50:11.743 When traffic is heavy, queues grow, and every packet in one waits. 00:50:11.793 --> 00:50:19.693 That waiting is queuing delay — and since queues grow and shrink moment to moment, the delay varies packet by packet. 00:50:19.743 --> 00:50:25.043 Delay that varies — you have met that: it is exactly the jitter from Session 1. 00:50:25.093 --> 00:50:29.573 Now you know where jitter is born: in the queues of packet switches. 00:50:29.623 --> 00:50:36.893 Nothing is wasted in silence, because nothing was reserved — but what you share, you sometimes wait for. 00:50:36.943 --> 00:50:40.433 That queue comes back with formulas in Session 6. 00:50:40.483 --> 00:50:48.579 For today, own the two mechanisms: reserve-then-talk, and store-and-forward. 00:50:48.629 --> 00:50:51.249 The head-to-head, one row at a time. 00:50:51.299 --> 00:50:54.789 Six rows, and every row is the same trade. 00:50:54.839 --> 00:50:56.309 The path. 00:50:56.359 --> 00:51:01.989 Circuit: one reserved path, held for the whole call. 00:51:02.039 --> 00:51:08.239 Packet: every packet finds its own way — two packets of one message may never see the same routers. 00:51:08.289 --> 00:51:09.569 Setup. 00:51:09.619 --> 00:51:15.589 Circuit: required, before the first word — no setup, no call. 00:51:15.639 --> 00:51:17.139 Packet: none. 00:51:17.189 --> 00:51:18.339 Send when ready. 00:51:18.389 --> 00:51:19.999 Capacity. 00:51:20.049 --> 00:51:24.749 Circuit: reserved end to end, for you alone. 00:51:24.799 --> 00:51:29.329 Packet: shared with everyone whose traffic happens to cross yours. 00:51:29.379 --> 00:51:31.039 In silence. 00:51:31.089 --> 00:51:37.419 Circuit: the reserved circuit sits idle — capacity wasted, by design. 00:51:37.469 --> 00:51:42.979 Packet: nothing idles — the instant you fall silent, others use the line. 00:51:43.029 --> 00:51:44.679 Silence costs nothing. 00:51:44.729 --> 00:51:45.979 Delay. 00:51:46.029 --> 00:51:51.929 Circuit: fixed and small once the call is up — no queuing anywhere. 00:51:51.979 --> 00:51:56.909 Packet: variable — queuing delay, and its variation, jitter. 00:51:56.959 --> 00:51:58.959 Circle this row in your notes. 00:51:59.009 --> 00:52:02.569 Every performance formula we build later lives here. 00:52:02.619 --> 00:52:04.079 Best for. 00:52:04.129 --> 00:52:09.359 Circuit: long steady flows — a century of phone calls. 00:52:09.409 --> 00:52:13.129 Packet: bursty computer traffic — the Internet. 00:52:13.179 --> 00:52:22.129 The summary sentence is the same one the topologies taught: what you reserve is what you waste; what you share is what you queue for. 00:52:22.209 --> 00:52:29.799 One trade, two scales — a wire in a lab, or a planet's worth of routers. 00:52:30.039 --> 00:52:31.379 Fifth checkpoint. 00:52:31.429 --> 00:52:32.759 Pause and answer. 00:52:32.809 --> 00:52:34.029 One. 00:52:34.079 --> 00:52:40.579 'A WAN connects hosts across a city.' Two things are wrong with that sentence — name both. 00:52:40.629 --> 00:52:41.789 Two. 00:52:41.839 --> 00:52:47.409 Two office LANs joined by one leased point-to-point line — what have you built? 00:52:47.459 --> 00:52:48.639 Three. 00:52:48.689 --> 00:52:55.599 Which switching style wastes capacity in silence — and what does the other one do instead of wasting? 00:52:55.649 --> 00:52:59.699 Pause now. 00:52:59.749 --> 00:53:01.189 Answers. 00:53:01.239 --> 00:53:07.029 The first: a WAN interconnects connecting devices — switches, routers, modems — not hosts. 00:53:07.079 --> 00:53:13.189 And you lease it from a communication company; you do not own it. 00:53:13.239 --> 00:53:17.289 The second: an internetwork — an internet, small i. 00:53:17.339 --> 00:53:20.989 Private, and perfectly legal without anyone's permission. 00:53:21.039 --> 00:53:27.289 And the third: circuit switching wastes — the reserved circuit idles whenever nobody speaks. 