What makes WiFi faster at home than at a coffee shop? How does Google order its search results from the trillions of webpages on the Internet? Why does Verizon charge $15 for every GB of data we use? Is it really true that we are connected in six social steps or less? These are just a few of the many intriguing questions we can ask about the social and technical networks that form integral parts of our daily lives. This course is about exploring the answers, using a language that anyone can understand. We will focus on fundamental principles like “sharing is hard”, “crowds are wise”, and “network of networks” that have guided the design and sustainability of today’s networks, and summarize the theories behind everything from the social connections we make on platforms like Facebook to the technology upon which these websites run. Unlike other networking courses, the mathematics included here are no more complicated than adding and multiplying numbers. While mathematical details are necessary to fully specify the algorithms and systems we investigate, they are not required to understand the main ideas. We use illustrations, analogies, and anecdotes about networks as pedagogical tools in lieu of detailed equations. All the features of this course are available for free. It does not offer a certificate upon completion.
Traffic Analogy
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從本節課中
Random Access in Wifi Networks
In this lesson, we will investigate WiFi, another type of wireless network. Rather than having stringent power control algorithms as we saw for cellular, WiFi relies on "random access" methods to manage interference among users in the same location.
教學方
Christopher Brinton
Lecturer
Mung Chiang
Professor
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.
