Traffic Analogy

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來自 Princeton University 的課程

Networks Illustrated: Principles without Calculus

39 個評分

Princeton University

Networks Illustrated: Principles without Calculus

39 個評分

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. Please note that per Princeton University policy, no certificates, credentials or reports are awarded in connection with this course.

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.

Unlicensed Spectrum5:01

Traffic Analogy4:43

WiFi Standards5:34

WiFi Deployment6:24

Accessing WiFi10:55

Interference7:49

Controlled vs. Random Access6:49

Random Access Protocols & ALOHA7:00

ALOHA Successful Transmission5:15

ALOHA Throughput7:12

ALOHA Inscalability8:19

CSMA Carrier Sensing5:54

CSMA Backoff7:49

CSMA vs. ALOHA5:41

與講師見面

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

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.

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