00:53:27.339 --> 00:53:36.289 Packet switching shares the line instead, so nothing idles — and the price appears as queuing: delay, and jitter, instead of idle copper. 00:53:39.954 --> 00:53:41.534 Section six. 00:53:41.584 --> 00:53:42.704 The answer. 00:53:42.754 --> 00:53:48.914 Back to the two numbers I asked you to write down in the first minute — and this time we compute both. 00:53:48.964 --> 00:53:53.238 Get your guesses out. 00:53:53.288 --> 00:53:54.988 The GPL lab. 00:53:55.038 --> 00:53:56.428 Forty computers. 00:53:56.478 --> 00:54:01.508 Question one: every computer wired directly to every other. 00:54:01.558 --> 00:54:07.698 That is a full mesh with n equal to forty, and you now own the formula. 00:54:07.748 --> 00:54:12.218 Forty times thirty-nine over two — seven hundred and eighty cables. 00:54:12.268 --> 00:54:21.038 And the ports, which we never halve: thirty-nine ports on the back of every machine, one thousand five hundred and sixty port-ends in total. 00:54:21.088 --> 00:54:23.118 Compare that with your first guess. 00:54:23.168 --> 00:54:31.028 Quadratic growth is easy to underestimate: forty machines feels like it should need about forty of everything. 00:54:31.078 --> 00:54:34.998 Question two: what the lab actually uses. 00:54:35.048 --> 00:54:36.188 A star. 00:54:36.238 --> 00:54:40.708 Forty cables — one per machine — into one switch in the corner. 00:54:40.758 --> 00:54:43.938 One port per machine, plus the switch's forty. 00:54:43.988 --> 00:54:50.848 If your guess was 'about forty', you were right for exactly the right reason: star cost is linear. 00:54:50.898 --> 00:54:56.868 Seven hundred and eighty against forty — nineteen and a half times fewer cables. 00:54:56.918 --> 00:55:02.978 The discount is real, and so is its price: everything now depends on the one switch. 00:55:03.028 --> 00:55:11.978 Which is why it lives in a padlocked cabinet with spare power — the star's bargain from section three, playing out in a real room you walk past every day: collect the fragility into one box, then guard the box. 00:55:16.838 --> 00:55:18.778 That is the arithmetic answer. 00:55:18.828 --> 00:55:24.297 One more reading of the same two columns. 00:55:24.347 --> 00:55:31.567 Put the whole session on one slide: the two columns of the bet, face to face. 00:55:31.617 --> 00:55:34.667 Build cost, for n devices. 00:55:34.717 --> 00:55:40.027 Bus: one cable and n taps — the cheapest thing a human being can build. 00:55:40.077 --> 00:55:42.287 Ring: n short hops. 00:55:42.337 --> 00:55:44.247 Star: n cables and a hub. 00:55:44.297 --> 00:55:50.557 Mesh: n times n minus one over two — seven hundred and eighty for our lab. 00:55:50.607 --> 00:55:53.687 Failure cost, for one break. 00:55:53.737 --> 00:55:55.207 Bus: everything. 00:55:55.257 --> 00:55:58.967 Ring: everything, in the simple ring. 00:55:59.017 --> 00:56:02.257 Star: one device — unless it is the hub. 00:56:02.307 --> 00:56:04.177 Mesh: one pair's direct link. 00:56:04.227 --> 00:56:11.067 Now read the two columns against each other, top to bottom, and notice they are almost perfectly inverted. 00:56:11.117 --> 00:56:16.957 The cheapest build has the worst failure; the most expensive build has the mildest one. 00:56:17.007 --> 00:56:22.987 And that inversion is not a coincidence — it is one sentence read twice. 00:56:23.037 --> 00:56:27.117 The bus is cheapest because everyone shares a single conductor. 00:56:27.167 --> 00:56:36.117 Read the same sentence again as a failure analysis: because everyone shares a single conductor, a break leaves no second path — and the break echoes noise both ways, so the intact half has no usable channel either. 00:56:41.847 --> 00:56:45.597 The very sharing that saved the money concentrates the failure. 