So, as you'll see throughout this course, we tend to draw analogies to transportation networks. Because they lend themselves very well to scenarios you can relate to everyday of our lives. Here, let's consider the idea of a traffic light versus a stop sign. So when you're driving, what happens when you come up to a traffic light as opposed to what happens at a stop sign? And they operate differently. There are different ways of regulating traffic throughout an intersection. Or having some, some form of coordination. Let's look first at the idea of a traffic light, and we'll see that it's kind of similar and analogous to the idea of a cellular network. So, this will be the difference that we look at between cellular and WiFi. So, when you're sitting at a traffic light, if the light's red, you know that you can't go through the intersection because it's completely dedicated to whoever is running perpendicular to you on the other side. And only these cars can go through when you're sitting at traffic light. And you may have gotten upset at one time or another because you may see other people not going the other way. So, the other side isn't being used at all. And you may think well this is really inefficient right now cause I could just go through the intersection cause no one else is coming. Why do I have to sit here and wait? Well, the reason is that it's dedicating the resource at that time to the other side to whoever's coming perpendicular to you. So, it doesn't matter who's going through there, how many people are going through there, or any of that, it's dedicated to them at that specific time. So, while traffic lights may not work very well when you have low traffic demand. But let's forget for a second the idea that the lights will, will change much more rapidly when there's less demand. Just assume that you, you know, you have to sit at the intersection for a relatively long time. They work well in case that we have high demand, because they regulate the flow. Okay. So, all the traffic will be flowing one way and they could stay up to speed and not have to keep stopping constantly. And not have to stop every time they hit another intersection, ideally. And it would get regulated nicely, and then once it's your turn then you can go and everybody else on your side can go. Otherwise, if they were all stop signs on the way home from work, we'd have a much more bumpy ride. And so, it works well in this situation. And that's kind of analogous to a cellular network. And perhaps it's better that we look at the other case of stop sign first rule we see that. But when we saw with cellular, the way that they allow people to access the air at the same time, in this case the resource would be the intersection. Whenever you're allocated a certain time you can use the network, you have that resource for the time that you needed or the time that's assigned to you. And no one else can use your frequency in the case of FDMA or time in the case of TDMA or code, in the case of CDMA. No one else can use the research that was assigned to you when you're using it. But you still have to wait assignment, just like the other traffic. They cannot go the other way while light is green, but once their light turns green and your's turn red, you also have the same problem. You can't go through the intersection at that time because then it switched and being used for them. Now, with WiFi, it's more like an intersection governed by a stop sign. Still we have the intersection as the resource, or in the case of networking, we're talking about the air at this point, but it's, it's a shared resource. So, the way that we share the network medium is by allowing everybody to share the same resource. So, this prevents, for instance, wasted periods. Because if you're sitting at a stop sign and you see that no one else is coming, you look both ways first, but then you can just go straight through. You don't have to wait until the light turns green or anything like that. So, it's going to prevent these quote unquote wasted periods that we talked about before and that's the idea behind the stop sign. But we still need some coordination. So, you have to do some work yourself because you have to like look both ways, for instance, so that you don't crash into anybody or else something like this could happen and that's not what we want to happen. But we'll see how WiFi has to deal with the idea of collisions. In the case of WiFi, we're talking about colliding frames or messages which is clearly nowhere near as serious as when cars crash with one another. But stop signs are really analogous to what we do in WiFi. So, in cellular, the way that we share the network medium is by dedicating resources. So, don't get confused when I say sharing down here, then. Because that this is the way that we're sharing the network medium is by sharing the resources. In this case, we saw the intersection. We can all share the intersection at different times. While, while we're in the intersection, no one else can be there or else we'll have a collision. But we allow anyone to access the network medium as long as they follow some sort of coordination. So if they see that no one else is coming, or they see no one else is transmitting, as in the case of the WiFi, then we say, well, go ahead.
