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En provenance du cours de Princeton University

Networks Illustrated: Principles without Calculus

43 notes

Princeton University

Networks Illustrated: Principles without Calculus

43 notes

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.

À partir de la leçon

PageRank by Google

In this lesson, we will take a look at PageRank, Google's famous algorithm for ordering the results on its search page. PageRank is a prime example of how coming up with the right "ranking" of a set of items is a difficult yet important question in networking.

New Word in the Dictionary5:01

Search Engines7:03

The "Random Surfer"7:38

Importance Equations5:31

PageRank Example Calculation10:40

PageRank Example Summary5:14

Dangling Nodes & Disconnected Graph6:22

Robust Ranking3:24

Rencontrer les enseignants

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

Alright, hello and welcome back. Today we're going to talk about an

interesting topic that probably impacts you a lot in your daily life.

Which is how Google ranks webpages. And in this chapter we're going to use a

little more math than we used in the previous chapters.

we're going to have to solve a system of equations, a very simple one.

But we'll walk through it step by step and really explain to you how to do that

and we'll gain an appreciation for why that's very important.

So I just wanted to point that out. And this chap, this this lecture will

also be shorter than the previous two lectures that we looked at.

So, previously in the last two, we were looking at first cellular devices, and we

looked at. How your cellphone knows what to

transmit, and what power to transmit at. And then with WiFi, we looked at the

random access protocols and different ways of sharing under WiFi.

So here were shifting gears and we're talking about a web based application,

which is Google. And we're going to look at Google Search

specifically and focus on that. So this is united under the general theme

that Consensus Is Hard which is the second principle that we go through.

In the optional reading available for this course.

Under that window, we also have what's called AdWords which is Google's way of

selling ad spaces to people. And we look at that as well.

We're not going to have time to look at that in this course.

But we do do that in the new book and perhaps in the future, offering of this

course we'll go back and revisit that. Page Rrank, which is also what we're

going to be doing today. We're going to be looking at that.

And then, another one is consensus formation on Wikipedia.

So, how they follow the principle and process of rough consensus, to come up

with a consensus on things. So you can see how each of these are

really a consensus problem. With AdWords you have to figure out

basically which people should get assigned to which ad spaces.

With Page Rank we have to come up with a consensus as to which webpages should be

ranked higher and displayed first in the search results page.

And on Wikipedia we have to come up with a process of rough consensus.

We have to follow the principle of rough consensus in order to come up and figure

out which edits should be kept. And which ones should be tossed away when

there's conflicting opinions resulting. And so, but here we're going to be

focusing again on the Page Rank aspect. So let's talk a little bit about Google

today and some of the things that we see with Google.

First thing is that it's really ubiquitous, it happens everywhere,

everybody is constantly using Google Search.

And in fact, you may sometimes say, well, I Googled that or I Googled this today.

And you're probably thinking, well, that's not a proper term but, in fact, it

is. because in 2006, Google itself was

declared a verb in the dictionary. And so it is actually a word and you can

use it, and it really means even beyond Google just means a search, on a search

engine in general. So you can say you googled something and

you, even if you're using one of it's competitors like Bing Or something like

that. And so now Google is a huge company.

They have, like, 50,000 employees, we can just write that down, 50,000 employees.

And it's actually a little higher than that now I believe, or at least at this

time in beginning in 2013 it was around 50,000.

And in the first quarter of 2013, it's revenue was greater than 14 billion, so,

it's huge revenue. But something that may not sense to you

right now at least is, well, how does Google make money?

Because searching on a webpage, searching on Google is free, right?

You don't have to pay for that. So, how is it that they're making all

this huge revenue and how do they stay in business and clearly how do they do so

well with their business? Well, it turns out that Google's revenue

comes from something that you may not have known specifically but it comes from

ads. Ads are a huge factor in Google's

revenue, specifically ads on Google sites themselves make up 85% of its revenue.

At least in this study that was done in 2011.

I believe that was the case and ten percent comes from ads on other sites.

So really we have 95% of the revenue, about, coming from ads themselves, which

is just huge. But, now where are the ads, you may have

already known this, but just to clarify, when you do a search, I've searched here

online education to Google. And you can see ads on the top here,

which are, they're colored. And this this shaded region over here.

They're also on the sidebar, and there actually is a shaded region, even below.

So running down this page, there is a, there's a whole shaded region below this,

down here. But, our focus, in this lecture, we're

not going to look at how Google makes money.

So we're not going to be focusing on ads in this lecture, just to point that out.

It's not going to be our focus. We're going to be focusing on how the top

few results get ranked and why they're ranked the way that they are.

So why does online education, US News and World Report, appear first?

What is it about that website that makes it first, the highest ranked?

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