00:56:45.647 --> 00:56:47.967 So you never pick the cheapest network. 00:56:48.017 --> 00:56:50.857 You pick the failure you can afford. 00:56:50.907 --> 00:56:53.187 Mesh: cannot afford to build. 00:56:53.237 --> 00:56:55.057 Bus: cannot afford to break. 00:56:55.107 --> 00:57:04.057 The star loses one device per cut and puts all the remaining risk in one lockable box — that is why the GPL lab is a star, and why almost everything is. 00:57:08.702 --> 00:57:11.302 Four quick calls to close the content. 00:57:11.352 --> 00:57:18.772 Pause on each, decide, then let the answer appear. 00:57:18.822 --> 00:57:24.712 First: forty lab PCs and one box in the corner — which topology? 00:57:24.762 --> 00:57:29.112 Pause and decide. 00:57:29.162 --> 00:57:30.522 Star. 00:57:30.572 --> 00:57:36.622 The box in the corner is the giveaway — the box IS the topology. 00:57:36.672 --> 00:57:41.692 Second: a full mesh of twelve routers — how many links? 00:57:41.742 --> 00:57:46.602 Pause and compute. 00:57:46.652 --> 00:57:48.652 Twelve times eleven over two — sixty-six. 00:57:48.702 --> 00:57:55.192 If you got one hundred and thirty-two, you just bought every cable twice. 00:57:55.242 --> 00:57:59.132 That is the most common error on this question. 00:57:59.182 --> 00:58:05.352 Third: a landline phone call in 1995 — circuit switched or packet switched? 00:58:05.402 --> 00:58:09.762 Pause and decide. 00:58:09.812 --> 00:58:11.142 Circuit. 00:58:11.192 --> 00:58:18.922 A reserved path, held from hello to goodbye — the telephone network's century-long philosophy. 00:58:18.972 --> 00:58:25.172 And fourth: two campus LANs plus one leased line between them — what have you built? 00:58:25.222 --> 00:58:29.942 Pause and decide. 00:58:29.992 --> 00:58:32.582 An internet, small i. 00:58:32.632 --> 00:58:38.402 Two or more connected networks — and nobody needed permission from anybody to build it. 00:58:38.452 --> 00:58:44.064 That is rather the point of the whole design. 00:58:44.114 --> 00:58:49.994 Before the recap: five mistakes, all of them harvested from real midterm papers. 00:58:50.044 --> 00:58:52.074 Ten seconds each. 00:58:52.124 --> 00:59:01.074 'A mesh with n devices needs n times n minus one links.' No — n times n minus one over two. 00:59:01.184 --> 00:59:03.704 Each cable serves both of its ends. 00:59:03.754 --> 00:59:08.724 The ports are the ones you do not halve: n times n minus one. 00:59:08.774 --> 00:59:16.564 'A star dies when any link fails.' No — one link failure isolates exactly one device. 00:59:16.614 --> 00:59:19.544 It is the hub whose death kills the whole system. 00:59:19.594 --> 00:59:23.934 Do not hand the star's one real weakness to the wrong component. 00:59:23.984 --> 00:59:30.674 'Cut a bus and each half keeps working.' No — a break stops all transmission. 00:59:30.724 --> 00:59:39.674 The damaged point reflects signals back as noise, in both directions, and that noise collides with every transmission on the intact half. 00:59:40.594 --> 00:59:48.584 'Cut one mesh link and nothing at all is lost.' Also no — and notice this is the opposite temptation. 00:59:48.634 --> 00:59:55.054 That pair's dedicated direct path IS lost; what survives is everyone else's links. 00:59:55.104 --> 00:59:59.884 Precision in both directions: not a catastrophe, not nothing. 00:59:59.934 --> 01:00:02.484 And the vocabulary pair. 01:00:02.534 --> 01:00:08.934 'A WAN is a big LAN' — no: a WAN interconnects connecting devices and is leased. 01:00:08.984 --> 01:00:17.934 'internet is short for Internet' — no: an internet is any connected set of networks; the Internet is the specific global one. 01:00:20.814 --> 01:00:25.074 Let me close by naming exactly what you should be able to do now. 01:00:25.124 --> 01:00:34.074 Define — network, host, connecting device and link, and judge any network by performance, reliability and security — including why throughput and delay fight. 01:00:35.224 --> 01:00:42.864 Count — a full mesh: n times n minus one over two links, n minus one ports per device. 01:00:42.914 --> 01:00:46.164 Halve the links, never the ports. 01:00:46.214 --> 01:00:54.014 Predict — what one failure costs each topology: a link, a tap, a station, the hub, the backbone. 01:00:54.064 --> 01:00:57.844 Blast radius from geometry alone. 01:00:57.894 --> 01:01:04.614 Fix — a ring: the dual ring for a dead cable, the bypass switch for a dead station. 01:01:04.664 --> 01:01:13.614 And distinguish — LAN from WAN, internet from Internet, circuit from packet switching — and name the trade each choice makes: what you reserve is what you waste, what you share is what you queue for. 01:01:21.924 --> 01:01:26.144 If any of those feel shaky, the section that covers it is still there. 01:01:26.194 --> 01:01:31.497 This is a video — use it as one. 01:01:31.547 --> 01:01:33.287 Five last questions. 01:01:33.337 --> 01:01:35.307 Pause and answer them on paper. 01:01:35.357 --> 01:01:36.567 One. 01:01:36.617 --> 01:01:42.807 A full mesh of ten devices — how many links, and how many ports per device? 01:01:42.857 --> 01:01:44.007 Two. 01:01:44.057 --> 01:01:50.107 In the lab's star of forty machines: one cable is cut — who is down? 01:01:50.157 --> 01:01:52.547 The switch dies — who is down? 01:01:52.597 --> 01:01:53.777 Three. 01:01:53.827 --> 01:02:00.747 Which topology is the only multipoint one, and what does one break in its backbone do? 01:02:00.797 --> 01:02:01.987 Four. 01:02:02.037 --> 01:02:07.107 A 1995 landline call — circuit or packet switched? 01:02:07.157 --> 01:02:10.157 And what is wasted while nobody speaks? 01:02:10.207 --> 01:02:15.447 Five. internet versus Internet — one sentence each. 01:02:15.497 --> 01:02:19.547 Pause now. 01:02:19.597 --> 01:02:22.287 Take your time on these. 01:02:22.337 --> 01:02:29.017 One — ten times nine over two: forty-five links, and nine ports per device. 01:02:29.067 --> 01:02:32.307 Two — one machine; then everyone. 01:02:32.357 --> 01:02:39.217 Three — the bus; a break stops all transmission, because the break echoes noise in both directions. 01:02:39.267 --> 01:02:44.797 Four — circuit switched; the reserved circuit sits idle while nobody speaks. 01:02:44.847 --> 01:02:51.067 Five — an internet is any two or more connected networks; the Internet is the specific global one. 01:02:51.117 --> 01:02:58.850 If you got all five, you are where this session wanted you. 01:02:58.900 --> 01:03:00.410 So take this with you. 01:03:00.460 --> 01:03:03.680 Topology is a bet on which failure you can afford. 01:03:03.730 --> 01:03:10.780 Ask that question whenever anyone shows you a network diagram; it is the question the diagram was drawn to answer. 01:03:10.830 --> 01:03:19.780 Before the next session, read Forouzan sections 2.1 and 2.2 — protocol layering, and the TCP/IP (Transmission Control Protocol / Internet Protocol) suite. 01:03:21.520 --> 01:03:24.110 And one question for the next session. 01:03:24.160 --> 01:03:29.970 What does the post office know about what is inside your envelope? 01:03:30.020 --> 01:03:35.790 That question is the whole of Session 3: protocol layering and encapsulation. 01:03:35.840 --> 01:03:37.730 Still no calculator needed. 01:03:37.780 --> 01:03:39.770 And one date for the diary. 01:03:39.820 --> 01:03:44.070 Weekly Online Quiz A1 opens on the course website on Sunday, the first day of Week 2. 01:03:44.120 --> 01:03:49.640 Twenty minutes, one attempt, and it closes on Saturday at midnight. 01:03:49.690 --> 01:03:57.190 There is one of these every teaching week from now on, and your best eight of the ten count towards your grade. 01:03:57.240 --> 01:04:01.775 See you in Session 3.